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Properties and applications of FRP in strengthening RC structures: A review
Accepted Manuscript Properties and applications of FRP in strengthening RC structures: A review
Y.H. Mugahed Amran, Rayed Alyousef, Raizal S.M. Rashid, H. Alabduljabbar, C.-C. Hung PII: DOI: Reference:
S2352-0124(18)30110-3 doi:10.1016/j.istruc.2018.09.008 ISTRUC 331
To appear in:
Structures
Received date: Revised date: Accepted date:
21 July 2018 20 September 2018 21 September 2018
Please cite this article as: Y.H. Mugahed Amran, Rayed Alyousef, Raizal S.M. Rashid, H. Alabduljabbar, C.-C. Hung , Properties and applications of FRP in strengthening RC structures: A review. Istruc (2018), doi:10.1016/j.istruc.2018.09.008
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ACCEPTED MANUSCRIPT Properties and applications of FRP in strengthening RC structures: A review Y. H. Mugahed Amran1, *, Rayed Alyousef2, and Raizal S.M. Rashid3, H. Alabduljabbar2 and C.-C. Hung4 1
Department of Civil Engineering, Faculty of Engineering, Amran University (AU), Quhal, Amran Province, Yemen. Department of Civil Engineering, College of Engineering, Prince Sattam Bin Abdulaziz University, 11942 Alkharj, KSA 3 Department of Civil Engineering, Faculty of Engineering, University Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia 4 Department of Civil Engineering, National Cheng Kung University ,1, University Rd, Tainan City 701, Taiwan
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Corresponding author, Tel: +967 775105108, Fax: +967 01 229939, E-mail: [email protected]
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Abstract
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In civil and structural engineering, building structures with robust stability and durability using sustainable materials is challenging. The current technological means and materials cannot decrease weight, enlarge
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spans, or construct slender structures, thus inspiring the exploration for valuable composite materials. Fiber reinforced polymer (FRP) features high-strength and lightweight properties. Using FRP motivates civil
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engineers to strengthen existing RC structures and repair any deterioration. With FRP, a system that can
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resist natural disasters, such as earthquakes, strong storms, and floods, can be developed. However,
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deterioration of structures has become a critical issue in modern construction industries worldwide. This paper reviews the FRP design, matrix, material properties, applications, and serviceability performance. This
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literature review also aims to provide a comprehensive insight into the integrated applications of FRP
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composite materials for improving the techniques of rehabilitation, comprising the applications toward the repair, strengthening, and retrofit of concrete structures in the construction industry today.
Keywords: Applications, Aramid/Basalt/Glass/Carbon Fiber Reinforced Polymer (AFRP, BFRP, GFRP, CFRP), Beams, Columns, Deterioration, Fiber Reinforced Polymer (FRP); Joints; Matrix, Properties, Rehabilitation, Reinforced Concrete Structures; Serviceability, and Strengthening Structures.
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Introduction Typical Materials of FRP 2.1 CFRP 2.2 GFRP 2.3 AFRP 2.4 BFRP 3 Matrix of FRP 3.1 Epoxy 3.2 Vinyl ester 3.3 Polyester 4 Mechanical Properties 4.1 Compressive and impact strengths 4.2 Flexural strength 4.3 Shear strength 4.4 Tensile strength 4.5 Creep rupture 4.6 Modulus of elasticity 5 Physical Properties 5.1 Density 5.2 Strength-to-weight ratio 5.3 Rigidity and stiffness 6 Durability Properties 6.1 Corrosion resistance 6.2 Fatigue resistance 6.3 Brittleness 6.4 Moisture content 6.5 Specific gravity, water absorption, and fraction of fibers 7 Functional Properties 7.1 Electrical and thermal conductivity 7.2 Temperature effect 7.3 Coefficient of thermal expansion 7.4 Fire resistance 8 Serviceability of FRP 8.1 Crack width 8.2 Deflection 8.3 Fatigue 9 Design of FRP 10 Strengthening techniques 11 The bond characteristics of FRP 12 Applications of FRP 12.1 Flexural strengthening 12.2 Shear strengthening 12.3 Column confinement and ductility improvement 13 Conclusion 14 Acknowledgment 15 References
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ACCEPTED MANUSCRIPT 1
Introduction
The initial use of fiber reinforced polymer (FRP) was known as reinforcement bars in 1975 particularly in Russia (Figure 1) [1, 2]. FRP is also recognized as fiber reinforced plastic, comprising materials that utilize either synthetic or natural fibers to automatically improve the stiffness and strength of a polymer model [2].
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FRPs employed to strengthen and reinforce structures are enormously strong, rated 8 times robust than
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classical steel reinforcement bar [3]. Glass fiber reinforced polymer (GFRP) is used as prestressing tendons
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to strengthen a 9 m-long, fastened wood bridge [4]. Relevant investigations on the use of FRPs as reinforcing bar to substitute the use of steel plate bonding for bridge restoration and strengthening instigated in Europe in
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the 1980s. But in the United States, FRP composites were engaged for structural strengthening for
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approximately 25 years [5]. During this period, FRP composite was accepted as a mainstream construction material parallel with the sum of accomplished FRP strengthening projects. The use of FRP for
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strengthening, rehabilitation and retrofitting has attained more reputation among design consultants over
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traditional strengthening methods, such as setting up of supplementary structural steel frames and components [6]. FRP is mainly worked as interior reinforcement, for instance rebar, or exteriorly-bonded
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reinforcement to reinforce concrete, timber, steel and masonry structures [7]. In Japan, FRP bars attained a
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significant support for the duration of the 1990s from the study on fascinatingly ascended train support structures [8]. FRP also has a unique tensile strength characteristic higher than that of steel hitherto weighs
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merely one quarter [1-8]. In 1996, the Japanese was the first team who announce the design guidelines for FRP in the strengthening of reinforced concrete (RC) structures [8, 9]. Later, the use of FRP as a structural reinforcement has enlarged exponentially, and the design supervision and guidance were authored by officialdoms worldwide [10, 11]. Structural strengthening with exteriorly-attached FRP reinforcement, in particular, with extra-high given strength carbon fiber reinforced polymer (CFRP), has been proved by design codes for seismic advancements of structures for several years. For instance, the growth of economic
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ACCEPTED MANUSCRIPT and efficient approaches to repair, upgrade, strengthen or reinforce the current RC bridges has acknowledged significant interest recently [12, 13]. The inspiration to strengthen an existing RC bridge classically derives from only two sources: a need to upgrade the strength of the bridge to preserve pace with upsurges in the weight of design automobiles and a desire to overhaul deterioration that has resulted over years of
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serviceability [14-20]. The structural imposed loading of elements may be increased via wrapping them with
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the reinforcements of FRP [4, 21, 22]. However, FRP has raised its market growth in recent years, and this
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market is expected to rapidly grow in the forecasted period. This study is aimed to technically review the FRP design, matrix, typical materials, and properties, such as impact, flexural, shear, and tensile strengths;
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strength-to-weight ratio; rigidity; electrical and thermal conductivity; and fatigue, corrosion, and fire
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resistance. This literature review also provides a comprehensive insight into the integrated applications of FRP composite materials for repair, rehabilitating, retrofitting and strengthening RC structures in the present
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construction industry.
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Typical Materials of FRP
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Figure 1: Continuous development of FRP matrix composite from the early 1970s to present [2]
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The mutual FRP composite reinforcements utilized in civil engineering are made through a pultrusion
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technique from carbon fiber (to produce CFRP), glass fiber (to produce GFRP), basalt fiber (to produce BFRP), and aramid fiber (to produce AFRP) [23, 24]. E-GFRP is the cheapest material of all structural FRPs
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and is thus the greatest consumed [25]. Unlike E-GFRP, BFRP costs higher due to lack of manufacturer capacity; however, its cost is reasonable given its superior strength to GFRP, alkalis resistance, and almost infinite resource [4]. Figure 2 illustrates the overall comparison between FRP materials and steel reinforcements based on stress–strain behavior. AFRP is not a popular structural bar because of low compressive strength regardless of fiber alignment direction and high charge [26, 27]. Aramid fiber is the best selection for ballistic-resistant fabrics since the fiber efficiently engrosses effect [28]. CFRP has the
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ACCEPTED MANUSCRIPT uppermost strength between FRP materials and broadest variety of strengths [29]. The variety is as a result of the source of carbon and production techniques. However, CFRP exhibited the highest resistance to fatigue and creep failure than other FRP materials [30]. The high charge of CFRP is counted by its great strength and resistance to fatigue and cyclic failures [31-33]. FRP materials are reviewed in detail in the subsequent
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subsections.
CFRP
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Figure 2: Comparison of FRP materials with steel [4]
Carbon fibers have diameters limited between 5 and 10 micrometers. The fibers are comprised largely of
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carbon atoms that bond both in crystals, which are less or more aligned similar to the long axis of the fiber
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given that the crystal arrangement offers high strength-to-volume ratio [21, 34]. CFRP is an enormously light and strong FRP that contains carbon fibers and possesses extremely high tensile strength and strength-to-
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weight ratio (20% the mass of steel) (Table 1). CFRP also has an ultra-elastic modulus similar to steel, which
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is popular in the aerospace and infrastructure industries. The reinforcement of CFRP composite is carbon fiber that affords the strength, and the matrix is generally a polymer resin, for instance epoxy, that attaches
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the bars together. Although CFRP can offer 50%–60% mass reduction compared with alike elements in steel,
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the cost is 2 to 10 times greater when the costs of materials and processing are considered, as claimed by William [35]. ACI [5, 23, 36] reported that the creep strain for CFRP at 20 °C, and regular humidity rests
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under 0.01% after 3000 h at load stages of 80% of comparative ultimate strength. CFRP is developed to strengthen existing RC structures, such as bridges, to avoid replacing constructions that function satisfactorily for many years [21]. Table 1: Typical properties of CFRP [37]
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GFRP
Glass fibers, which are also known as fiberglass and usually added at 0.5%–2.0% by weight to the composite, are referred to as fiberglass reinforced plastic [38]. GFRP is a sort of plastic compound that precisely uses 5
ACCEPTED MANUSCRIPT glass fiber constituents to instinctively increase the stiffness and strength of plastics [39-40] (Table 2). The resin affords a supplemental protection to the fiber thanks to the interaction between different materials [41]. GFRP has become a staple in the building industry since the mid-1930s [42]. Properties of GFRP rely on the features of the type of polymer matrix, reinforcing fiber, fiber content, fiber orientation, and the bonding
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between fiber and matrix [43]. GFRP also has extremely high strength-to-weight ratio; low weights of 9.67
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kg/m2 to 19.52 kg/m2; and resistance to salt water, chemical effect, and alkaline environment. In addition,
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GFRP has a great thermal insulation property, excellent heat resistance, and low cost [44]. The increase in the thickness of GFRP plates with more than 6.35 mm increases the strength associated with anchorages
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provided at the ends of the plates by 40% to 100% [45]. The creep strain of GFRP is approximately 0.3%–
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1% [46]. Furthermore, GFRP is mostly used in the construction of secondary structures, such as bridges, domes, and building frames or nonstructural elements, such as masonry walls [47].
AFRP
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Table 2: Typical properties of GFRP [37]
Aramid fibers are artificial high-performance fibers with molecules branded by moderately stiff polymer
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chains and categorized as heat-resistant and strong synthetic fibers [48, 49]. AFRP is one of the most useful
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fibers in textiles and fiber reinforced composites (Table 3). It has strong synthetic fibers, great strength and elastic modulus, heat resistance, 40% lesser density than GFRP, and slightly higher cost [50]. AFRP absorbs
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moisture and remains sensitive during manufacturing until impregnated with a polymer matrix [51]. AFRP is a better option given its high resistance to alkaline environments and more economical than CFRP reinforcing bars [51]. The breaking strength of AFRP at high loading rate is comparable and 40% higher than other FRP materials, resulting in only 13% reduction in strength after 100,000 cycles [48]. The creep strain of AFRP is 0.15%–1% [49]. AFRP is often used in concrete structures [56], but the industries continue to restrict the use of AFRP in lightly loaded structures because aramid fibers own extremely low compressive strength and high tensile strength [50]. 6
ACCEPTED MANUSCRIPT Table 3: Typical properties of AFRP [37]
2.4
BFRP
Basalt fibers are materials made from extremely fine fibers with nearly 10 and 20 micrometers in diameter. These materials are composed of minerals such as plagioclase, pyroxene, and olivine [37, 51, 53]. BFRP is a
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new promising technology for the construction industry and an alternative to GFRP bars [54]. BFRP
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composite is one of polymeric matrixes that can assist improving rigidity, strength, matrix interface, thermal
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conductivity, and resistance to heat, chemical and physical corrosion [55]. BFRP is also recognized for it’s a great tensile strength, elongation at fracture, and alkalis resistant in ultra-high concrete CFRP and GFRP [53,
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56, 57] (Table 4). The modulus of elasticity of the BFRP is mainly relied on the chemical empathy and
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conformation of the single BFRP fiber. Basalt is plentiful and encompasses equal to 33% of Earth’s crust [55]. The long-standing tie strength-preservation estimates of the bars after 50 years of serviceability in
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moist, dry, and moisture-saturated environs with mean yearly temperatures limited to 5 °C and 35 °C array
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from 71% to 92% [58]. BFRP could reduce automobile body self-weight by 40%–60%. However, the charge of the whole process is presently not economically feasible, and its cost is alike to that of GFRP [32].
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Considering the advantages of basalt fiber, applicable applications exist in the production of basalt–epoxy
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compounds, which are also frivolous and feature robust load-bearing characteristics that are valuable in
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weighty vehicle industries and strengthening materials for structural RC members [37, 55, 58-60]. Table 4: Typical properties of BFRP [44]
Matrix of FRP
The resin is the interaction agent of several composites of FRP and is also recognized as matrix. The most typical resins are thermosetting and thermoplastic polymers [59]. The selection of resins during production process is crucial since the choice impacts the mechanical characteristics of composites. Thermoplastic polymer is not applicable to be used for civil engineering resolutions on account of its low creep and thermal resistances. Though, thermosetting resins, for instance polyesters, epoxies, and vinyl esters (Table 5), which 7
ACCEPTED MANUSCRIPT are the utmost used resins, display suitable thermal constancy and resistance to chemical and endure low creep and stress reduction, as prescribed by ISIS Design Manual 2007 [24, 59] and revealed in Table 5. In addition, FRPs are composites that consist of fibers and matrix (Figure 3) [61]. Fibers are the elements that carry the applied loads, and the matrix guarantees the consistency of the fibers, re-transition of applied loads
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to the fibers, and defense of fibers from exterior environment [62].
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Table 5: Properties of thermosetting resins of FRP matrix [59]
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Figure 3: Typical composite geometry of FRP [61, 62] Epoxy
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Epoxy is any of the basic adhesive components or cured end products of epoxy resins. It is commonly
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applied on the surface of the strengthening area of RC elements with a little 1%–3% addition of a compound [20, 67]. Epoxy typically requires extra remedial agent at a greatly high ratio of mastic to toughened, which
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is generally 1:1 or 2:1 [67]. The main types of epoxies are non-glycidyl and glycidyl epoxy. Glycidyl epoxy
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resins are known as glycidyl ether, glycidyl ester or glycidyl amine, Non-glycidyl epoxy resins are either cycloaliphatic or aliphatic resins [64]. Epoxy is utilized efficiently as a bonding agent, sealing resin for
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moistening out mechanical textiles, and coating. Epoxy features tremendous thin-film cure properties and
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demonstrates superior micro-cracking resistance to polyester resin. Epoxy also provides 3.5% to 4.5% tensile protraction at failure [20]. For example, Sikadur 330 (1.31 kg/L) comprises two components (A + B),
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namely, a liquid epoxy resin (white paste, A) and a hardener (gray paste, B) (Table 6) [63]. The amount of adhesive and room temperature should be carefully monitored during epoxy applications. Epoxy provides excellent fiber bonding (matrix to fiber), improving flexural and compressive strengths, increasing interlaminar shear and impact strength, and enhancing damage tolerance [20, 65]. Table 6: Technical properties of epoxy (Sikadur 330, 1.31 kg/L)
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Vinyl ester
Vinyl ester is mastic formed by the esterification of an epoxy resin with an insatiate monocarboxylic acid. The retort product is then melted in a retrograde solvent, for instance styrene, to 35%–45% content by mass [71]. This resin is mostly used in bonding the GFRP and basalt fiber reinforced polymer/plastic (BFRP)
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applications [52, 60]. Vinyl ester additionally delivers enhanced FRP toughness and fatigue resistance over
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epoxy and polyester [72]. The material design with FRP decks primarily differs in fiber architecture and resin
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type. Thus, vinyl-ester resin is preferred for internal FRP reinforcements due to its excellent environmental resistance [73]. Moreover, the tie strength of GFRP bars by means of vinyl ester is higher than that of BFRP
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bars using epoxy [35]. Patnaik et al. [52] studied BFRP bars produced with the misty layup method by vinyl-
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ester resin and a fiber amount segment of 50%. The elastic modulus of BFRP bars utilizing cross-section characteristics is 43–45 GPa, and the regular rupture strain is higher than 2.5%. On the contrary, Chen et al.
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[74] used GFRP bars of 9.5 mm diameter with vinyl ester resin and contained a bar surface that was
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marginally sand layered and helically giftwrapped with a fiber content of higher than 70% by mass. Vinyl esters cured at ambient temperature display lesser creep resistance than those post-cured at 93 °C [75, 76]. Polyester
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Polyester is a resin that is most widely used in FRP composite industries because it is less expensive, resistant to corrosion, fast curing, convenient, and tolerant of temperature and catalyst extremes [73].
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However, it bears drawbacks, such as low modulus of elasticity and enhancement of up to 5%–15% only [77]. Polyester can also cause a creep [78]. Polyester has 1%–2% tensile elongation at failure compared with 3.5%–4.5% for typical epoxy resins [20, 65]. The mechanical and functional properties upsurge with the increase in the content of glass in hybrid GFRP composite with polyester resin [79-81]. Therefore, polyester is a potential candidate for structural composite applications [82]. A polyester resin is primarily infused in GFRP composites, which are used in the face sheets of sandwich-bridge decks [11]. Polyester resin is
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ACCEPTED MANUSCRIPT preferred for its minimal cost, whereas vinyl-ester resins are favored for saturated environs [73]. The improvement in eventual strength of 115% in tension and consistent 43% upgrade in modulus for incessant filament arbitrary mat glass polyester content are detected in examining for compression and tension by Fernie and Warrior [83]. Mechanical Properties
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Mechanical properties are also utilized to classify and identify materials of FRP bars. The most common
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properties considered are impact, flexural, shear, and tensile strengths, creep rupture, and modulus of elasticity. FRP composite bars have been widely used in construction in the last few decades. Table 7 lists
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the most commercially available FRP bars and their mechanical properties.
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Table 7: Mechanical properties of several classes of FRP materials [6]
Compressive and impact strengths
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Strengthening the core structural RC elements is crucial since the refurbishment of weakened structures
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would impose special techniques, advantages, and technological materials with different features that impact the structures, leading to a huge cost [36, 84, 85] (Table 8). The cost-efficiency factor is realized over a
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strength efficacy measure of intended sample (SEff), as shown in Equation (1) [31]. AFRP and CFRP have
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the lowest and highest compressive strength, respectively, in comparison with the other typical FRP materials [27]. However, the composite strength is high if basalt fibers are either positioned on the face or
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equipped by substitute layers within the composite as a sandwich form [55]. The compressive strength of epoxy-based composites is higher than that of polyester-based composites, indicating that strength of whole the composites with and without fillers with polyester as matrix is less than that of the epoxy laminates [86]. 𝑆𝐸𝑓𝑓 =
𝐶𝑜𝑙𝑢𝑚𝑛𝑠 𝐶𝑜𝑛𝑐𝑟𝑒𝑡𝑒 𝐶𝑜𝑛𝑓𝑖𝑛𝑒𝑑 𝐹𝑅𝑃 𝑜𝑓 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑡ℎ𝑒 𝑖𝑛 𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑃 𝐶𝑜𝑙𝑢𝑚𝑛𝑠 𝐶𝑜𝑛𝑐𝑟𝑒𝑡𝑒 𝐶𝑜𝑛𝑓𝑖𝑛𝑒𝑑 𝐹𝑅𝑃 𝑡ℎ𝑒 𝑜𝑓 𝐶𝑜𝑠𝑡 𝑇
[31]
(1)
However, impact strength is a measure of the sum of energy that a FRP composite material that could be captivated before fracturing under a high rate of deformation at a specific shock loading under impact. The
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ACCEPTED MANUSCRIPT strength of a GFRP compound is condensed to 9.2% at ambient temperature [87]. For example, Wu and Li [88] showed that the impact strength of samples slowly decreased from 45.0 MPa to 38.8 MPa at ambient temperature and at 300 °C, respectively, exhibiting a 13.8% reduction in strength. It is also reported that the strength of hollow columns wrapped with CFRP is improved by 66% (1 layer) and 123% (3 layers)
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compared with that of the GFRP material, which only increased by 36% (1 layer) and 105% (3 layers) [84].
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However, that of the filled column that comprised of a circular hollow section filled with concrete and
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wrapped with CFRP increased by 154% (3 layers) compared with 144% (3 layers) with that of GFRP. The interior moment of the column is increased, causing a reduction in the compressive strength capacity [18].
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Al-Sunna [7] found that the service load of beam wrapped with CFRP corresponds to a stress scale in the
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upper concrete fiber of about 40% of the compressive strength of concrete compared with the non-fibered sample. Through the ultra-high performance fiber RC showed compressive strengths with values of at least
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115% larger than that of ultra-high performance concrete [89]. Mastali and Dalvand [90] studied the use of
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252 cylinders and cube samples on the reinforcement of plain concrete using jacketing/U-wrapping method and reported an increase of approximately 31.10%, 47.07%, and 65.10% in the impact compressive strength
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of samples with recycled carbon fiber of 0.25%, 0.75%, and 1.25%, respectively. In addition, the
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compressive strength of sandwich structure strengthened with CFRP is approximately higher by 24.68% than that of none-strengthened samples [91]. Another marked fast upsurge in the strength is obtained for the
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material with lengthy bars and high volumes of CFRP/prepreg waste (PW), that is, 83% for PW and 80% for CFRP (long) [92]. The increase in fiber length found the maximum improvement in the strength of nearly 3% in the sample [90]. However, the energy engrossed in breaking the test sample, indicating by the position of sample, is restricted in the vice of the machine of testing. The notch of the sample faces the striker, and the root of the notch is under a similar level with the parallel face of the vice, calibrating dial bounded to the testing machine [93]. Impact strength is equal to energy absorbed (Joule capacity, J) by the pendulum
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ACCEPTED MANUSCRIPT hammer at the instance of impact over the width of notched face (mm), which is multiplied by the length below the notch in the pendulum hammer after breaking the specimen (mm), as shown in Equation (2) [94]. The thickness of FRP layer considerably affects the compressive strength of the strengthened concrete element zone [29]. 𝐸𝑛𝑒𝑟𝑔𝑦 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑𝑖𝑛 𝑗𝑜𝑢𝑙𝑒𝑠
(2)
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𝐼𝑚𝑝𝑎𝑐𝑡 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ = 𝑊𝑖𝑑𝑡ℎ𝑜𝑓𝑛𝑜𝑡𝑐ℎ𝑒𝑑𝑓𝑎𝑐𝑒 × 𝐿𝑒𝑛𝑔𝑡ℎ 𝑏𝑒𝑙𝑜𝑤 𝑡ℎ𝑒 𝑛𝑜𝑡𝑐ℎ kJ/m2 [93, 94]
Flexural strength
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Table 8: Qualitative comparison of several fibers used in design of the composites [85]
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FRP reinforced members are generally over-reinforced, that is, the proportion of FRP bar to concrete is larger than the balanced ratio; hence, concrete crushing of the member controls the failure mode [6, 36]. However,
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when the ratio of FRP reinforcement to concrete is less than the balanced ratio, the FRP rupture encounters a failure mode, which is not a preferred ductile failure mode [6, 55]. The reduction factor of flexural strength is
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limited between 0.55 and 0.65 in the basis of the ratio of proposed reinforcement to the neutral reinforcement
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ratio because of the deficiency of ductility in FRP reinforced failure modes [12]. The strength reduction
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factor for FRP rupture at failure is 0.55. However, when the failure is by concrete devastating, the reduction factor of flexural strength increased to 0.65, where the ratio of the neutral FRP reinforcement is smaller than
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1.4 times the proposed reinforcement ratio [12]. The flexural strength section wrapped with CFRP evidently
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decreased with increasing delamination factor [95]. However, the flexural strength of the FRP material is determined using ACI 440 similar to ACI 318 because FRP rebars do not yield similar to steel bars [5, 36, 96]. The flexural aptitude of the reinforced sections is practically supposed to be restricted by the rupture strain of the composite structures [21]. Certain studies investigated the variables and parameters influencing the flexural strength of the compounds, including the length of fiber, thermal treatments, binder content, and pre-activation of the fibers prior creation. For instance, the lengthiest fiber at 10,000 microns has the lengthiest distance between binder interaction points, causing in the feeblest composite [97] (Table 9). Park
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ACCEPTED MANUSCRIPT and Jang [98] presented carbon fibers, along with fibers of polyethylene (PE) within an epoxy form; to formulate a hybrid layered composite scheme. The hybrid-based composite is powerfully relied on the location of the fiber of reinforcement. Moreover, the placement of CFRP at the marginal sheet delivers a great grade of flexural strength. The increase in recycled CFRP fiber length in self-compacting concrete
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results in the maximum recorded increase in the flexural strength by almost 6% compared with that found in
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the constant fiber length [90]. Mastali and Dalvand [90] claimed that increasing carbon fiber content (1.25%)
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improves flexural performance of the beams by 10% and 27.23% more than the mean flexural strengths of 0.75% and 0.25% specimens, respectively. This study also reported that the flexural strength of
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reinforcement of plain concrete beams increased by around 31.20%, 50.93%, and 66.93% with recycled
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carbon fibers of 0.25%, 0.75%, and 1.25%, respectively. Basalt fibers and polyvinyl alcohol (using externally bonded reinforcement (EBR) method) are used to provide substantial backing and resist cracking, thereby
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increasing the break robustness of matrix and improving the flexural strength by nearly 27% at the yielding
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zone compared with that of control specimens [99]. Another study developed flexural beam strengthened using a dubbed carbon skin-basalt core composite and a basalt peel-carbon core compound (EBR method).
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The results showed 245% and 32% increase in flexural modulus and flexural strength, respectively, of former
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composite compared with the later composite [100]. Using AFRP provides an expressive magnitude of ductility for FRP-reinforced beams [101]. It is found that the silica/polyester composites enriched the flexural
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strength from 115 MPa to 156 MPa at dry phase [102]. Besides, using a twin layer carbon–glass fiber composite system (using near surface mounted (NSM) method) to strengthen RC beams increases the strength capacity by 114% on the strengthened beam compared with that of reference control [103]. Carbon fiber exhibited a lower strain than glass fiber, and CFRP samples have less ductility than GFRP samples. The combination of glass fiber and PE fiber shows no destructive impact on the flexural strength of the samples [104]. For flexural strengthening, the plates, tow sheets, and bars, are some of FRP reinforcement products
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ACCEPTED MANUSCRIPT that used to bond the tension side of a concrete, timber, or even masonry, substrate with epoxy mastic. Strengthening of structural flexural elements improved the load-bearing strength of up to 40%. Thus, Equation (3) is employed to compute the cross breaking strength of flexural strengthened members. Cross breaking strength =
1.5𝑊𝐿 𝐵𝐷 2
, kN, [86] ,
(3)
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W = loading, kN B = breadth in mm D = thickness, mm L = span between supports, mm
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-
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where
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Table 9: Summary of previous studies on flexural tests
Shear strength
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The interpretation of shear strengthening of RC structures by means of outwardly bonded FRP shields significantly relies on the bond performance at interaction between the FRP sheets, FRP jacket thickness,
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concrete substrates, number of layers, and epoxy materials with fibers [105-112]. The behavior should be
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parallel to the principal tensile stresses (Table 10) [11, 113-115]. The increase in the amount of moisture
PT
absorbed on the epoxy leads to loss of desired shear strength of the strengthened concrete element [116, 117]. However, to resist shear forces, an FRP reinforced member must contain ties or stirrups, or, if not practical as
CE
in the situation of RC tanks, rely only on the concrete resistance to shear loading [114]. RC design without
AC
shear reinforcing leads to deep sections where shear is critical [118]. Although a deep member may not initially be desired, it corresponds to the upsurge in the cracking moment of the element [119]. A cracking moment that is 25% larger than the applied service moment allows the use of gross moment of inertia to compute deflections and the full section of the concrete member to resist shear loads [120]. The tensile strengths of FRP rebar can be much greater compared to that of steel. Most FRP bars have significantly small modulus of elasticity or stiffness. Decreased stiffness indicates the necessity of deep members or additional reinforcement to mitigate long-term deflections and limit crack widths [118]. Shear strength design of FRP
14
ACCEPTED MANUSCRIPT uses ACI 318 methods [96], whereas ACI 440 [36] does not allow for dowel action of FRP rebar to resist shear in comparison to ACI 318 that facilitates shear resistance of steel bars. Moreover, the textile is usually formulated to reveal a certain load–strain profile in its 45° directions, permitting the ideal impact to beam shear strength [121]. However, the rosette strain gages exhibited that the sheet led to the beam shear strength
T
after shear cracking. The contribution of shear of the FRP is obtained in the basis of the failure modes, and
IP
the maximum strain is restricted to 0.004 for failure attributable to FRP rupture and 0.002 for attach perilous
CR
applications [119]. The maximum recorded 45° strain reading for the sheet at beam sides is 0.40% compared with its ultimate strain of 1.2% [121]. The ultimate strengthening intervention in hollow concrete bridge
US
column restores its ductility, strength, and flexural shear strength ratio [122]. Certain studies showed that the
AN
fiber shawl had a lower effect on filled columns than on hollow columns (Table 10). This effect is also notable in hollow columns, and GFRP is less effective than CFRP. Another study indicated that shear
M
strength of the reinforced beams, which are supported using CFRP sheets, is increased parallel with eventual
ED
stiffness and strength of the beam associated with that of the control beam and the condensed ductility of the RC beams [86]. It is also reported that the shear strength of GFRP composite decreases to 13% at the case
PT
when the temperature is elevated to 200 °C [87]. Meanwhile, the influence of bucky paper interleaves
CE
produced from carbon nano-fibers on inter-laminar mechanical characteristics of CFRP (using embedded through-section (ETS) method), exhibiting 104% and 31% enhancement in mode II fracture toughness and
AC
interlaminar shear strength, respectively [123]. On the contrary, Han et al. [124] found that the concrete structures strengthened with polyfunctional epoxy resin and CFRP tendons with different diameters (using near surface mounted (NSM) method) increased the shear strength by approximately 30%–40%. Investigation reported that a sudden reduction in the shear strength may be occur when the BFRP composites are engrossed and matured under hot salt water at 40 °C [125]. For shear strengthening, FRP reinforcements are glued to the external of the beams in a vertical U-shaped conformation as an exterior stirrup.
15
ACCEPTED MANUSCRIPT Strengthening of shear walls, such as under- RC walls and unreinforced masonry walls, can be achieved by bonding FRPs to either both or one sides on the wall in either a horizontal, vertical, or X pattern. However, the nominal shear strength is practically calculated using Equation (4) [126]. Vn = Vs + Vc
[126]
(4)
𝐴𝑠 𝑓𝑦 𝑑 𝑠
,
IP
𝑉𝑐 =
T
𝑉𝑐 = 2 √𝑓𝑐′ 𝑏𝑑
US
As = area of flexural reinforcement, mm2 d = distance from extreme compression fiber to centroid of flexural reinforcement, mm b = width of beam, mm s = horizontal spacing of shear reinforcement, mm fy = yield stress of flexural or shear reinforcement, N/mm2 f 'c = concrete compressive strength, N/mm2
AN
-
CR
where
4.4
M
Table 10: Summary of previous studies on RC sections strengthened by CFRP and steel
Tensile strength
ED
FRPs are used for interior bar and strengthening of RC structures that utilize artificial fibers in a polymeric
PT
matrix to afford tremendous tensile strength parallel to the direction of fibers [127]. The fibers are aligned in a parallel, straight, and unceasing configuration within the matrix [43]. However, if radial bursting stresses
CE
become higher than the concrete tensile strength in the element, then cracks develop and the bond between
AC
the concrete and bar is negatively influenced [59]. These FRPs are rarely recognized in the community of civil engineering as ultra-high-strength compounds and can be computed using Equation (6). Tensile strength values of FRP fibers are commonly indicated depending on the matrix, the interface of section, moisture absorption, fiber orientation, and types of fibers [117]. For example, the tensile strength increases with rising weight ratio of fiber by a certain volume [128]. However, CFRP fibers are reported to own greater tensile strength and lesser weight compared with the other typical FRP fibers (Figure 4) [90, 128-154]. This condition leads to significant weight reduction and enlarged span of prestressed structures [124] given that 16
ACCEPTED MANUSCRIPT CFRP is a purely elastic-brittle material [146]. Wu and Victor [81] fabricated a hybrid composite and bonded its interface by CFRP and engineered cementitious composite (ECC). At strain capacity of 1.7%, the tensile strength of ECC at room temperature was resulted of about 4.9 MPa. Thus, the tensile strength of ECC was improved from 3.5 MPa to 4.9 MPa (by about 40%) in comparison with the mortar, meanwhile, the strain
T
capacity enhanced from 0.011% to 1.7% (by 153 times). The adhesive tensile strength of CFRP decreases
IP
with the adding of nanoclay at higher temperatures. For instance, the tensile strength of CFRP is reduced by
CR
13.9% after 744 h of exposure [155]. It is also reported that the decisive temperatures of CFRP tendons are 330 °C, and the residual 88.7% tensile strength is at 100 °C. In addition, the tensile strength shows a drop of
US
approximately 21% from 75 °C to 100 °C, although nearly no change is seen when the temperature elevates
AN
from 25 °C to 75 °C [157]. However, the basalt fibers have first-rate tensile strength and protraction at break [88, 89] (Table 18). Investigations invented and improved the tensile strength of basalt/polypropylene
M
composites (polypropylene-g-maleic anhydride (PP-g-MA) with chopped basalt fibers) [158], basalt/epoxy
ED
composites (tourmaline micro/nano particles (0.5–2 wt.%)) [159] via the vacuum aided resin transform molding method and basalt/vinyl ester composites (by modifying the basalt fiber surface via a silane
PT
precursor as a coupling agent [160]. These laminates of TM/basalt/epoxy display that the tensile strength
CE
upsurges by 16%, while the increase of 153% and 27% is found for the flexural modulus and tensile strength, respectively [166]. Wu et al. [161] revealed that GFRP increased the tensile strength of hybrid FRP
AC
composite by 36% compared with the use of BFRP, which is 2.56% greater than that of polyparaphenylenl benzobisoxazole (PBO) composites. Other experimental outcomes showed that the impregnation of carbon fibers with neat epoxy or nanocomposite system lead to substantial gains in terms of tensile strength by approximately 57% and 58% and ultimate strains by approximately 44% and 42% [162]. Studies on GFRP composites revealed that tensile and short beam strengths of vinyl-ester composites are slightly affected, whereas the tensile strength of polyester composites sharply drops by 80% [163]. However, using MBrace
17
ACCEPTED MANUSCRIPT saturant and Sikadur 30 epoxy reports a 40% reduction and 46% upsurge in the eventual tensile strength of CFRP compound samples, respectively [87]. Besides, the tensile strength of the GFRP polyester-based composites and GFRP epoxy-based composites reduce by nearly 19.71% and 22.8% with wheat husk fillers and rice husk fillers, respectively [87]. This result resolved that the tensile strength of GFRP in hybrid
T
strengthening is more dispersed than that attained with merely CFRP or GFRP strengthening [164]. 𝑊
ultimate tensile strength of FRP, N/mm2
fy =
yielding strength of steel, N/mm2
US
ffu =
(5)
CR
where
IP
𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ, 𝑓𝑓𝑢 = 𝐵𝐷 , kN, [86],
4.5
AN
Figure 4: Factored FRP tensile strength [26, 154, 165] Creep rupture
M
Understanding the limits of a fiber is a crucial feature when choosing the form of RC composite for structural
ED
elements to withstand long-term loading [167]. Cyclic and continuous loading on FRP in exceeding of its ability to endure those loads may inspire fatigue failure, long-term deflection, or creep rupture in the
PT
structural section [168]. In structural elements; the stresses in FRP bars recommended to be less than the
CE
creep-rupture stress range to eradicate the deflections motivated by creep [169]. ACI [62, 96] and other design codes [10, 12, 119, 120, 126] endorse a reduction factor to be applied for the FRP ultimate tensile
AC
strength to decrease fatigue and creep rupture failures (Table 11). The reduction factors prescribed in the ACI codes for carbon, aramid, and glass FRPs are revealed in the following section. A reduction factor for basalt FRP is also guided in the basis of the findings of the research led by Anil [31] and Wang and Wu [168]. Wu et al. [167] reported that CFRP tendon has the greatest and poorest creep rupture performances, exhibiting with its high and low cost, respectively. Ascione et al. [168-170] and Banibayat [171] reported that the creep test only sustains three hours of impact when the BFRP tendons are below a stress rate of less than 70% and
18
ACCEPTED MANUSCRIPT when creep rupture occurs on the tendons. The creep test can withstand 500 h when the stress scale decreases to 65%. Guohua et al. [166] tested carbon GFRP composite tendons and found that the creep rate is 19.05% when the level of stress is 0.8 ffu, which is the ultimate tensile strength. The rate of creep is lower than 10% when the stress scale is lesser than 0.7 fu. Thus, the rate of creep lessens with the reduction in the level of
T
stress.
IP
Table 11: Creep ruptures reduction factors [5, 26]
CR
The creep rupture reduction factors considerably affect the usable strength of the FRP system [26, 171]. Figure 5 displays the tensile strength of several FRPs increased by the suitable creep reduction factors
US
parallel with the limit of standard stress of 80% of the yielding strength of steel. Carbon FRPs have larger
AN
usable strength that equals a condensed volume of FRP for a specified application, which can balance the rise in material and manpower costs [36, 136].
M
Figure 5: Comparison of tensile strength with creep reduction factor [6, 23]
ED
Creep of FRP materials are typically categorised into three different zones, as depicted in the schematic in Figure 6 (b) [24, 168]. The primary creep zone may exist instantly after the first elastic strain, exhibiting that
PT
the creep strain quickly rises with time [168]. The secondary creep zone is mainly significant for analysis due
CE
to that the structure will continue serviceable in this zone [172, 173]. The tertiary zone accords with noticeable material damage in the RC structure. Meanwhile, the typical improvement of strains under
AC
continued loading with time, as revealed in Figure 6 (a). The figure displays two profiles that signify two closely equal load heights of 43% and 45% (a). The primary creep zone, instantly after the first elastic strain, expecting to speedily grow with time, does not occur for BFRP bars. The secondary creep zone of constant strain over a certain period obviously advanced, followed by 8%–10% spear of strain at closely 32%–34% of the whole time to failure. Another region marginally improved, but the tertiary zone for BFRP bars seemed
19
ACCEPTED MANUSCRIPT over a tremendously short time period, and failure is sudden beyond the tertiary zone. This figure illustrates that the creep history of BFRP bars is faintly dissimilar from the ideal creep history shown in Figure 6 (a). Figure 6: Comparison of strain versus time [24] 4.6
Modulus of elasticity
T
The design of the elastic modulus is known by ACI 440.1R as the mean modulus of a manufacturing lot [5,
IP
56, 69]. Compared with conventional materials such as metals, FRP materials mandate distinct amendment in
CR
codes and standards because of their low ductility and modulus [11]. The elastic modulus of the plate material is significantly imperative when the plate is not prestressed before bonding since only stiff plates
US
can release the stresses in the standing interior steel reinforcement [173]. However, due to differences in
AN
engineering properties, this elastic modulus particularly showed marked variances in the extent and magnitude of cracking, extent of deflection, and failure mode [101]. The lowest modulus of elastic is
M
attributed to GFRP and AFRP bars (Table 2), and the largest ones are exhibited by CFRP (Table 1) because
ED
they are sensitive to adhesive thickness of the test, epoxy matrices due to moisture absorption, fiber length, and the percentage of fibers [97, 103, 116, 117]. For example, SikaWrap 230C, a type of CFRP with nominal
PT
thickness equals to 0.131 mm and a modulus of elasticity equals to 234 GPa, equals 0.009 for either CFRP
CE
coupons due to the maximum elongation [175]. Likewise, the modulus and strength of cement mixture are predicted to improve by 39% and 56%, respectively, with the addition of carbon fibers treated by silane
AC
[176]. The glass/epoxy-based composite revealed a substantial improvement in the modulus and strength as the rates of strain are enlarged [177]. However, synthetic fibers, largely polymeric fibers, typically have small modulus of elasticity. Meanwhile, basalt-epoxy-based composites presented alike modulus to glass– epoxy-based composites [178]. The reduction in elastic modulus of FRP rebars indicates to larger cracks, increase in steel cables and higher localized impacts in comparison with those of steel RC beams, thus, this reduction can maintained by the addition of bagasse fiber with glass fiber to the mix design [7, 179-181]. In
20
ACCEPTED MANUSCRIPT addition, BFRP fibers have higher strength-to-weight ratio and modulus of elasticity than GFRP fibers [23, 180, 182]. Hawileh et al. [183] studied that the elastic modulus of hybrid GFRP composite is reduced by nearly 28% when subjected to various temperatures limited between 25 °C to 300 °C. The fatigue loading was found to reduce the bond strength between the high modulus of steel and CFRP by nearly 30% to 20%,
T
respectively [87]. The modulus increases by about 8%–21% nearly linearly as the rate of strain of CFRP and
IP
epoxy composites increases under dynamic loading condition [139]. Other experimental outcomes show that
CR
the impregnation of carbon fibers with neat epoxy or nano-composite system led to significant gains in terms of Young’s modulus, which slightly increased by 6% and 8%, respectively, using the new matrices [173].
US
Moreover, using GFRP fiber is 80 to 160 times higher than those of the matrices, such as polyvinyl chloride
5
AN
(PVC), polypropylene (PP), PE, phenolic, epoxy, and polyester resin [153]. Physical Properties
M
FRP composites kept growing at a remarkable rate because these materials are utilized with naturally
ED
circulating materials or engineered produced from more than one constituent substance with significantly several physical properties, such as density, rigidity, strength-to-weight ratio, and stiffness (Table 12), as
PT
detailed in the following subsections.
5.1
Density
CE
Table 12: Typical properties of different FRP materials [5]
AC
Fiber constituent with low density and high strength, such as carbon, basalt, boron and aramid, were purchased to please the high performance tasks of air travel and space investigation in the 1960s and 1970s [11]. Regardless of the benefits of natural fibers over ordinary materials such as low density and cost, they suffer from low processing temperature [152]. The bulk density of the material is not an important criterion because the density of all fiber composites considered is lower than that of steel [34, 85, 174] (Table 12 and Table 8). Cured composites with the highest modulus and density are the strongest, and thermal treating drones away the binder, causing in a frailer composite with lesser density [97]. However, CFRP and GFRP 21
ACCEPTED MANUSCRIPT are largely used for structural applications in aeronautics thanks to their high strength despite their low density [95]. CFRP has a relatively low density (1.6 g/cm³); thus, X-Ray radiation can easily get through, leading to comparatively high resolution and still providing acceptable contrast between delamination and CFRP [95]. On the contrary, sheet basalt plastic is generally produced with thickness (t) between 1.5 and 2.5
T
mm and density (ρ) between 1360 and 1380 kg/m3 (one-third that of steel) [42]. The density of warp yarn in
IP
basalt-epoxy composites is also greater than that in glass epoxy; thus, basalt-epoxy composites can reinforce
CR
structural concrete element and show even higher tensile strength [184, 185]. Besides, a synthetic thermoplastic fiber of PE terephthalate (PET) with high density PE (HDPE) is used to produce platform
US
mooring ropes [52]. The adhesive has high adhesive paste, recognized as Sikadure-30, with a average
AN
viscosity material and have density of 1.31 kg/L for a resin blend associated with FRP materials for strengthening concrete structures [162]. Another composite was exhibited high paste strength between the
M
HDPE matrix and the basalt fiber [55]. It is also reported that the use of 58 w/t% flax fiber reinforced
ED
polyethylene bio-composites to reinforce HDPE is linearly showed a low density PE [186]. Also, the use of short hemp fibers in HDPE can reduce the tensile modulus of all composites [187]. However, the density of
PT
any designed composite can be computed theoretically using Equation (6) [188].
CE
𝜌𝑐 = 𝑉𝑓 𝜌𝑓 + 𝑉𝑚 𝜌𝑚
[188]
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑏𝑒𝑟
AC
𝑉𝑓 = 𝑇𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒𝑙 𝑎𝑚𝑖𝑛𝑎𝑡𝑒 𝑉𝑚 = 𝑉𝑐 =
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑡𝑟𝑖𝑥 𝑇𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒𝑙 𝑎𝑚𝑖𝑛𝑎𝑡𝑒
𝜌𝑐
−
𝜌𝑒𝑥𝑝
𝜌𝑐
,
where -
ρm = density of matrix Vf = fiber volume fraction Vm = matrix volume fraction Vc = volume fraction of voids ρc = density of composite laminate 22
(6)
ACCEPTED MANUSCRIPT
5.2
-
ρf = density of fibers
-
ρexp = density of a composite laminate (ASTM-D792) [196]
Strength-to-weight ratio
FRP materials are mainly used to repair, overhaul and strengthen aging infrastructures affords a motivating alternative to conventional techniques because these materials have low density, which can provide high
IP
T
strength-to-weight ratio [21]. This characteristic provides important functional and economic wealth,
CR
extending from strength improvement and mass reduction to durability characteristics, apart from its importance in transportation and many structural applications [50, 190]. Also, FRP systems provide mobility
US
to specialists to stratify the strengthening technique on any curved, flat, or geometrically unbalanced surfaces due to their exceptional formability [113]. However, CFRP and GFRP have high strength-to-weight ratio,
AN
leading to three to seven times higher than steel and half that of aluminum; these materials are also 80%
M
lighter than steel. It is also reported that fiberglass is a robust, durable, and lightweight composite material [2,
ED
55]. Composites and fiberglass have the greatest strength-to-weight ratio presented for element production [11]. The increase in the ratio of fiber weight to an optimum value and the bond with resin fabric resulted in
PT
superior mechanical characteristics. However, the extra addition of fiber ratio adversely affects the relative
CE
properties [153]. It is found that the combination of sisal/carbon fiber hybrid composites with divergent fiber weight ratios through chemical resistance test (NaOH treatment) showed that hybrid composite does not
AC
resist the carbon tetra chloride [78]. In general, FRP composite has superior resistance property to chemical attack and chloride ion compared with steel bars [44, 56, 78, 153, 163, 182, 192-198]. Table 12 presents the weights of aramid, carbon, and glass fibers. Aramid has strength-to-weight ratio lower than that found in carbon and glass fibers [5]. Carbon and aramid fibers have high strength-to-weight ratio when examined in the alignment of the fibers direction. Meanwhile, glass has low strength-to-weight ratio and still moderately high, similar to carbon or aramid [2] (Table 13) [188]. By contrast, basalt fibers have greater strength-toweight ratio and elastic modulus than E-glass fibers [23, 180]. The strength-to-weight ratio and density of 23
ACCEPTED MANUSCRIPT FRP are clear features in transportation, handling, and insulation; these features reduce the weight of the concrete structures and any other relevant products. Table 13: Comparative chart of glass, aramid, and carbon fibers
5.3
Rigidity and stiffness
T
FRP composites are considered by specific strength and stiffness that surpasses that of the same metal
IP
structures [173]. Many researchers found that FRP composites have a significant applicable use for several
CR
civil engineering applications, and they generally develop to become the first material between the other alternatives for rehabilitation and retrofit of RC structures given their high stiffness-to-weight ratio. FRP
US
material is preferable to other traditional materials [113]. However, the concrete element stiffness or its yield
AN
load cannot easily be increased unless large cross sections of these materials are used to participate considerably to the element load prior the steel yielding mode [156]. The slope of the load-deflection curve
M
in all zones is governed by the stiffness and rigidity of the samples. Therefore, the increase in deflections
ED
indicates the retraction of stiffness given that damage is accrued with load cycling [21]. The joints of FRP lattices control bond stiffness and rigidity, thus affording a occasionally bonded strengthening system in
PT
cases where nominal connection occurs between the cross-over points [191]. The composite materials with
CE
high rigidity, great stiffness structural fibers, and frivolous, such as boron, carbon, and aramid, have significant mechanical characteristics and durability than the composites alone [11, 114]. A large FRP cross
AC
section may not be economical and may result in a brittle response of the element down to sudden debonding of the strengthening material from the concrete surface [121] (Table 14). The bond strength measures the efficiency of the grip among FRP bars and concrete, and the comparative slip between concrete and FRP is 0.125 mm at unrestricted end of the bar in a shear test on plain bars (Figure 7). Figure 7: Bar-pullout bond test apparatus [192] The pre-compression influence from reduction on the bar does not impact the cracked stiffness, and thus, have no great effect in the overall load-deflection behavior after cracking [193, 194]. For instance, the 24
ACCEPTED MANUSCRIPT toughness of the reference steel-reinforced beam is similar to that of the AFRP-reinforced beams before cracking. Yet, the toughness is numerous times greater than the corresponding reference values after the post-cracking range [101, 175]. It is also reported that the energy ductility of the CFRP-strengthened beam and GFRP is 2.6% and 33%, respectively, since the beam reinforcement that upsurges eventual load capacity
T
significantly increases stiffness and rigidity and decreases deflection [103]. In addition, the stiffness and
IP
strength of the GFRP epoxy-reinforced based composites are 18% higher than that of the BFRP epoxy-
CR
reinforced material [184]. On the contrary, the rigidity and stiffness of carbon fiber are twice that of aramid and five times that of glass fiber [27]. Besides, the stiffness and strength of the natural FRP composites are
US
mainly relied on fiber loading [153]. The composites made of PE fibers along with carbon fibers within an
AN
epoxy matrix resulted superior structural features of the hybrid-based composite relies largely on the alignment of the reinforcing fiber [98]. The strength and stiffness of CFRP cables are close to that of steel
M
[157, 181]. Moreover, the steel-FRC and the FRP grid are effective in increasing the toughness of the steel
ED
deck plate by 47%, 9.45%, and 63.16% with a 4 mm-thick and 60 mm-thick CFRP grid, respectively [188]. Furthermore, the FRP bars stiffness exhibited insignificant changes as a result of freeze-and-thaw (FT)
PT
exposure [23]. The decrease in stiffness is great when the amount of moisture collected by the FRP
CE
composite specimen is large [116, 117]. CFRP/GFPP shows a reduction of 42% in stiffness for the saturated condition when compared with the dry condition [196]. The mechanical properties of GFRP reinforcing bars,
AC
particularly the strength and the stiffness of the composites under elevated temperature, reduced substantially due to the stiffness and strength of the resin lessened hastily when the temperature surpassed its glass transformation temperature [87]. Table 14: Summary of previous studies on FRP material bonding with surface of RC elements
6
Durability Properties
To consider the material durability, existing design codes classify ecological reduction factors for any FRP property that can be used in the design. Therefore, durability properties such as corrosion and fatigue 25
ACCEPTED MANUSCRIPT resistances, brittleness, and moisture content as well as specific gravity, water absorption, and fraction of fibers are scientifically reviewed in the subsections. 6.1
Corrosion resistance
Reinforcements of FRP are initially utilized in RC structures that entail a better corrosion resistance [11].
T
Using FRP composite materials also improves the enactment, condensed drag, used and enriched durability
IP
and resistance of corrosion in RC structures. FRP structure also requires smaller work crews, lighter
CR
equipment, and lighter supporting structures during installation. These advantages translate into better engineered systems such as low stress applications that perform better, last long, cost-effective, and decrease
US
long-term maintenance costs when compared to steel material [50, 157]. Thus, these materials are widely
AN
used in engineering applications [197]. However, FRP is not recommended in dealing with concentrations of corrosion resistance (90 °C to 15% concentration) beyond 50% (Table 15) [50]. Corrosion resistance is
M
controlled by the laminate structure and the resins used. A wide variety of thermoset resins are available to
ED
satisfy a wide range of service requirements, such as polyester or vinyl ester resin, reinforcements (mat and fiberglass roving), and additives (UV inhibitors, pigments) [198, 199]. Yang [37] introduced brominated
PT
epoxy vinyl-ester resins and E-glass that delivered fire retardancy in addition to corrosion resistance as a key
CE
requirement for FRP equipment in many civil, structural, and industrial applications. The results shown that basalt fibers have higher corrosion resistance and greater chemical durability, and could be used in a
AC
chemical environment for long-term service due to these excellent features [182, 198]. In addition, glass fibers under stress are less sensitive to a corrosive environment [200]. However, corrosion resistant fiberglass panels can be manufactured in thicknesses of 3.18 mm to 31.75 mm [199]. Investigations depressed using carbonation fillers in FRP composites envisioned for acid facility since these fillers can increase infusion and diminish resistance to corrosion [182, 199]. The resin-rich veil plies and the mat layers in the corrosion barrier contain almost 90% and 70%–75wt% resins, respectively, creating an efficient boundaries for
26
ACCEPTED MANUSCRIPT corrosion and permeation. A gel coat with UV inhibitors can be applied to the exterior of the panel during manufacturing to improve weathering characteristics. Table 15: Comparison of resistance to corrosion by several composites in corroded environs [76]
6.2
Fatigue resistance
T
Structural element is designed to resist bending and straighten repetitively; thereby it ultimately fails
IP
attributable to fatigue. For instance, CFRP is marginally critical to fatigue and inclines to fail calamitously
CR
without prior signs of distress, thus presenting low fatigue strength values and acceptable damping characteristics compared with epoxy-based composites [117]. Pultruded rods based on carbon CFRP have
US
been progressively utilized in structural applications in several engineering areas down to their outstanding
AN
properties, for example lightweight and great fatigue resistance [208]. Meier [34, 85, 174] soaked CFRP composite in water to nearly 100% saturation. After 12 million cycles, the first steel reinforcement failed due
M
to fretting fatigue. Study exhibited the tensile fatigue performance of the hybrid-BFRP and hybrid-FRP based
ED
composites [161]. A 45% capacity drop is observed due to fatigue load. Similarly, AFRP is more resistant to fatigue. However, this material is susceptible to damage by ultraviolet radiation [23]. GFRP fibers have
PT
higher resistance to fatigue, relying on the setup and the type of glass, thereby indicating higher fatigue
CE
strength than CFRP fibers [188]. Equation (7) is used to reveal the best distribution fits of fibers in FRP
analysis [202].
AC
composites for the life of fatigue and the identical 95% confidence intervals based on previous research
f(x) =
1 0.5987√2𝜋𝑥
[−
(𝐿𝑛𝑥−4.202)2 0.7169
]
[202]
(7)
The use of BFRP based on hybridization for fabrication of a long-span cable-based bridge of a 70 mm-thick steel fiber, resulting that the impact of fiber hybridization improves in the whole fatigue performance [203, 204]. Another research reported no such reduction in bond strength for samples with great modulus CFRP sheets at the case when the ratio of load is larger than 0.55, along with the total 10 million of fatigue cycles
27
ACCEPTED MANUSCRIPT [87]. The isophthalic and bisphenol fumarate resins are classified the greatest fatigue resistance compared with the vinyl-ester resin [161, 205, 206]. The effective design of hybrid-CFRP laminates composite consisting of wave sheets and a tinny stainless steel plate was tested under tension loading [208]. The findings shown that the CFRP thickness and loading conditions are the main parameters influencing the
T
mode of failures and resistance to fatigue as well as the crack propagation control depends on the number of
IP
layer of side bonded plates and its thickness [205]. Zheng X.H. et al. [206] examined the fatigue behavior of
CR
the carbon fiber laminate (CFL) border exposed to humidity and temperature variations. The results exhibited that the accepted temperature and RH negatively influenced the bond performance of CFL and the fatigue
US
life reduced by a bigger stress level. However, the presence of resin can reduce the slip in pultruded FRP
AN
composites under fatigue loading [207]. Studies investigated the damage behavior of BFRP and FRPstrengthened RC composites tested to fatigue loading, and the results shown that the deflection of beam, the
M
strain can be computed with a high degree of accuracy and the cracking pattern and interfacial debonding are
ED
the key damage forms at the low fatigue stress levels for BFRP [209, 210]. Furthermore, the performance of glass fiber based-reinforced GF/epoxy structures entrenched with shape memory alloy (SMA) under cyclic
PT
loadings [211]. However, the findings displayed that the fatigue of SMA structures is twice greater than
6.3
Brittleness
CE
GF/epoxy composites due to the robustness of laminates of SMA composite.
AC
Concrete is the most brittle material. However, polymers exhibit brittleness and non-ductile properties. Thus, these materials cannot display a continuous rate of strain protraction under the tensile test [177]. The FRP brittleness is deliberated when forecasting the performance of retrofitted elements [212]. This brittleness cannot permit the stress redistribution in RC elements, and hence, the ordinary design philosophies are unacceptable for FRP RC members. The resin and reinforcement are the two main constituents of the FRP structures [155]. For instance, the thermosetting resin is generally very brittle [73, 163]. The properties are
28
ACCEPTED MANUSCRIPT massively enriched by the addition of reinforcing fiber such as glass fiber, carbon, or aramid [46, 48, 59]. The issues of brittleness and weakness in the CFRP composites can be maintained by a hybridization method, that is, by swapping the sheets of the carbon fibers with flexible fibers [213]. Reportedly, the carbon composite laminates can decrease its tensile strength with increasing modulus on account of the material
T
brittleness at higher modulus can cause great stress concentrations [50]. Thereby, the strain at rupture is
IP
highly lower and the carbon fibers exhibited greater brittleness than glass fibers [95]. Study reported that the
CR
upsurge in brittleness of CFRP laminates as the temperature is condensed [87]. UV photons result photooxidative reactions that adjust the vinyl-ester matrix and chemical composite of polymers, causing in
US
material weakening and contributing to extreme brittleness and likely micro-cracking [215]. Tetta et al. [216]
AN
reported that the configuration of two layers FRP-mortar system is more efficient by almost 92% than when of a single FRP were used in U-wrapped formulation, but the breakability of the FRP-mortar system still
Moisture content
ED
6.4
M
persisted. This problem can be maintained by incorporation with other appropriate fibers.
The FRP mechanical and electrical properties are greatly significant; these features largely rely on the
PT
attendance of moisture in the structures [217]. The moisture propagates parallel the GFRP rod, contributing
CE
to thermal failures [218]. The engrossed moisture approval content (Mt) can be computed in line with its weight after exposure (w
after exposure)
and before exposure (w
before exposure),
as presented in Equation (8).
AC
Moisture content is recognized as the function of the square root of time. The moisture saturation level (Mt) is roughly 0.77%, which is consistent with the gravimetric practical results attained in the previous research [219]. 𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (𝑚𝑡 ), % =
(𝑤𝑏𝑒𝑓𝑜𝑟𝑒 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 ) − (𝑤𝑎𝑓𝑡𝑒𝑟 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 ) (𝑤 𝑎𝑓𝑡𝑒𝑟 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 )
× 100 [117]
(8)
The carbon fiber volume fraction is 60% + 1% for all materials reported [116]. The diagrams show that the moisture absorption tends towards an equilibrium value, which depends on the material. The maximum 29
ACCEPTED MANUSCRIPT moisture content of carbon fibers (CF, polyetheretherketone, and PEEK,), epoxy (EP), and resin is approximately 1.6 wt. %, 2.5 wt. %, and 0.3 wt. %, respectively [123]. The specimens absorbed moisture, and the curves reached the saturation level fast when the water temperature is high [123]. For example, the elongation values of the 90% CFRP laminate composite decreased with increasing moisture content [116,
T
117]. Adams and Singh [220] suggested that the temperature of FRPs decrease to 32 °C and 40 °C when the
IP
moisture content is 2% and 1.6%, respectively. Investigations studied the acceptance level of BFRP
CR
composites engrossed in salt water and the impact of moisture captivation BFRP structures aged for 240 days [221, 222]. The results exhibited that the Young’s modulus and tensile strength of the composite reduced
US
marginally. Sergio et al. [21] utilized CFRP composites to improve the flexural capacity of RC beams
AN
subjected to moisture and environmental actions. After eight months of beam exposure, results showed no negative impacts on the interaction between the surface and the composite of the concrete. It is also reported
M
that when the GFRP composites exposed to a moist environment with comparative moisture of 80% at 50 °C;
ED
the result of moisture relies on the type of composite used. For example, the vinyl-ester-based composites and the modified polyester-based composites have the greatest and worst valuable moisture dispersion
PT
characteristics, respectively, and the epoxy-based materials had tolerable absorption rates. However, these
6.5
CE
composites progressively engross higher moisture in a non-Fickian manner [217]. Specific gravity, water absorption, and fraction of fibers
AC
FRP grating has almost two-thirds the specific gravity (SG) of aluminum and nearly one-fourth that of steel [76]. PE fiber has a SG of 0.97. Therefore, it is the merely reinforcing fiber found that is nimbler than water, whereas flame-retardant resins have high SG [199]. The high SG of FRP composites has the tendency to physically reduce permeation. Furthermore, the SG design criterion is one of the main parameters involved in the construction of a filament-wound layer thickness [68]. The SG is usually measured in line with Indian Standard IS: 10192-1982 [223]. The sample should be equipped for the test with accuracy rate of 40 ± 1 mm
30
ACCEPTED MANUSCRIPT with similar thickness to that of the laminate of 4 mm. The sample is first weighed in air by swinging it with the assistance of a filament restricted to the hook of the balance, and then the weight (Wair) at air state is recorded Equation (9). The weight (Wwet) is also noted by gauging the sample in fresh purified water. The sample attains the water temperature by immersing it in water for sufficient time. However, no air foams
𝑊𝑎𝑖𝑟
IP
[217]
𝑎𝑖𝑟 − 𝑊𝑤𝑒𝑡
(9)
CR
Specific gravity = 𝑊
T
should twig to the sample.
The absorption of water test was performed in line with Indian Standard IS: 1998–1962 [152]. A square test
US
sample of 38+0.5 −0.0 mm was established. The sample weight was initially recorded in air (Wwet), and then the
AN
sample was engrossed in purified water for a retro of 24 ±1 hour. The sample was wiped appropriately and weighed within two minutes after removal from water. This weight was then recorded as (Wair), as shown in
M
Equation (10), as follows:
𝑊𝑤𝑒𝑡 − 𝑊𝑎𝑖𝑟 𝑊𝑎𝑖𝑟
ED
Water absorption =
× 100 [159]
(10)
PT
The volume fraction of glass content (glass fibers) was found as stated by ASTM D2584-08 [217]. The test
CE
samples should be prepared. A dried ceramic pot is weighed (W1), and then weighed again together with the sample (W2). Then, the pot with the sample is sited in a soften heater at a temperature of less than 600 °C.
AC
When the carbonaceous material vanished, the crucible is cooled to a room temperature, and then weighed again with glass fibers left alone (W3). Finally, the glass content is computed using Equation (11), as follows: 𝑊 −𝑊
Ignition loss = 𝑊2 − 𝑊3 × 100 2
1
Glass content = 100 — Ignition Loss [226]
31
(11)
ACCEPTED MANUSCRIPT Several studies reported that the whole composites revealed the same volume fractions of fibers with the regular fractions limited between 53%–57% with moderately slight scatter, with the exclusion of the high seed-glass/vinyl ester structure. The more seed-glass/vinyl ester structure samples had almost 11% greater volume fraction of fibers. However, the rate of moisture content depends on the volume fraction and
T
diameters of fibers and hastily increases by increase the sum of FRP laminate layers (Table 16). The ECR
IP
(high seed)-glass/vinyl-ester composite specimens exhibited significantly higher volume fractions of fibers
CR
by approximately 66% than the other seven composite systems, between 53%–57%. The rates of moisture absorption and the high moisture contents found from those samples are highly lower than the corresponding
US
information for the other structure (Table 17).
AN
Table 16: Fiber volume fraction corresponding to the number of laminate layers of CFRP/GFRP [85] Table 17: Volume fractions of fibers (%) and rate of water absorption for different fibers [217]
Functional Properties
M
7
ED
The functional properties of the FRP composites are largely affected by the choice of fiber. The selected fibers are required in the design for composite applications in industries, civil and structural engineering,
PT
compromising with metallic structures. The functional properties of FRP composite materials include
CE
thermal and electrical conductivity, temperature effect, thermal expansion coefficient, and fire resistance, as reviewed in the subsections (Table 18).
7.1
AC
Table 18: Functional properties of different FRP materials
Electrical and thermal conductivity
The thermal conductivity (merely 1/900 that of aluminum and 1/187 that of carbon steel) is obviously benefit at the processes of packing, using, or transferring liquids at higher temperature. Loss of heat develops much smaller, and the hazard that hot apparatus foxes for labors is condensed [83, 206]. CFRP is an electrical insulation and conductive, while, GFRP and AFRP cannot connect electricity [224]. AFRP is utilized for guy lines in broadcast towers. Although it is not transmitting, it can engross water, and the water behaves 32
ACCEPTED MANUSCRIPT electricity. Therefore, in such composite materials, a waterproof layer is used to the AFRP. Galvanic corrosion is an anxiety when it is in interaction with metallic materials because carbon fibers do not conduct electricity [225]. The low thermal conductivity of inorganic polymer coatings also greatly contributes to fire resistance [72]. Studies reported that the use of carbon nano-fiber (CNT)-coated alumina fibers and sic fibers
T
increased the thermal conductivity by 200% and 50%, via the IR microscopy technique, respectively [233,
IP
227]. Liang et al. [228] developed CNT on carbon fibers using the 3-Omega technique and reported
CR
improvements of 33% in thickness. Forty five boaters with carbon fiber poles and masts have educated to isolate their aluminum hooks and networks to reduce corrosion. Veedu et al. [227] reported enhancements in
US
the electrical conductivity on sic fibers with developments of 440% and 360% in the through-thickness and
AN
in the in-plane directions, respectively. Besides, basalt fibers are used to replace asbestos as heat insulators because of their weak thermal conductivity but having a high protection against fire hazards [229, 230]. Temperature effect
M
7.2
ED
In general, the ecological degradation experiments on temperature impacts for FRP materials are restricted to a justly short period, generally not exceeded five years [219]. Though, the predicted service life of
PT
infrastructures, for instance bridges surpasses 50 years where the long-term performance of FRP structures
CE
cannot be effectively anticipated [219]. In terms of the temperature effects, the mechanical performance significantly decreases when the test temperature approaches the glass transformation temperature (Tg) of
AC
FRP composite materials [231]. In particular, the variation of the mechanical performance of CFRP, GFRP, BFRP, and AFRP reinforcing bars subjected to low temperatures ranges from 100 °C to -200 °C, 0 °C to −100 °C , 75 °C to 200 °C, and 25 °C to 180 °C, whereas the elevated temperatures range from 200 °C to 600 °C, 23 °C to 315 °C, 200 °C to 800 °C, and 180 °C to 482 °C [55-60, 63, 74-77, 80-90, 99-103, 163, 177, 190, 196, 264]. Karbhari et al. [232] exposed concrete cylinders reinforced with CFRP sheets to 200 freeze-thaw (FT) cycles limits from 22.5 °C and −20 °C, resulting in a more sudden rupture between the
33
ACCEPTED MANUSCRIPT concrete and reinforcement instead of control. Another research exposed 50 cylinders to 200 and 100 FT cycles between 20°C and −20 °C, respectively, at 70% comparative moistness. The results exhibited closely no impact on the values of connection strength, excluding for the bar of 19.1 mm diameter that revealed 15% reduction after 200 cycles. Davalos et al. [233] contacted up to 30 FT at 60 °C and strengthened with CFRP
T
and GFRP bars. The results showed that the bond strength condensed by around 18% after contact. It is also
IP
reported that the RC beams reinforced with five CFRP and two steel-prestressed bars tested under fatigue
CR
loading at −28 °C [234]. The main findings presented that at a low temperature resulted a slip between the concrete and the bars at load with up to 90% of the eventual strength capacity of the RC beams. It is also
US
found that the concrete cubes reinforced with FRP bars at different temperature (20, 70, 50, 100 °C)
AN
increased the micro-cracking in concrete and decreased the ultimate tensile load with up to 16% in comparison with the control as a result of the weakness of the FRP matrix under elevated temperature that
M
instigated the bond failure [235, 236]. Similarly, another experimental result exhibited a reduction in the
ED
bond strength of FRP bar between 20% and 40% at 100 °C, 75% at 150 °C, and 90% at 220 °C [237]. Besides, the cylinders fabricated with GFRP bars and exposed to 40 °C and 60 °C and left humid for 4
PT
months [238]. The findings showed 18% and 20% reduction in the bond strength when exposed to 40 °C and
CE
60 °C, respectively, attributable to the increase in the diameter of GFRP bars. Accordingly, the contact between temperature and moisture speeded the environmental deprivation on the FRP composites [219].
AC
Moreover, it indicated that glass fibers have thermal insulating properties that are 10 times lower than that of basalt fibers. Temperatures beyond zero may cause alterations in mechanical characteristics and generate further micro-cracks in FRP materials. Micro-cracking at low temperatures promotes the increase of water absorption at elevated temperatures while the transverse micro-crack increases in the expansion of freezing water found in voids and cracks [88, 124, 144-150]. Such freezing exposures contribute to a material deprivation through increasing brittleness and reducing debonding [183,144-151, 190, 196]. The E-
34
ACCEPTED MANUSCRIPT glass/vinyl ester and carbon-vinyl ester composites at low temperature thermal cycling can cause a significant reduction of the structural properties FRP [156, 157, 161, 183, 190, 196, 220, 236]. However, the possible contact to fire of the RC structures must also be deliberated during the stage of design [157, 161, 183, 236, 237]. RC generally provides a great resistance to fire at low cost. The use of flammable FRP
T
materials is, consequently, problem that recommended to be considered extremely [33, 88, 126]. All FRP
IP
composite materials are disposed to deprivation of mechanical characteristics at elevated temperatures [146,
7.3
CR
150, 183, 220, 134, 237]. Coefficient of thermal expansion
US
The coefficient of thermal expansion (CTE) may be detrimental when an FRP liner is used to a steel
AN
substrate and CTE value is twice that of steel/carbon [199]. FRP bars have higher CTE than concrete in the transverse direction (Table 19). In unidirectional structures, the CTE in the transverse direction is larger than
M
that in the longitudinal directions (Table 20) [126, 220]. Gaitonde [239] reported that the thermal expansion
ED
coefficients of carbon fiber strengthened polyether ether ketone (PEEK) exhibited that the worst errors for modulus values are -1.4% at -100 °C and -2.3% for the PEEK material. Similarly, the CTE of epoxy
PT
composites perpendicular to the fibers is approximately (60 ×10-6) per °C; hence, the shear modulus is
CE
underestimated by 2.9% at −100 °C. At high temperatures, the FRP bar entrenched in concrete expands, causing higher radial bursting stresses than tensile strength and thereby initiating cracks and negatively
AC
affecting the interaction between the concrete and the bar [59]. The crack mechanism arising from the limited tensile deformation capacity of concrete directly influences aesthetics, stress transfer, and structure durability [5, 6, 30, 36, 238]. Differences in surface configurations and mechanical properties exist when traditional steel reinforcement is compared with FRP reinforcement. Therefore, differences in the cracking behavior such as crack pattern, crack width, and crack spacing are expected [198, 119]. However, after an initial linear elastic behavior of RC ties and once the load causing first cracking is attained, cracks appear randomly due to
35
ACCEPTED MANUSCRIPT non-homogeneity of concrete [101, 240]. At cracked sections, concrete stress drops to zero and strain compatibility is lost [59, 238]. Stress is transferred from reinforcement to concrete and, at some distance, the compatibility condition is recovered due to the action of bond forces. The required strain compatibility recovering distance is short when the bond behavior between the two materials is excellent [125, 219, 233].
T
During this crack creation stage, fresh cracks seem as the load is enlarged, thereby reducing the average
IP
crack spacing [11, 21, 114, 174]. This behavior remains valid until the cracking stabilization load is attained
CR
[174]. From this point, no cracks can appear, and the average crack spacing remains constant. An increase in load is obtained during the opening of the existing cracks [238]. AFRP and CRFP are resistant to high
US
temperatures have no dissolving points and utilized for protecting fabrics and clothing near fire [27, 48, 51].
AN
Although GFRP is extremely resistant to elevated temperatures, it ultimately melts [84, 177]. Furthermore, CFRP and AFRP are used to make protecting welding blankets, firefighting and hands while using a knife
M
[26, 27, 45, 101, 241].
ED
Table 19: CTE of different FRP materials Table 20: Transverse and longitudinal CTE of prestressing steel tendons and CFRP tendons [124]
Fire resistance
PT
7.4
CE
Fire resistance is a major issue that results in the limited application of FRP materials. Thus, rejoinder under fire contact is a main anxiety as a consequence of the extensive applications of FRP strengthening in RC
AC
structures, particularly in buildings, where fire resistance is one of the RC design requirements [242]. FRP is utilized as a material system that must gratify the requirements of the local design code to prove its fire safety [243]. The FRPs fire resistance can be enriched by means of fire-resistant polymers [242, 244-246]. FRP materials must still exhibit sound behavior associated to high temperature, particularly predicted to the fire design given these considerable features, thereby leading to the inactive protection of buildings in case of fire [244]. Reportedly, the fiberglass bars with thermoset resins are generally utilized in corrosion resistant coats, owning a low rate of flame when fire is sustained by outdoor source [242-244]. Fire retardant 36
ACCEPTED MANUSCRIPT thermoset resins classically comprise either bromine or halogens molecules [244-247]. All matrix resins, for instance polyesters, epoxies, and vinyl esters, used for FRP composites are classified as non-fire resistant materials. It is reported that the FRP-strengthened RC beam can stand more than 3 h when expose to fire, exhibiting a low rate of fire resistance than that of an un-strengthened RC beam [242, 245]. Another study
T
revealed that when the FRP reinforcing system is secured with fire insulation of 25 mm diameter, the FRP-
IP
strengthened beam can stand more than 1 h 60 min compared with that of an un-strengthened beam [28].
CR
Qinghua et al. [126] reported the fire resistance of CFRP tendon with the matrix epoxy resin (transition temperature ranges from 90 °C to 110 °C). Therefore, the disintegration of epoxy resin at elevated
US
temperature leads to the reduction of at least 50% of the ultimate strength followed by the failure of the
AN
CFRP tendon when the temperature exceeded 250 °C. Basalt fibers do not produce toxic substances when subjected to fire, thereby overcoming a serious drawback of conventional fibers [23]. The BFRP bars with
M
different types of heat-resistant epoxy and vinyl resins can resist up to 100 ºC is because of the impregnation
ED
and adequate cohesion between the basalt fibers and the heat-resisting resins [244]. The mechanical characteristics of FRPs weaken with elevating temperature. The maximum temperature is the glass transition
PT
temperature Tg of the matrix of polymer, presenting typically in the array of 65 °C to 120 °C for matrices
CE
utilized in infrastructure applications [37, 54, 242]. The longitudinal properties are considerably influenced by low temperatures compared with the transverse properties due to the anisotropy of unidirectional FRP
AC
structures; therefore, the shear and transverse stiffness and strength reduce quickly above Tg [242, 243]. The deprivation of the mechanical properties of FRPs at elevated temperature is naturally ruled by the characteristics of the matrix polymer [242, 243]. Figure 8 displays the temperature-dependent strength of carbon, aramid and glass fibers in the basis of the data reviewed from previous studies [197]. This figure advocates that carbon fibers are moderately unresponsive to high temperatures, while aramid and glass fibers experience substantial weakening of strength at elevated temperature [124, 197, 246]. The relatively robust
37
ACCEPTED MANUSCRIPT fire resistance of any RC beams strengthened with glass FRP can possibly accredited to the clear concrete cover of 70 mm that was afforted for the FRP bars. Several literature reviews and theoretical studies have also been presented [156, 157, 161, 183, 190, 196, 220, 236, 242-247]. Table 21 presents the summary of imperative comparison and rankings for steel and FRP, particularly their practicality and durability
IP
RC members in current codes and standards are extremely limited.
T
properties. Furthermore, a review of the literature indicates that fire design guidelines for FRP-strengthened
CR
Figure 8: Modification in FRP bond strength with temperature [197] Table 21: Imperative comparison and rankings for steel and FRP [188] Serviceability of FRP
US
8
AN
Serviceability frequently governs the design of concrete elements that are internally strengthened with FRP reinforcements by reason of the hardened properties of FRP composite materials. Three main serviceability
M
criteria, namely, crack width, deflection, and fatigue, should be satisfied. These factors should be considered
ED
when the shear strengthening application is being designed. The reasons are given in the following subsections. Crack width
PT
8.1
CE
The cracking phenomenon in FRP composites mainly depends on the fiber type, fiber breaking, fiber surfaces, matrix debonding, matrix interface, moisture content, and thermal cycles, which all contribute to
AC
mechanical degradation [198, 212]. Corrosion crack propagation is also associated to the toughness of the resin, fiber, and matrix used to bind the FRP composites [198]. The toughness (Gc) of a composite is the amount the energy captivated per unit area of crack under Equation (12) [50]. If the crack is solely 𝑓
promulgated straight over the toughness of matrix (𝐺𝑐𝑚 ) and toughness of fibers (𝐺𝑐 ), then a simple rule-ofmixtures may be expected. However, if the length of the fibers is less than lc, then the system will not fracture.
38
ACCEPTED MANUSCRIPT 𝑓
𝐺𝑐 = 𝑓𝑓 𝐺𝑐 + 𝑓𝑚 𝐺𝑐𝑚
[50]
(12)
where m = matrix f = fiber lc = critical fiber length, mm ff = fraction volume of fibers fm = fraction volume of fibers = 1 − ff
T
-
IP
Crack width is computed based on the same perception for FRP rebar because this parameter is for steel-
CR
reinforced members. However, the parameter is modified by a bond quality coefficient (kb) [5, 6, 23, 36], as
US
expressed in Equation (15). The bond between FRP bars and concrete is generally lower than that of steel bars because of less prominent deformations and long-term effect on crack widths, particularly in GFRP
AN
composite structures. The design recommends that the crack width should be increased by the bond quality coefficient (kb) equal to 1.4, except when a FRP bar manufacturer can prove by testing that the bond with
M
concrete results in a decreased bond quality coefficient [5]. The maximum probable crack width (w) in mm is
PT
ED
given by Equation (13).
𝑓
𝑠
𝐶𝑟𝑎𝑐𝑘 𝑤𝑖𝑑𝑡ℎ, 𝑤 = 2 𝐸𝑓 𝛽𝑘𝑏 √𝑑𝑐2 + (2)2
-
β
(13)
= is the ratio of the distance from the neutral axis to the extreme tension fiber to the distance from the neutral axis to the centroid of the tensile reinforcement dc = the concrete cover thickness measured from the center of the closest reinforcing bar to the extreme tension fiber, mm s = longitudinal bars spacing, mm
AC
-
[96]
CE
where
𝑓
High aging temperatures that approach the glass transformation temperature of samples can increase the structural performance by depreciate the materials by prompting thermal cracks, which do not appear in the actual use of FRP structures [125, 219, 133]. Width of crack is usually smaller steel-RC beams than in in glass FRP-RC beams on account of the small modulus of elasticity of FRP bars [11, 114]. Reportedly, the 39
ACCEPTED MANUSCRIPT short-fiber press shaped flanges can be disposed to cracking and an inappropriate for tough services [199]. It is also reported that the largest crack width in CFRP-reinforced and AFRP beam tested under cycling load, is greater than the reference values for steel-reinforced beams due to the decrease of their strength at a postcracking variety and the crack width should not exceeding 0.051 mm [21, 101]. Meier [174] reinforced the
T
flexural beam by using CFRP epoxy structures with laminate of 2 mm thickness, therefore, the finding
IP
shown a reduction from 3.85 mm to 2.58 mm at nonlinear range. Salakawy and Benmokran [247] fabricated
CR
bridge deck slabs strengthened with FRP reinforcements. The results showed that the FRP reinforcement ratio increased from 100% to 200% and the crack spacing decreased by 4% to 10% and 44% to 49% for slabs
US
reinforced with carbon and glass FRP reinforcements, respectively. The measured crack widths for FRP-RC
AN
slabs and steel-RC structures are recorded within the permissible code as 0.5 mm and (0.4 and 0.3 mm for interior and exterior exposures), respectively (ACI 440) [5]. Deflection
M
8.2
ED
Serviceability problems, for instance deflections, mostly govern the design due to the low elastic modulus of FRP bars [51]. The deflection control of cracked RC elements relies on the effective moment of inertia of the
PT
section (Ie), as prescribed in most the design codes [247]. This technique is in the basis of the hypothesis that
CE
the moment–curvature profile of cracked FRP RC element residues elastic under the increment of applied load, with a rigidity of flexural (Ec Icr), and the tension hardening is insignificant [119]. Computation of the
AC
long-term deflection is in the line ACI 318 [96]. In this approach, the applied unfactored moment surpasses the cracking moment (Mcr), and the moment of inertia is condensed by the diminished tension stiffening in FRP-reinforced sections relative to that in steel-reinforced sections. The degree of tension stiffening in the FRP-reinforced sections decreased with the volume of reinforcing associated with the balanced reinforcing ratio [254]. For a one-way slab under flexural loading, the greatest deflection is provided in Equation (14).
40
ACCEPTED MANUSCRIPT 𝛿𝑚𝑎𝑥 =
24𝐸𝑐 𝐿𝑐𝑟
𝑎 3
𝑎
𝐿𝑔 3
[3 (𝐿 ) − 4 (𝐿 ) − 8𝜂 ( 𝐿 ) ] , and 𝜂 = (1 −
𝐿𝑐𝑟 𝐼𝑔
)
[119]
(14)
P = applied load, kN L = span of the slab, mm a = shear span Lg = distance, mm; support to point where Ma = Mcr in simply supported slabs Lcr = Cracking length of the section, mm
T
where -
𝑃𝐿3
IP
This section presents the observation of common inclines in the load–deflection profile thru the testing of the
CR
reinforced samples. In Figure 9 and Table 22, Sergio et al. [21] analyzed that the samples in zone A remain
US
uncracked, whereas zone B parallels to the cracked concrete and linear performance of the FRP laminates and reinforcement. The deflection is governed by the volume of FRP composites and reinforcement on the
AN
cross section [21]. However, zone C is measured by the stiffness and strength of the FRP structures and the straw- hardening strength of the bars. The deflection curve in region C is large for the strengthened element
M
because of the stiffness of CFRP composites. Furthermore, Olofin and Liu and Nor et al. [181, 248] found
ED
that when the tensile strength (3500–7000 MPa) and elastic modulus (230–650 GPa) of CFRP are higher than those of steel, then the deflection of structural members and the elongation at failure is improved in the
PT
range between 0.6% and 2.4%. For clarity, the restrictions in deflection of serviceability are forced on RC
CE
members to confirm their structural reliability under service load conditions. Under the same condition, RC members strengthened with FRP reinforcements are resulted larger deformations than of those of steel-
AC
reinforced member attributable to the low elastic modulus of the FRP rebars [249-252]. Figure 9: Qualitative depiction of the load–deflection profile of the RC members [21] Table 22: Typical behavior of specimens strengthened with different material properties [21]
8.3
Fatigue
FRP is an excellent-performing material with high strength fatigue resistance, which is extensively and widely used in engineering applications [197]. The material FRP composite elements experience progressive deformation subjected to a constant load over a retro in a process known as creep, followed by fatigue failure 41
ACCEPTED MANUSCRIPT mode [24]. The knowledge obtained from investigation regarding the CFRP steel composite system may not be appropriate to the CFRP concrete composite as a result of the distinguished variation between the fatigue failure mode and debonding mechanisms for connections and steel members [94]. Studies on the influence of fatigue on FRP-RC elements specify that the fatigue failure mode restricts the volume of stress permitted on
T
FRP rebar [30, 87, 135, 138-143, 150] (Table 23). Another study examined the effect of fatigue loading at
IP
several load ratios that array from 0.15 to 0.55 on the interaction between CFRP and steel sheets with
CR
nominal modulus (240 GPa–640 GPa). The findings exhibited that the decrease in bond strength for the nominal modulus CFRP sheet is almost 20% to 30% (Figure 10) [138, 222]. The impact of fatigue loading on
US
joint rigidity decreases less than 10% of the rigidity as a result of the accrued damage resulted by the
AN
increasing of fatigue loads.
Figure 10: Fatigue damage zone [138]
M
Investigations of BFRP and GFRP-RC composites under fatigue loads, thus, the results showed 36% increase
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in modulus/tensile strength of BFRP compared with the GFRP [161, 203]. The BFRP rupture strain is also found 2.56% greater than that of PBO composites. It is also reported that the AFRP composites exhibited
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higher resistance to fatigue, and isophthalic and bisphenol fumarate resins are known the lowest fatigue
CE
resistance compared to with those of the vinyl-ester resin [27, 199]. Moreover, samples made from Araldite 420 and Sikadur 30 showed no reduction in bond strength at temperatures less than −40 °C [30, 87, 130].
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Meanwhile, the bond strength of MBrace saturant is reported deceasing by 40% when the temperatures released from 20 °C to −40 °C Table 23: Summary of tests of beams under fatigue loading
9
Design of FRP
Design loads on a RC structure are determined using the same methods, whether reinforced using steel or high-strength FRP reinforcing bars [249]. Steel and FRP- RC members are analyzed using similar methods to satisfy the strength and serviceability criteria, namely, factored moment, factored shear, crack width, and 42
ACCEPTED MANUSCRIPT long-term deflection [5, 12, 250]. FRP is used mainly to strengthen existing RC structures, and this process involves multiple assessments and requires a good sympathetic of the current RC structural circumstances along with the use materials to overhaul the building before to FRP installation [23, 251-254]. The applicability of FRP for a reinforcing project could be measured by sympathetic FRP, its features, and more
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significantly, its restrictions. For instance, GFRP is the toughest and greatest resistant to twisting forces when
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polymer fibers are equivalent to the applied force and vice versa [8]. Figure 11 illustrates the analysis and
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design of FRP-strengthened RC members, for example beam, deck-slab, and plate-girder-bridge [255]. In FRP design, the moment–curvature profile of a RC section reinforced with FRP composites is computed by
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dividing the section into horizontal portions and assigning precise material properties to each portion [21].
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The techniques utilized to measure the strain and stress in each portion and compute the identical curvature and moment are labeled in Figure 11. Interior force equilibrium is developed for sequences of extreme
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concrete compressive strains. Curvature and moment can be computed at each point identical to the extreme
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strain of concrete given to the extreme fiber under compression, as revealed in Equation (1). The method utilized to calculate curvature and moment of a reinforced section is noticeable in the following steps [12,
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21]: (1) set the supreme compression strain in the concrete (εc max) to a value between the greatest usable
CE
concrete strain (εcu) and zero, (2) approximate the primal neutral axis location (h/2), (3) compute the strain shape based on the maximum compression fiber strain and the location of the neutral axis and then calculate
AC
the identical interior stress elements by means of the material prototypes approved, (4) inspect equilibrium in the horizontal direction via the interior stresses, (5) regulate the depth of neutral axis (c) until force equilibrium is attained, (6) measure the interior curvature and moment; and (7) upsurge (εc
max)
for the
purpose of accuracy. However, the interior stress in each portion is computed at mid-thickness and presumed constant throughout the portion. The distribution of stress is estimated by sequences of squares with a depth equivalent to the
43
ACCEPTED MANUSCRIPT potion thickness and height identical to the stress computed from the stress–strain profile [Equation (2)]. The force envelopment from each portion is calculated with the cross section width at the portion mid-plane (bi) and the portion thickness (t-portion). This envelopment is showed for non-rectangular cross sections in Figure 11. The interior force elements are timed by their distance to the neutral axis zi to compute for the
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interior moment. Meanwhile, the curvature is computed by dividing the extreme compressive strain by the
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neutral axis depth. In the last step, the model primarily estimates that CFRP composites residue connected to
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the surface of concrete [23]. Under this assumption, the supreme stress that possibly established in CFRP is identical to the stress of rupture fpu. The envelopment of CFRP composite to the complete tensile force
𝜀𝑐 𝑚𝑎𝑥 𝑐
-
(17)
PT
h = overall depth of beam; ds = effective depth; b = the beam width; c = the neutral axis depth εcu = concrete crushing stain; Acs = effective area of compression steel under compression fc = concrete compressive strength; fsc = stress in compressive steel; fy = steel yield strength As = effective area of steel under tension; Efrp =tensile elasticity modulus of CFRP bar; Internal moment, kip-in; Ø = Curvature, 1/mm; Fi = Internal tensile, kip zi = distance from internal force component to neutral axis, Fi, mm. εc max = strain at the extreme
CE
-
[21],
(16)
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where
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Ø=
[21]
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𝑀 = 𝛴𝑖 𝐹𝑖 𝑍𝑖
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develops zero when stress of rupture is reached [21].
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compression fiber; c = neutral axis position, mm; Afrp= effective area of CFRP rebars; εfrp = strain in CFRP bar; α1 and β1 = compressive stress block parameters in compression zone of concrete Figure 11: Internal stresses and forces in strengthened section [12, 21, 249, 255] Furthermore, Table 24 shows the guidelines strategies of the FRP as proposed by various codes for strengthening RC structures at different situations such as flexure, shear, and column confinement. The table is basically briefly presented the related parameters controlled the design strategy of the FRP such the concrete strength, the quality of the concrete surface, the glue line thickness, and the stiffness, the effective 44
ACCEPTED MANUSCRIPT bond length and width of the FRP sheets. It can also be used as a sample guideline for engineers, researchers, and material producers and ease them to develop their understanding of FRP strengthening technology. Table 24: Guidelines strategies of the FRP sheets as proposed by various standards for strengthening RC structures
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10 Strengthening techniques
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Externally bonded reinforcement (EBR) and Near-surface mounted (NSM) strengthening approaches are the
CR
most common and lately utilized encouraging strengthening approaches for RC structures [223, 256-258]. The NSM approach is performed through the following steps: (1) splits of 4-5 mm width and 12-15 mm
US
depth are cut by a diamond knife cutter over the surfaces of concrete of the members to be reinforced, (2) the
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splits are gutted by compressed air, (3) CFRP layers are gutted by acetone, (4) epoxy resin is formed in line with provider commendations, (5) the splits are filled with the epoxy resin, (6) epoxy resin is used on the
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laminates surface, and (7) laminates are presented into the splits and the extra epoxy resin is removed. To use
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wet lay-up slices of a CFRP laminate by the EBR approach, research achieved the subsequent processes [223, 259-263]: (1) On the regions of the surfaces of beam where the slices of the laminate should be stuck,
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emery was used to eliminate the superficial paste; (2) the remains were apart by compressed air, (3) a layer of
CE
primer was used to normalize the surface of concrete and improve the strength capacity of the concrete
266].
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substrate. Lastly, (4) the slices of the laminate were pasted using epoxy resin onto the beam surfaces [264-
11 The bond characteristics of FRP Bond FRP plate or sheet for strengthening and rehabilitation concrete members is became more popular in civil and structural engineering fields due to the remarkable characteristics, such as anti-corrosion, and light weight, compared with the traditional strengthening techniques [16-27, 119, 120, 304, 305]. However, the bonding with the concrete is essential to the exterior reinforcement technique for either repairing or
45
ACCEPTED MANUSCRIPT strengthening reinforced concrete (RC) elements. In most of strengthening cases, the bond of interface is critical in transforming stresses from the existing RC structures to the externally bonded FRP composites [223, 256, 257]. In structural RC system, sheet bonding has a superior potential for construction flaws, due to the mixing of resins and the curing of FRP composites. The Japan Concrete Institute (JCI), Canada, and the
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Great Britain, recognized a technical committee on retrofitting technology that concerned on the bond
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properties between the existing RC structures and retrofitting materials for both glue bonding and
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overlapping retrofitting technologies [217]. The current applications of the sheet bonding system include shear strengthening, flexural strengthening, and column wrapping [113, 121, 219, 271, 293-295, 306, 308,
US
311-315]. In the case of column wrapping, the failure of bond interface is not a main concern compared to
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the fracture of FRP sheets is the major mode of failure [306, 311, 312, 315]. Thus, it has been recently recommended that FRP materials should be used with a high fracture strain capacity [3-5]. However, for RC
M
members strengthened with FRP sheets for shear and flexure; debonding of the FRP from concrete is
ED
governed the overall structural failures [121, 271]. In the flexure strengthening cases, the debonding of FRP sheets from the concrete substrates are having more complex corresponded with several failure mechanisms
PT
unlike with the shear strengthening cases that are found in line with results obtained in pull-out shear bond
CE
tests and; respectively [273, 277, 278]. To evade the mid-span debonding failure, the most design guidelines endorse limits on the strains in the FRP sheets because the mid-span debonding is always occurred due to the
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interaction of the concrete cover, the FRP sheets and the steel reinforcement. Through the results of the pullout tests, it is reported that the concrete strength, the quality of the concrete surface, the glue line thickness, and the stiffness, the bond length and width of the FRP sheets are having a major impact on the bound strength of FRP composite system despite the diversity of reinforcing materials used (i.e., carbon fiber, steel, and glass fiber composites), the different concrete compressive strengths [217]. Several studies reported that the stiffness of the FRP sheets (elastic modulus × thickness) can lead to influence the bond strength [39-40,
46
ACCEPTED MANUSCRIPT 103, 151, 181]. For instance, it has been obtained that bond layers with lower elastic moduli, but good stiffness, could contribute to greater interface bond strengths [97, 116, 117]. It is also found that the sheet width does not affect the moderate bond strength of interfaces when the width of sheets limited between 50 mm to 200 mm. It is reported that the bond strength increases as the sheet bond length is increased.
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Nevertheless, the effective bond lengths are varying considerably between 45 mm and 275 mm [217].
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Moreover, the design guidelines of FRP for strengthening RC members with respect to local bond
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characteristics and the role of the interface bond characteristics on RC member behavior will be informative to engineers, researchers, and material producers and motivate them to advance their understanding of FRP
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retrofitting and strengthening technology. Moreover, a summary of the bond strength equations as pre
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various design standards and the commendations afforded by several scientists are given in Table 25. Table 25: Bond provisions as proposed by various standards for strengthening RC structures
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12 Applications of FRP
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Large-scale research projects undertaken in 1993 in the USA and Canada involving CFRP/GFRP composites confirmed the decision to use such materials in construction (Table 26) [256-267]. The USA is not only the
PT
largest producer and user of FRP composites but also leads the world’s composite technology development
CE
and implementation. Nearly half of the world claim for FRPs exist in the US and account for 4.2 billion in 2002 (Table 26 and Figure 12). The US composite industry is expanding despite the overall slow-down in the
AC
US economy in the past couple of years, and the FRP manufacturing is predicted to improve at yearly rate of 4% to 5% over the next five years (Figure 12) [267]. The US has higher than 13,000 services that process composites, hiring 236,000 persons, and backing over 524 billion to the state’s economy (Figure 12). Reportedly, about half of the global demand for FRPs resides in the United States, accounting for 4.2 billion pounds in year 2002, but the main market share comprises almost 21% in construction, 32% in transportation, 12% in corrosion-resistant application, 10% in marine businesses, 10% in electronic
47
ACCEPTED MANUSCRIPT industries, and the residual 0.6% is utilized in aerospace and aircraft industries, as indicated by SPI composite association (Figure 12) [268]. In civil engineering sector, the advanced application of composite materials has grown gradually because of economic constraints, complex technique involves at substituting the advanced composite systems instead of conventional structural systems to [16, 32, 33, 38]. The
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developing field of regenerated engineering, articulating the character of FRP structures in civil engineering,
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is separated into 1) rehabilitation, comprising the applications in the direction of overhaul RC structures and
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(2) new building with the entire FRP resolutions or novel composite FRP/concrete composites [269]. The operational efficiency of FRPs in the reintegration of existing RC systems is also proved with large-scale
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structural investigation to retrofit and strengthen unreinforced and reinforced brickwork walls for seismic
AN
loads [19, 148, 169]. However, FRP composites have been efficaciously utilized for the seismic upgrade of RC structures, including their interior and exterior structural elements due to its high resistance to corrosion
M
[13, 17, 18, 122, 270]. These composites comprise justifying brittle failure modes, such as shear failure of
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columns/beams; withstand buckling of the longitudinal steel bars of confined column, shear failure of unconfined column–beam joints, and lap joint failure [106, 271]. These FRP systems upsurge the energy
PT
dissipation capacities and global displacement of the RC structure and increase its global performance [189].
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FRP composites are typically used for external strengthening and internal bars (dowels, rebar, and posttensioning tendons) [20, 63, 85, 103, 108, 109, 113-115, 134]. Externally bonded or close-surface-mounted
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FRPs are commonly used for the structural rehabilitating and overhaul of RC, timber, masonry, and steel structures [107, 111, 112, 114, 120, 256, 263]. FRPs utilized as internal reinforcement are included in roads, bridges, slopes, tunnels, and marine environments [15, 19, 126, 201, 229]. Internal bar with FRPs grasps a specific benefit in channel diaphragm walls, where steel bar damages the surface of a channel boring machine. FRP bars are utilized in hospitals as medical scanning device, such as magnetic resonance imaging, is applied in Maglev railway bonds and constructions and based on a large electric motors since such bars are
48
ACCEPTED MANUSCRIPT charismatically transparent [2, 32, 57, 81, 153, 154, 185, 238]. Furthermore, structural repair with exteriorly bonded FRP bar, particularly with high strength carbon FRP, has been proved by codes for seismic upgrades of RC structures for many years [10, 12, 19, 23, 36, 119]. The eccentric and axial loading of columns could be improved by wrapping columns with FRP bars [115, 121, 197, 268, 269]. Besides, FRP composite system
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afford a useful tool to strengthen, repair and retrofit RC structures and are suitable for shear strengthening,
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flexural strengthening, confinement of column, and improvement of ductility, as all are reviewed in
CR
following subsections.
Figure 12: Current markets and applications of FRP materials
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Table 26: FRP merits and suitability of applications [195]
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For a practical case study on the utilization of FRP in practice, Kaitbay fence [267] is deemed one of the ancient places in Cairo constructed from stones and utilized as small shops. The lintels utilized in the front
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entrances of the shops were made of stones with shear keys and without mortar. Weakening of the joints
ED
between the stones and excessive deflection of the lintels was reported and as a result one of the stones fell off. However, CFRP laminates were utilized as tension bars at the bottom surface of the lintels. Holes were
PT
penetrated at the two ends of the lintel. Therefore, the CFRP laminates were strained by means of a rotating
CE
device at the penetrated holes, while connected to the bottom face of the lintel and the area of CFRP laminates was formed to preserve the self-weight of the stone lintels. Another example is the bridge deck
AC
slab [214], the system was required strengthening to prevent cathodic corrosion protection (Figure 13). Smart-deck slab (carbon textile-reinforced mortar, CTRM) consists of two layers of an epoxy-resin permeated carbon grid in combination with a high-performance mortar and textile material for the strengthening. CTRM layer was installed between the surface of the road and the reinforced concrete bridge deck slab surface (Figure 13 a). It was also mounted in segments to gain known sectors in the longitudinal direction in order to leak in the surface of the road and to prevent the damage of the CTRM layer (Figure 13 b). However, the tendency of the damage depends on the traffic volume. The two CTRM layers were made 49
ACCEPTED MANUSCRIPT with carbon reinforcement of 35 mm thick, installed at a distance of 15 mm, and fitted with electrical attachments for the monitoring. The system was also strengthened with a carbon grid with a 38 mm mesh opening as well as epoxy-resin incorporated with carbon nanotubes, aiming to increase the electrical conductivity. However, on the basis of the test, the fatigue and ultimate capacity of the strengthened bridge
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deck slabs and beams were increased, due to the use of the CTRM layers that offered an advanced method
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applied for strengthening evaluates by integrating the benefits of light, glued CFRP strips and the better bond
CR
features of an additional concrete layer.
Figure 13: Smart-deck Bridge; and (b) the location of sawn segment in a supporter slab [214]
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12.1 Flexural strengthening
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For flexural strengthening, FRP bar products, for instance plates, tow sheets, and rebars, are attached to the tension side of a masonry, concrete, and timber substrate with epoxy resin/ fibers parallel to the principal
M
stress alignment [11, 103]. Consequently, numerous researchers studied the use of diverse methods to
ED
eliminate debonding as a mode of failure due to the methods explained in Figure 14. The use of CFRP composites in the type of strips can provide a cost-effective method for the flexural strengthening of deficient
PT
RC slabs [270]. In flexural strengthening, these methods principally used 0° FRP fibers and increased the
CE
load bearing strength of slabs up to 40% [21, 121]. Besides, beams strengthened with FRP showed that the flexural strength increases by 36% to 57%, the flexural stiffness upsurges in the range of 45 to 53%, and the
AC
flexural ductility and rigidity reduce as a result of the elastic behavior of FRP composite to tensile rupture and the damaged degree [103, 280]. The RC beams strengthened with CFRP sheet is identified to be liable to the premature, fragile, and enormously adverse failure [277, 278], since the process avoids the entire application of the strength characteristics to the tensile of polymer. Some beams strengthened with CFRP sheet can fail by localized debonding of strengthening from its fastening region or regions with extreme concentration of shearing and/or flexural cracks or by premature forms of failure [279]. Lately, the attention
50
ACCEPTED MANUSCRIPT on the RC beams strengthening via exteriorly bonded FRP composites was largely increased; this approach improves the shear capabilities of RC beams [280-281]. Meanwhile, shear failure was reported to occur due to the influences of the ratio of longitudinal tensile reinforcement [6], effective span to depth ratio [281-283290], and the volume, orientation, spacing of CFRP strips [283]. Although considerable investigations have
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been directed on flexural strengthening of RC beams by using FRP materials, these studies focused on the
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effect of the size and depth of the beam and thickness of concrete cover, the FRP thickness, width, and type
CR
on the modes of failure [293-295]. However, it is reported that the beam size has no such effect on the ductility and rigidity of the RC beams strengthened with CFRP sheets significantly and the incorporation of
US
GFRP and CFRP is improved by the ductility and rigidity through adjusting the modulus ratio [171, 296].
AN
Meanwhile, the CFRP tensile strength is less circulated in hybrid strengthening than in GFRP [103]. Attari et al. [103] revealed the use of twin-layer CFRP–GFRP composite materials to RC beams increased the
M
strength capacity by 114% compared with the control beam, but increases by 84% and 72% compared with
ED
the reference beam as reported by [91] Mehmet and Zarringol [63], respectively. Similarly, the upsurge on flexural strength of RC beams was reported by 38.86% for three CFRP layers, 46.6% for two layers, and
PT
15.5% for one layer compared to the control beam [84]. But, the increase on flexural strength of the RC
CE
beams strengthened with GFRP sheets was by 45% and by 27% when the RC beams strengthened with BFRP sheets relying on the number of layers used and the reduction of displacement is 53.6% [21, 70].
AC
However, the reduction on the flexural strength capacity can be improved by the addition of carbon fibers, PE fibers, fiber orientation and epoxy resins [98]. Figure 14: Flexural strengthening techniques (e.g., CFRP composites) [26, 121, 167] 12.2 Shear strengthening For shear strengthening, the design procedures of RC structures with exteriorly bounded FRP are found in quite a lot of documents [5, 12, 24, 119, 120]. For shear strengthening, FRP bars are attached to the external
51
ACCEPTED MANUSCRIPT of beams in a vertical U-shape formation as an exterior stirrup [113, 121, 219, 271]. The walls shear strengthening, for example under-RC, walls, and unreinforced masonry walls, could be achieved by warping FRPs to both or one sides on the wall in a vertical, horizontal, or x pattern (45°) [216, 279]. Shear strengthening is completed as very tinny edges with merely two or one sheets that are 0.5–1.0 mm thick and
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attains substantial seismic enhancements, particularly for a response of in-plane shear wall [297]. Existing
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epoxy resins are robust; hence, surface failures commonly happen in the concrete, in particular, at a weak
CR
joint in RC member that requires being shear strengthened [269]. The EBR method is applied as continuous jacketing or strips. Three main formations of FRP strengthening, namely, U-wrapping, complete wrapping
US
and side bonding (Figure 15) [273]. The complete wrapping of structural RC elements is recognized as the
AN
most effective method for FRP shear strengthening due to its feasibility at the presence of some geometric restrictions [113]. Some studies were used different methods on the strengthening of RC beams in shear
M
under, for instance exterior prestressed, bonded steel plate and fiber materials [282–291, 298-300]. Externally
ED
applied FRP, comprising aramid, carbon, and glass fibers, have been widely used for shear and flexural strengthening of RC beams/ columns [283-300]. Reportedly, the use of CFRP for shear strengthening of RC
PT
beams showed an increase in the shear strength by 19%-122% attributed to the orientation of the FRP at 45°
CE
and CFRP sheet, compared to the control beams [301, 302]. The shear strength of FRP-strengthened beams is generally computed by the addition an individual elements of shear resistance from the concrete, FRP, and
AC
steel stirrups. It is reported that the use of U-wrap CFRP shear strengthening system in RC beam is increased the shear capacity by 50% for one CFRP layer [303] and 92% for two CFRP layers [268], accredited to the shear-span-to-depth ratio that limited to equal to 3 or greater than 2. For example, RC beams of ratio of 1.5; CFRP shear strengthening showed no such increasing in the shear strength. Another study examined bridge desk slabs strengthened with CFRP bars with ratios higher than the balanced reinforcement ratio, the shear strength capacity was increased by 81% to 111% compared to the control slabs [247]. But the study on the
52
ACCEPTED MANUSCRIPT influence of bucky paper interleaves formed from carbon nanofibers on the interlaminar hardened characteristics of CFRP exhibited 31% and 104% enhancement in interlaminar shear strength [123]. Liu et al. [222] studied the allowable level of BFRP composites absorbed in salt water for 240 days. The finding displays no reduction in shear strength of the BFRP composites even after 199 FT. However, a unexpected
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12.3 Column confinement and ductility improvement
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Figure 15: Shear strengthening [11]
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reduction in shear strength happened after being immersed in hot salt water at 40 °C.
FRP composites are a progressively suitable substitutive to steel reinforcement for RC structures, comprising
US
cast in-situ and pre- and post-tensioned bridges, columns, beams, precast concrete pipes, and other elements
AN
[18]. The FRP and steel reinforcement’s performance, comprising resistance to corrosion, is illuminated on Table 15. Column structures also benefit from FRP reinforcement [18, 84, 197, 255, 274, 275] (Table 27).
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The use of structures as original reinforcement to strengthen other structures is specified increasingly by
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structural design engineers in private and public construction industries [14, 38, 91]. Presently, column wrapping by FRP is a mutual solution in seismic retrofitting. This method, related with other conventional
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jacketing choices, provides several advantages, for instance reversibility, ease of application, and great
CE
corrosion resistance. Besides, this tactic is approved in the retrofit of RC elements with numerous purposes, for example providing ductility, preventing bar buckling, increasing shear capacity, and enhancing hollow
AC
elements [257, 268]. The efficiency of FRP wrapping in lap splice areas for rectangular and circular cross sections has been verified in numerous studies [110, 257, 268, 269]. Moreover, many authors afforded analytical clarifications concerning the mechanisms involved the improvement in the bond between the concrete and the lap-spliced bars attributable to wrapping, mainly to FRP wrapping [256, 269] and provided further confinement to the concrete by using FRP [1-15]. The main advantage of FRP is its orthotropic performance that restrains the bound between the concrete and jacket in the axial direction; this performance
53
ACCEPTED MANUSCRIPT contributes to the initially activation of its confinement [16-27, 304, 305]. Investigations revealed that the ductility and strength of columns are enhanced by allocating the longitudinal bars around the core perimeter and reserving these bars with laterals, for instance ties and the smaller volumetric ratio of ties decreases the concrete core confinement and [306, 307]. The larger volumetric ratio of tie flaws in concrete steadiness
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generates a weak zone between the concrete cover and the core [308]. Saatcioglu and Grim et al. [309] study
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the column strengthened with welded reinforcement grids; the results showed a shortage of confinement
CR
caused by ties and recommended to be improved by expanded welded wire mesh, metal mesh, ferrocement and FRP to restrain the concrete core [310]. Thus, ferrocement is widely utilized in the repair, overhaul and
US
rehabilitation of current concrete columns [306, 308, 311-315]. Ho et al. [312] strengthened RC columns
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with upgraded ferrocement jackets, including wire mesh, and rendering material. However, few studies considered hollow RC with CFRP confinement compared with any section of solid column [269]. CFRP
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confinement has been verified to improve the concrete ductility and strength and delays the buckling of
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reinforcing bars and concrete cover spalling [268]. The uniform distribution of bars in mortar enhances several engineering properties, for instance durability, ductility, in-plane strength, and the crack resistance.
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The application of CFRP confinement does not increase the ductility of the hollow column but also avoids
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any slippage caused by insufficient overlapping. Kus and Hadi [274] compared the confinement of circular hollow RC column with CFRP sheets. The results found significant findings where reinforced columns
AC
without CFRP confinement show brittle failure mechanism. By contrast, a more sustained, higher ultimate load and a larger axial deflection are observed in CFRD. Another study led by Lignola [276] showed the initial point of buckling in compressive reinforcement with 15% confinement of CFRP strength compared with the column without confinement. Yeh and Mo [277] reported that the ductility factors obtained from their experiment ranged from 3.3 to 5.5, which is sufficient to sustain under seismic load. Pavese [13] discovered that CFRP strengthening permitted the completion of 4.8% drift for concrete that is crushed and
54
ACCEPTED MANUSCRIPT bars that are buckled at the column base. This finding was also proven by another experimental study regarding the seismic performance of CFRP confinement that efficiently enhanced the strength capacity of hollow RC columns [270, 278]. Table 27 shows the extensive summary of the researcher’s parameter. One, three, and four CFRP layers were used to the hollow column section in previous research [14, 262, 267, 270].
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Ductility increases with the thickness of FRP jacket, restrains all shear cracks, and modifies the mode of
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failure of the sample from shear to flexure [14, 275]. Lignola [275] conducted experiments on seven hollow
CR
prismatic columns, considering the effects of FRP confinement. Results showed that using additional layers of FRP can increase the ductility of columns and prevent shear cracking. The performance of circular hollow
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reinforce concrete columns is not jeopardized with a minimum two layers of wrapped CFRP [267].
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Moreover, a minimum of one layer of wrapped CFRP for hollow column section was used by several studies [16, 17, 40]. All such works reported that the performance of the CFRP-wrapped hollow section is enhanced
M
even when only one layer of CFRP is utilized. Kus and Hadi [274] also stated that CFRP confinement in the
ED
loop direction can increase the ductility and strength of a circular hollow RC column relative to those of the reference control. Hassan et al. [267] studied the effectiveness of FRP laminates in enhancing the strength of
PT
uniaxially loaded high-strength concrete columns by increasing the flexural capacity by up to 23% and 59%
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for small and large eccentricity-loaded specimens, respectively. Moreover, the use of ferrocement jackets including steel bars, in strengthening of circular and square RC columns were reported to effectively improve
AC
the seismic performance [307, 313-316]. Table 27: Summary of parameter studies on CFRP strengthening confinement of hollow RC bridge columns
13 Conclusion FRP is a composite system composed of a matrix of polymer strengthened with fibers, which are generally carbon, glass, basalt, or aramid. FRP composites have protracted as a substitute material to produce reinforcing bars for RC structures because FRP reinforcement bars provide benefits, such as noncorrosive and nonconductive properties, over steel reinforcement. Unique guidance on the construction and 55
ACCEPTED MANUSCRIPT engineering of RC structures strengthened with FRP bars is needed as a result of other changes in the mechanical and physical behaviors of FRP composites against steel. This comprehensive literature review shows that the majority of investigations are limited to the evaluation of composite characteristics of FRP rather than the characteristics of its materials and their influence on the strength and ductility of FRP
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composite matrix. The selection of FRP materials to strengthen any concrete structural element should
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consider its influences in terms of properties and long-term service and its reaction with the ecosystem.
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However, FRP composites obtained a great reputation worldwide; thus, global construction agencies have authored design and construction codes, in particular, for the use of FRP reinforcements as concrete rebar.
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This literature review aims to provide fundamental information reference on the basis of knowledge reviewed
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from global conducted studies, simulation work, and practical applications of FRP reinforcement. This review guides researchers, designers, and manufacturers about the latest developments in scientific research,
M
particularly, in FRP composites, and the level of its performance for practical use. Moreover, this review is a
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continuation of research on resolving their limitations. However, the differences between the current review papers on FRP in strengthening RC structures comprise basically of superficial discussions handling only
PT
few functions or properties with no depth in any aspect. No existing study has touched the applications and
CE
properties of FRP materials in a wide range and provided meaningful guidelines for researchers, designers, and manufacturers. Meanwhile, this review mainly covers an extensive study on FRP design, matrix,
AC
material properties, applications, and serviceability performance. These literature reviews provide a comprehensive insight into the integrated applications of FRP composite materials for improving the rehabilitation techniques, comprising the applications for the repair, overhaul, strengthening, and retrofit of RC in the construction industry at present. The aforementioned review mainly intended to evaluate the current material properties of FRP. However, this review is expanded to enhance the FRP design matrix and combine two FRP materials to improve the
56
ACCEPTED MANUSCRIPT techniques of rehabilitation, including strengthening, repair, and retrofit of structures. Moreover, the achievement of structural reintegration measures with innovative composite materials contributes to the advance of novel lightweight structural theories that utilize FRP composite materials to create novel structural RC composite systems. The use of FRP composites in the industries remains significantly
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advanced over time. Thus, many methods remain under investigation because of their application to
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strengthen current RC structures. The subsequent are the recommendations for further studies. 1)
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Investigations on the fatigue performance of strengthened RC elements are limited. 2) A literature review indicates that the fire design codes for FRP-strengthened RC members in available standards and codes are
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limited. 3) The effect of creep performance of the tendon of FRP on its long-standing work capability should
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be studied. Therefore, the durability and safety of the tendon in its typical service life can be confirmed. The affiliation between stress level and creep rupture time is also constructed based on the test data to expect the
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CFRP tendon capacity in long-standing work. A wide image from previous study findings on the application
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schemes of advanced composites in civil engineering is afforded in this paper to illustrate the scope of current growths and discover the practicality of upcoming applications in civil engineering sector. A
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prediction for the advanced composites application in civil engineering industries is produced by delineating
engineering society.
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14 Acknowledgment
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critical technical execution problems that should be determined prior to a comprehensive acceptance of civil
The authors gratefully acknowledge the financial support from the Department of Civil Engineering, Faculty of Engineering, Amran University, Yemen (RG: 2018110) and the Department of Civil Engineering, College of Engineering, Prince Sattam Bin Abdulaziz University, KSA (RG: 2017889), for this research.
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ACCEPTED MANUSCRIPT Ueda, Tamen, and Jianguo Dai. "Interface bond between FRP sheets and concrete substrates: properties, numerical modeling and roles in member behaviour." Progress in Structural Engineering
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and Materials 7(1) (2005) 27-43.
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ACCEPTED MANUSCRIPT Table 1: Typical properties of CFRP [37] Trade Name
Tensile Strength, (MPa) 1596 2068 2250 1200
Ultimate Tensile Strain 0.013 0.017 0.015 0.012
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V-rod Aslan Leadline Nefmac
Modulus of Elasticity, (GPa) CFRP 120 124 147 100
Tensile Strength, (MPa)
Ultimate Tensile Strain
710 690 600
0.015 0.017 0.020
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V-rod Aslan Nefmac
Modulus of Elasticity, (GPa) GFRP 46.4 40.8 30
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Trade Name
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Table 2: Typical properties of GFRP [37]
Tensile strength, (MPa) 2.3-3.4 3.3 3 2.8
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Modulus of elasticity, (GPa) 70-43 70 79 123
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Trade Name Kevlar Technora Twaron Heracron
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Table 3: Typical properties of AFRP [37]
Extension to break, (%) 1.4-4 4.3 3.3 2
Density, (g/cm3) 1.44-1.47 1.39 1.44 1.44
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Table 4: Typical properties of BFRP [44]
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Trade Tensile strength, Modulus of Coefficient of Thermal Expansion Elongation (%) Name (MPa) elasticity, (GPa) (x 10-6/°C) Rockbar 1000 50 2.24 2.0 BCR 1100 70 2.20 0.35-0.592 Annotation: Coefficient of thermal expansion of concrete = 10x10-6 /°C reliant on the concrete mixtures Table 5: Properties of thermosetting resins of FRP matrix [59]
Resin Epoxy Vinyl Ester Polyster
Specific Gravity 1.2-1.3 73-81 1.1-1.4
Tensile Strength (MPa) 55-130 1.12-1.32 34.5-103.5
Tensile Modulus (GPa) 2.75-4.1 3-3.35 2.1-3.45
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Cure Shrinkage (%) 1-5 5.4-10.3 5-12
ACCEPTED MANUSCRIPT Table 6: Technical properties of epoxy (Sikadur 330, 1.31 kg/L)
Technical values Design code 2 7 days (at +23 °C) ~ 3 800 N/mm ` (DIN EN 1465) [73] Component A+B mixed 1.30 ± 0.1 kg/L (ISO 527) [74] (at +23 °C) Tensile strength 7 days (at +23°C) ~ 30 N/mm2 ⸗ 2 Modulus of elasticity in tension 7 days (at +23 °C) ~ 4 500 N/mm ⸗ Elongation at break 7 days (at +23 °C) 0.9 % ⸗ Tensile adhesion strength > 2 N/mm2, Concrete fracture on sand (EN ISO 4624) [75] blasted substrate Coefficient of thermal expansion at −10 °C to +40 °C 4.5 × 10−5 1/K (EN 1770) [76] Glass transition curing Temperature Curing time 30 days Temperature +30 °C (EN12614) [77]
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Properties Modulus of elasticity in flexure Density
Table 7: Mechanical properties of several classes of FRP materials [6] Density, (g/cm3)
Steel Glass FRP Basalt FRP
500-500 600-1400 1000-1600
Aramid FRP
1700-2500
Carbon FRP
1755-3600
7.75 - 8.05 2.11-2.70 2.15-2.70 1.28- 2.6 1.39-1.45 1.55-1.76
Tensile Strength (MPa)
Specific Gravity
Elastic Modulus (GPa)
Strain at Break, %
480-1,600 1,035-1,650
7.8 1.5-2.5 2.7-2.89
(200) 35-51 45-59
1.2-3.1 1.6-3.0
1,720-2,540
1.38-1.39
41-125
1.9-4.4
1,720-3,690
1.0-1.1
120-580
0.5-1.9
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Yield Strength (MPa)
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Reinforcing Material
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Annotation: - Ultimate tensile strength, ffu - Tensile Modulus of Elasticity, Ef - Elongation at Break, εfu - The values of fiber volume fraction of FRPs are limited between 0.5 and 0.7. CFRP and GFRP have a tensile elastic modulus of at least 124 GPA and 39.3 GPa, respectively
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Table 8: Qualitative comparison of several fibers used in design of the composites [85] Criterion
Tensile strength Compressive strength Young’s modulus Long-term behavior Fatigue behavior Bulk density Alkaline resistance Price
Type of Fiber used in Composite Carbon Fibers Glass Fibers Aramid Fibers Very good Very good Very good Very good Inadequate Good Very good Good Adequate Very good Good Adequate Excellent Good Adequate Good Excellent Adequate Very good Good Inadequate $7.11-18.11/m2 $0.13-0.27/m2 $8-12/m2
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ACCEPTED MANUSCRIPT Table 9: Summary of previous studies on flexural tests
-
[108] [109] [110]
[111] [1]
-
[112]
-
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[26]
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[107]
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[21]
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-
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[106]
Experimental Parameters • Composite type • Anchorage at end of plate Composite type Shear span/depth ratio Effect of pre-cracking Surface preparation • Configuration of CFRP system • Fiber orientation Number of composite plates Anchorage at end of plates • Anchorage technique at ends of plates • Composite type Thickness and/or number of plies • Number of plies • Effect of pre-cracking • Anchorage by wrapping with CFRP sheets External anchorage for CFRP plates (to control slip) • Shear span to depth ratio • Plate end anchorage Existing reinforcement ratio Effect of composite area to steel ratio • Placement of CFRP system • Anchorage with vertical sheets
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Refs. [105]
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ACCEPTED MANUSCRIPT Table 10: Summary of previous studies on RC sections strengthened by CFRP and steel
Single lap joint with circular hollow section Single lap joint with circular hollow section Single shear pull test
Static
No. of CFRP layer 5
Static
1-5
Sheet
40–85
640
Static
1
Plate
350
165
-
Single shear pull test
Static
1
Plate
300–380
150
-
Static Static Static Static Static Static
1 3 1 1 1 1 1
250 40–80 50–200 300 30–250 500 25–250
160 240 338 197 479 195 151
-
Static
Plate Sheet Plate Plate Plate Plate Plate
[137]
Single shear pull test Double shear pull test Double shear pull test Double shear pull test Double shear pull test Double shear pull test Single lap, double shear, and T-peel test Pure bending test
Static
1
Plate
203
[19]
Double shear pull test
Fatigue
1
Plate
200
[30] [138] [20] [139141] [142]
Double shear pull test Double shear pull test Full-scale bridge girders
Fatigue Fatigue Fatigue
3 1 1
Sheet Plate Plate
40–60 60 457
155 166 and 640 479 112 205
Double shear pull test
Impact
1 or 3
Sheet
Pull-off test
Sheet
[143]
Double shear pull test
Impact 1 or 3 Large 3 deformation
E C
Ref. [127] [128] [129] [20, 130] [131] [132] [133] [134] [135] [136] [25]
[144, 145] [144, 146] [145] [147] [148] [15, 194] [150] [151]
Type of test
Type of loading
Double shear pull test
cyclic
Double shear pull test
Static
Double shear pull test Double shear pull test
C A Static Static
CFRP (plate or sheet) Sheet
Bond length (mm) 23–126
CFRP modulus (GPa) 230
Eco-condition
Araldite 420 , (1,901)
-
T P
Three types, (4,013 - 10,793) Araldite 420, Araldite 2015, Sika 30, Sika 330 (1,830 - 11,250) Two types (4,013 - 10,793) Araldite 420 (1,901MPa) SPSpa bond, (3,007) Sikadur 30, (4,500) and Sikadur 330, (3,800) Araldite 420 (1,901) and Sikadur 30 (9,282) Sikadur 30, (4,500) Tw types of epoxy ,(1,000)
I R
C S
U N
A
M
Araldite 420, (1,901)
-
-
Adhesive (modulus), MPa
-
Two parts epoxy system (1,240)
-
Sikadur 30, (2,689) Araldite 420, (1,901) Araldite 420 ,(1,901) Plexus MA555, (107) MBrace saturant, (2,229)
10–100
205
-
T P
100 and 150
205
Subzero
Sheet
D E
205
Temperature
Araldite 420, (1,901)
Sheet
100
205
Elevated Temperature
Araldite 420, (2,012); MBrace saturant (1,482) and Sikadur 30, (9,515)
1 or 3
Sheet
20–150
205
Elevated
3
Sheet
100
1 or 3
Sheet
20–100
640
Elevated Temperature
3
Sheet
1800
230
Seawater
Sikadur Hex 330, (50,350), SikadurHex 306 (44,900), and Tyfo SW-1, (62,500)
3
205
Temperature
Araldite 420, (1,901) and MBrace saturant (2,229)
Araldite420(1,901MPa) Araldite 420, (2,012); MBrace saturant (1,482) and Sikadur 30, (9,515) Araldite 420, (1,901)
Four-point bending test
Static
Double shear pull test
Static
1
Plate
200
418
Seawater
SP Spabond (2,980)
Double shear pull test Double shear pull test
Static Static
3 1 or 3
Sheet Sheet
100 30–100
205 205
Seawater UV ight
Araldite 420, (1,901) Araldite 420, (1,901)
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ACCEPTED MANUSCRIPT Table 11: Creep ruptures reduction factors [5, 26] Property Creep-Rupture Stress Limit, Ff,s
GFRP 0.20
BFRP 0.20
AFRP 0.30
CFRP 0.55
Table 12: Typical properties of different FRP materials [5]
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Strength to weight ratio 564 1013 993 1000 28
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Density of laminate g/cc 2.4-2.5 1.9-2.1 1.44 2.6-2.8 1-1.15
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Laminated strength , MPa 1500 1600 1430 1100 12-40
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Table 13: Comparative chart of glass, aramid, and carbon fibers
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Fibers Glass Aramid Carbon Properties 2 2 Cost $0.13-0.27/m $8-12/m $7.11-18.11/m2 Weight to strength ratio P E E Tensile strength E E E Compressive strength G P E Stiffness F G E Fatigue resistance G-E E G Abrasion resistance F E F Sanding/Machining E P E Conductivity P P E Heat resistance E F E Moisture resistance G F G Resin adhesion E F E Chemical resistance E F E Annotation; E=Excellent, G=Good, P=Poor, F=fair
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GFRP CFRP AFRP (Kevlar) BFRP Epoxy
Fibre strength , MPa 3450 4127 2757 3792 -
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Material
89
Material Young's Modulus, GPa 30-40 125-181 70.5-112.4 70-90 3
ACCEPTED MANUSCRIPT Table 14: Summary of previous studies on FRP material bonding with surface of RC elements
[109] [110]
-
[111] [1]
-
[112] [26]
-
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[108]
• Concrete crushing • FRP composite debonding • Shear at ends of plates Peeling-off at end of plates Shear/peeling-off • Peeling-off along concrete cover Debonding Crushing (fully wrapped beams) Debonding along concrete cover • Debonding if not anchored • Gradual slip if anchored Shear/Peeling along concrete cover Debonding • Concrete crushing (high, ρ) • FRP debonding (low, ρ) FRP rupture if transverse sheets are used along entire length Debonding when plates are placed on bottom of beams
-
Peeling-off related to thickness and stiffness of plates Unless anchored, plates peel off • Use anchoring system to avoid brittle mode of failure Full wrap required to achieve maximum strength without debonding
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[106, 107]
-
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-
• Debonding at adhesive-concrete interface • Shear-peeling at ends of plates Debonding of FRP composite
Main conclusions Stress concentration at end of plates needs more study Selection of bonding agent is critical • Improve concrete-FRP adhesion • Wrapping entire length effective as anchorage Fiber orientation has large effect on maximum strength Pre-cracking has negligible effect • Brittle failure modes need to be considered in design • Need to improve knowledge on adhesion performance
-
•
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Observed modes of failure Debonding after yielding of reinforcing steel
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[106, 107] [21]
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Ref. [105]
-
Anchorage required for adequate performance
Failure mode depends on shear span/depth ratio Anchorage required at ends especially for low a/d ratios • Unable to develop full FRP strength without anchorage Wrapping along full length of CFRP increases maximum load Bonding plates on bottom and sides improves performance
Table 15: Comparison of resistance to corrosion by several composites in corroded environs [76] Conc. HCI N Y Y N
Dilute HNO3 N N N N
Chloride Salts N Y Y N
Dilute NaOH N N N N
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Material Dilute H2SO4 Conc. H2SO4 Dilute HCI FRP (laminate) N N N Carbon steel N N Y Stain less N N Y Hastelloy N N N Annotation; Y (yes) =Corroded; N (no) = Unaffected
Table 16: Fiber volume fraction corresponding to the number of laminate layers of CFRP/GFRP [85] Property fiber type (Laminate type number) 1 2 3 4 5
Fiber volume fraction (%)
Footnote
70 51 66 70
Fiber type 1 is a hybrid composite of carbon/glass fibers Fiber type 2 is a hybrid carbon fiber/resin composites Fiber type 3 is a hybrid glass fiber/epoxy composites Fiber type 4 is a hybrid carbon fiber/vinyl ester composites Fiber type 5 is a hybrid glass/carbon fiber composites
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ACCEPTED MANUSCRIPT Table 17: Volume fractions of fibers (%) and rate of water absorption for different fibers [217]
55.4 ± 3.0 66.2 ± 2.5 55.9 ± 4.8 53.1 ± 4.8
3.45 2.29 3.20 4.12
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E-glass/modified polyester E-glass/epoxy E-glass/vinyl ester ECR (high seed)-glass/modified polyester ECR (high seed)-glass/epoxy ECR (high seed)-glass/vinylester ECR (low seed)-glass/epoxy ECR (low seed) -glass/vinylester
Rate of water absorption, Average value 1 mm 2 mm 4 mm 15.1 4.54 8.65 4.07 1.59 2.02 3.74 1.18 1.95 11.96 11.9 7.34
Volume fraction of fibers (%) 55.8 ± 5.4 53.1 ± 4.4 56.9 ± 3.5 56.4 ± 5.4
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Material
2.27 1.38 1.79 2.33
1.42 0.86 0.85 0.94
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Annotation: ECR-glass = E-glass with higher acid Corrosion Resistance Table 18: Function properties of different FRP materials
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BFRP GFRP CFRP 8.0 2.9-4.5 0.05 3.15 4.7-5.6 1.2 982° 650° 1100° 820° 480° 1000° 260° -60 -170° 0.031-0.03 0.034-0.04 0.035-0.04 1450° 1120° 1550° 1050° 600° 1300°-1670° 1.91 0.32 1.75 9-23 9-13 9-15 <0.1 <0.1 <0.1 100 100 100 95 92 94 82 52 80 0.2 0.7 0.05 5.0 6.0 5.0 2.2 38.9 15.7 Annotation: Part per million/degree Celsius of temperature (ppm/°C); Watt per meter per kelvin (W/m K)
Direction Longitudinal Transverse
AC
CE
PT
ED
Properties Thermal Linear Expansion Coefficient, ppm/°C Elongation at break, % Maximum application temperature, (°C) Sustained operating temperature, (°C) Minimum operating temperature, (°C) Thermal conductivity, (W/m K) Melting temperature, (°C) Vitrification conductivity, (°C) Glow loss, (%) Filament diameter, (microns) Absorption of humidity (65%RAH), (%) Stability at tension (20 C°), (%) Stability at tension (200 C°), (%) Stability at tension (400 C°), (%) H2O, (%) 2n NaOH (Sodium Hydroxide), (%) 2n HCI (Hydrochloric acid), (%)
Table 19: CTE of different FRP materials
Coefficient of thermal expansion (x 10-6/°C) Steel GFRP CFRP 11.7 6 to 10 -1 to 0 11.7 21 to 23 22 to 23
AFRP -6 to -2 60 to 80
Table 20: Transverse and longitudinal CTE of prestressing steel tendons and CFRP tendons [124] Property Units Longitudinal thermal expansion coefficient ̊C Transverse thermal expansion coefficient ̊C Relaxation ratio at room temperature % Annotation: Carbon fiber composite cable (CFCC)
Leadline -0.9 × 10-6 27 × 10-6 2-3
91
CFCC -0.5 × 10-6 21 × 10-6 0.5–1
Prestressing steel 11.7 × 10-6 11.7 × 10-6 8
ACCEPTED MANUSCRIPT Table 21: Imperative comparison and rankings for steel and FRP [188]
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Steel 4 2 3 3-4 3-5 4 3 4 5 3
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FRP Strength/stiffness 4-5 Weight 5 Corrosion resistance/ Environmental durability 4-5 Ease of field construction 5 Ease of repair 4-5 Fire 3-5 Transportation/handling 5 Toughness 4 Acceptance 2-3 Maintenance 5 Annotation: (1) Very Low, (2) Low, (3) Medium, (4) High, and (5) Very High
T
Merit/Advantage (Rating)
Property (Parameter)
Region
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Table 22: Typical behavior of specimens strengthened with different material properties [21] Material Behaviour Concrete Elastic Elastic Yielding
AC
CE
PT
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M
A B C
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FRP Elastic Elastic Elastic
92
Reinforcement Un-cracked Cracked Cracked
ACCEPTED MANUSCRIPT Table 23: Summary of tests of beams under fatigue loading Failure Parameters Ref.
Number of Specimens
Cross Sectional Shape
[21]
1
[243]
1
[244]
Composite Type
Mode of Failure
Rectangular
Number of Cycles 480,000
Tee
12,000,000
Steel Fracture
Rectangular
20,000 732,600 508,500
Steel Yield Steel Yield Steel Fracture
1,889,087
Steel Fracture
>11,968,200 150,000
No Failure Steel Fracture
2,000,000 1,800,000 1,756,000 3,000,000 3,215,000
CFRP Fabric Rupture CFRP Fabric Rupture CFRP Fabric Rupture CFRP Fabric Rupture CFRP Fabric Rupture
Steel Fracture
Hybrid (33% Carbon/ 67% E-glass) Sheet CFRP Sheet
5
[245]
6
Tee
T P
D E
E C
C A
93
Number of cycles indicates fracture of first reinforcing bar although cycling was continued
U N
CFRP Fabric
T P
After 10.7 x 106 cycles, testing was continued in an environmental chamber at 40ºC and 95% relative humidity Control Control Maximum load represented the same percentage of the ultimate load as for specimen 2 Stress range in the reinforcement approximately equal to the stress range of specimen 1 Stress range in the reinforcement approximately equal to the stress range of specimen 1 Strengthened after applying 150,000 cycles
I R
C S
CFRP Pultruded Plates
A
M
Annotation
Accumulation of damage was characerised by a loss of stiffness in the load-deflection plots after different numbers of cycles
ACCEPTED MANUSCRIPT Table 24: Guidelines strategies of the FRP sheets as proposed by various standards for strengthening RC structures Country Name of code Year of publishing
Title of the FRP design code/standard/guideline
Shear
Strengthening of RC structures
Technique
Flexural Column confinement
Europe countries Eurocode 8-3 2004 Currently no Eurocode for FRP Design of structures for earthquake resistance – Part 3: Assessment and retrofitting of buildings) -
(𝐸𝑓 𝑡
4𝑓𝑐𝑡𝑚
le,=
Should be between 40 -240 mm 𝑤𝑓
√
E C
1.5(2− (1+
The concrete strength, MPa
C A
𝑠𝑓
𝑤𝑓
)
)
D E
(𝑛𝐸𝑓 𝑡 0.58
Should not be less than 200 mm
Af = 2ntfwf
Canada
Australian
Japan
Italian
CSA S806-02 2004 Design and construction of building components with fibrereinforced polymers
HB 305 2008 Design guideline for RC structures retrofitted with FRP and metal plates: beams and slabs
JSCE -E 541 2002 Japanese Design and construction guidelines for seismic retrofit of existing reinforced concrete buildings
CNR-DT 200 2004 Guide for the design and construction of externally bonded FRP systems for strengthening existing structures.
T P
I R
C S
U N
A
23300
T P
Kp = Width of the FRP sheets, wf
China
ACI 440.2R CECS 146 2002 2003 Guide for the Technical design and specification for construction of strengthening externally concrete bonded FRP structures with systems carbon fiber for reinforced strengthening polymer laminate concrete structures Embedded Through‐Section Near Surface Mounted (NSM) Near Surface Mounted (NSM) Steel Fiber-Reinforced Concrete (SFRC overlay) Externally Bonded Reinforcement (EBR) Genetic Algorithm (GA) Jacketing (U-wrapping) Steel wire reinforced polymer (SWRP) le, = √
Effective bond length of the FRP sheets, le
American
M
𝐸𝑓 𝑡𝑓
le, = √
𝑐2 𝑓𝑐′
le, =0.7 √ le,=
25350 (𝑛𝐸𝑓 𝑡𝑓)
0.58
𝐸𝑓 𝑡
𝐸𝑓 𝑡𝑓
le, = √
𝑓𝑐′
200 mm (not including section cut away from edge)
𝐸𝑓 𝑡𝑓
le, = √
𝑓𝑐𝑡𝑚
2𝑓𝑐𝑡𝑚
Should not be less than 200 mm Kp =1.06
50mm ≤ wf≤ 250mm
Af = nf tf wf
wf = df –2Le
100 𝑚𝑚
Structural grade ≥ 30 MPa Should be at least 30 MPa Required a high quality surface Up to 2 mm 2-3 mm 1-2 mm 0.17 – 0.83 mm
Pmax = 𝑏𝑓 √2𝐸𝑓 𝑡𝑓 𝐺𝑓′′
√
2−
(1+
𝑤𝑓
)
𝑏 𝑤𝑓
≥1.0
)
400 𝑚𝑚
Quality of the concrete surface The glue line thickness, mm Up to 2 mm 2.0 -2.3 mm Up to 2 mm Thickness of FRP, tf Ratio of strength/stiffness of the 4-5 FRP sheets Annotation: Ef = elastic modulus of FRP, Le = effective bond length, f ´c =concrete strength, fck = characteristic strength of concrete, fctm = mean tensile strength of concrete, n = number of layers of FRP, tf = thickness of FRP, df and wf = effective depth and width of FR, respectively. sf is the spacing between the centreline of the FRP strips, and Af is the effective area of the FRP, nf is the number of FRP layer, 𝐺𝑓′′ Young's modulus of FRP, Kp Factor related to the width of the bond of FRP sheet, Pmax maximum load of the section, b = width of section soffit in mm.
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ACCEPTED MANUSCRIPT Table 25: Bond provisions as proposed by various standards for strengthening RC structures Name of design code
Related equation (Bond strength) ɛf =0.41 √
ACI 440.2R-07
Footnotes
𝑓𝑐′
-
(𝑛𝐸𝑓 𝑡𝑓)
ɛf =0.484 𝑘𝑝 √
CNR DT 200
𝑓𝑐′
𝑓𝑐𝑡
𝑤𝑓
(𝑛𝐸𝑓 𝑡𝑓)
ɛf =0.5 𝑘𝑝 √
Concrete Society TR 55
Kp =1.06 √
𝑓𝑐𝑡
2− (1+
(𝐸𝑓 𝑡𝑓)
(𝐸𝑓 𝑡𝑓) 0.2 𝑙𝑏
)
Kp =1.06 √
IP
√(𝑛𝐸𝑓 𝑡𝑓)
−
T
1
ɛf = kp fct (
CECS-146
)
≥ 1.0
400 𝑚𝑚
Gf = interfacial fracture energy taken as 0.5 N/mm in absence of test values
𝐺𝑓
ɛf = √
JSCE -E 541
)
𝑏 𝑤𝑓
𝑤𝑓
2−
𝑏
(1.25 +
)
𝑤𝑓 𝑏
)
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Annotation: kp retrofit geometry parameter (factor accounting for wf/w in design), Gf interfacial fracture energy, fct concrete tensile strength, lb provided anchorage bond length
Strength/stiffness Weight Corrosion resistance/ Environmental Durability Ease of field construct Ease of repair
Aerospace Marine, construction, pipes, bridges, reinforcing bars, automotive Aerospace, marine, construction, pipes, bridges, reinforcing bars, automotive Marine, boat industry, construction industry, aerospace Automotive, leisurely applications Buildings, bridges, pavements, kiln linings, wind mill blades, radomes Bridges, tunnels, underwater piles. Aerospace, marine, automotive, blast resistant FRP construction. Bridge decks, leisure products, marine boats Shapes, bridge decks, components and assembled FRP systems Bullet proof vests, vandalism and graffiti proof walls. Construction and aerospace industries Offshore and fire resistant applications.
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Fire
FRP applications Application
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FRP Very high High Very high Very high High High High Very high Medium Very high High low Low
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Parameter
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Table 26: FRP merits and suitability of applications [195]
Transportation/handling Toughness and impact
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Acceptance
CE
-
AC
Actual projects used FRP
-
Egyptian Museum in Cairo and Kaitbay Fence (Egypt) Bridge deck slab (Germany) Brisbane City Council - Bowen Bridge (Queensland) Melbourne Water – Maribyrnong and River Pipe Bridge - Melbourne Airport – Service Culverts, – Kororoit Creek Rd Bridge & Arden St Bridge (Victoria) Main Roads WA - Greenough River Bridge (Western Australia) Ouse River Bridge & Emu River Bridge (Tasmania) Superstructure strengthening of Alaskan Way Viaduct bridges and WSDOT Evaluation Project (United State)
95
ACCEPTED MANUSCRIPT Table 27: Summary of parameter studies on CFRP strengthening confinement of hollow RC bridge columns Dimensions, mm
[33]
[17]
Rectangu lar
Square
Rectangu lar
160 0
160 0
140 0
450 x 900
450x45 0
550 x 350
550x35 0
[26 1]
Circular & Square
300
150 x300
[19 7]
Square
100 0
120×80 ×8
300x750
300x300
340x140
AC
4
2
2
Tensile Streng th (MPa)
Wrapping Position
` Axial Load
0.165
1820
3000
Longitudina lly wrapped
250 & 500
3480
Unidirectio n al 3m up the column
3600 & 3900
Wrapped spaced at 100mm along height
3800
Wrapped 200mm & 100mm spaced
3750
Confined 500mm to the bottom column
0.137 5
0.117
0.117
-
-
11
0.111
3800
-
200
2& 3
0.111
300
3972
106 x106
2
0.165
200
1102
140×80× 8
3
-
-
3769
330x130
CE
[18]
Rectangu lar
140 0& 280 0
900x900
Densi ty (g/m3 )
96
Confined 500mm to the bottom column Wrapping Position
Materials properties (MPa)
Load
Concre te
Long. Reinf.
-
33
670
no data
-
30
418 & 420 (64ϕ2 2)
413 & 420 (10ϕ @ 200m
700
500
35
450
450
700
500
28
435 (40ϕ8 )
440 (2.6ϕ)
Lateral
T
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1500 x 1500
2
Thickne ss (mm)
CR
[16]
350 0
300x300 900 &
Loading (kN)
US
[14]
Circular & Square
450x45 0 1500 &
No. of Lay er
AN
[13]
900 & 135 0
Inner Dia (d)/Dimens ion
M
Square
Heig ht
ED
Section
CFRP
PT
Ref.
Outer Dia. (D)/ Dimensio n
480
72.32 (min) , 202.3 5
22.4
Transv. Reinf.
500
500
(20ϕ10)
(6ϕ@10)
413, 420 (6ϕ@10 0)
(max)
909.6
1000
30.4
418, 420 (20ϕ1 0)
7511563
-
40
8 mm steel rod
-
3436. 1
-
40
235
-
ACCEPTED MANUSCRIPT Highlights
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M
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FRP composite is highly resistant to chloride ion and chemical attack compared to steel bars.
PT
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FRP has a distinctive tensile strength property of being greater than that of steel, yet, it weighs only one quarter as much. FRP composites were accepted as a mainstream construction material for repair, strengthening and retrofit of concrete structures. Rehabilitation function is mainly influenced by the direction of polymers fibres used.
AC
97
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Y.H. Mugahed Amran, Rayed Alyousef, Raizal S.M. Rashid, H. Alabduljabbar, C.-C. Hung PII: DOI: Reference:
S2352-0124(18)30110-3 doi:10.1016/j.istruc.2018.09.008 ISTRUC 331
To appear in:
Structures
Received date: Revised date: Accepted date:
21 July 2018 20 September 2018 21 September 2018
Please cite this article as: Y.H. Mugahed Amran, Rayed Alyousef, Raizal S.M. Rashid, H. Alabduljabbar, C.-C. Hung , Properties and applications of FRP in strengthening RC structures: A review. Istruc (2018), doi:10.1016/j.istruc.2018.09.008
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ACCEPTED MANUSCRIPT Properties and applications of FRP in strengthening RC structures: A review Y. H. Mugahed Amran1, *, Rayed Alyousef2, and Raizal S.M. Rashid3, H. Alabduljabbar2 and C.-C. Hung4 1
Department of Civil Engineering, Faculty of Engineering, Amran University (AU), Quhal, Amran Province, Yemen. Department of Civil Engineering, College of Engineering, Prince Sattam Bin Abdulaziz University, 11942 Alkharj, KSA 3 Department of Civil Engineering, Faculty of Engineering, University Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia 4 Department of Civil Engineering, National Cheng Kung University ,1, University Rd, Tainan City 701, Taiwan
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2
*
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Corresponding author, Tel: +967 775105108, Fax: +967 01 229939, E-mail: [email protected]
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Abstract
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In civil and structural engineering, building structures with robust stability and durability using sustainable materials is challenging. The current technological means and materials cannot decrease weight, enlarge
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spans, or construct slender structures, thus inspiring the exploration for valuable composite materials. Fiber reinforced polymer (FRP) features high-strength and lightweight properties. Using FRP motivates civil
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engineers to strengthen existing RC structures and repair any deterioration. With FRP, a system that can
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resist natural disasters, such as earthquakes, strong storms, and floods, can be developed. However,
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deterioration of structures has become a critical issue in modern construction industries worldwide. This paper reviews the FRP design, matrix, material properties, applications, and serviceability performance. This
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literature review also aims to provide a comprehensive insight into the integrated applications of FRP
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composite materials for improving the techniques of rehabilitation, comprising the applications toward the repair, strengthening, and retrofit of concrete structures in the construction industry today.
Keywords: Applications, Aramid/Basalt/Glass/Carbon Fiber Reinforced Polymer (AFRP, BFRP, GFRP, CFRP), Beams, Columns, Deterioration, Fiber Reinforced Polymer (FRP); Joints; Matrix, Properties, Rehabilitation, Reinforced Concrete Structures; Serviceability, and Strengthening Structures.
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ACCEPTED MANUSCRIPT Content
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Introduction Typical Materials of FRP 2.1 CFRP 2.2 GFRP 2.3 AFRP 2.4 BFRP 3 Matrix of FRP 3.1 Epoxy 3.2 Vinyl ester 3.3 Polyester 4 Mechanical Properties 4.1 Compressive and impact strengths 4.2 Flexural strength 4.3 Shear strength 4.4 Tensile strength 4.5 Creep rupture 4.6 Modulus of elasticity 5 Physical Properties 5.1 Density 5.2 Strength-to-weight ratio 5.3 Rigidity and stiffness 6 Durability Properties 6.1 Corrosion resistance 6.2 Fatigue resistance 6.3 Brittleness 6.4 Moisture content 6.5 Specific gravity, water absorption, and fraction of fibers 7 Functional Properties 7.1 Electrical and thermal conductivity 7.2 Temperature effect 7.3 Coefficient of thermal expansion 7.4 Fire resistance 8 Serviceability of FRP 8.1 Crack width 8.2 Deflection 8.3 Fatigue 9 Design of FRP 10 Strengthening techniques 11 The bond characteristics of FRP 12 Applications of FRP 12.1 Flexural strengthening 12.2 Shear strengthening 12.3 Column confinement and ductility improvement 13 Conclusion 14 Acknowledgment 15 References
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1 2
2
3 4 5 5 6 7 7 8 9 9 10 10 12 14 16 18 20 21 21 23 24 25 26 27 28 29 30 32 32 33 35 36 38 38 40 41 42 45 45 47 50 51 53 55 57 58
ACCEPTED MANUSCRIPT 1
Introduction
The initial use of fiber reinforced polymer (FRP) was known as reinforcement bars in 1975 particularly in Russia (Figure 1) [1, 2]. FRP is also recognized as fiber reinforced plastic, comprising materials that utilize either synthetic or natural fibers to automatically improve the stiffness and strength of a polymer model [2].
T
FRPs employed to strengthen and reinforce structures are enormously strong, rated 8 times robust than
IP
classical steel reinforcement bar [3]. Glass fiber reinforced polymer (GFRP) is used as prestressing tendons
CR
to strengthen a 9 m-long, fastened wood bridge [4]. Relevant investigations on the use of FRPs as reinforcing bar to substitute the use of steel plate bonding for bridge restoration and strengthening instigated in Europe in
US
the 1980s. But in the United States, FRP composites were engaged for structural strengthening for
AN
approximately 25 years [5]. During this period, FRP composite was accepted as a mainstream construction material parallel with the sum of accomplished FRP strengthening projects. The use of FRP for
M
strengthening, rehabilitation and retrofitting has attained more reputation among design consultants over
ED
traditional strengthening methods, such as setting up of supplementary structural steel frames and components [6]. FRP is mainly worked as interior reinforcement, for instance rebar, or exteriorly-bonded
PT
reinforcement to reinforce concrete, timber, steel and masonry structures [7]. In Japan, FRP bars attained a
CE
significant support for the duration of the 1990s from the study on fascinatingly ascended train support structures [8]. FRP also has a unique tensile strength characteristic higher than that of steel hitherto weighs
AC
merely one quarter [1-8]. In 1996, the Japanese was the first team who announce the design guidelines for FRP in the strengthening of reinforced concrete (RC) structures [8, 9]. Later, the use of FRP as a structural reinforcement has enlarged exponentially, and the design supervision and guidance were authored by officialdoms worldwide [10, 11]. Structural strengthening with exteriorly-attached FRP reinforcement, in particular, with extra-high given strength carbon fiber reinforced polymer (CFRP), has been proved by design codes for seismic advancements of structures for several years. For instance, the growth of economic
3
ACCEPTED MANUSCRIPT and efficient approaches to repair, upgrade, strengthen or reinforce the current RC bridges has acknowledged significant interest recently [12, 13]. The inspiration to strengthen an existing RC bridge classically derives from only two sources: a need to upgrade the strength of the bridge to preserve pace with upsurges in the weight of design automobiles and a desire to overhaul deterioration that has resulted over years of
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serviceability [14-20]. The structural imposed loading of elements may be increased via wrapping them with
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the reinforcements of FRP [4, 21, 22]. However, FRP has raised its market growth in recent years, and this
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market is expected to rapidly grow in the forecasted period. This study is aimed to technically review the FRP design, matrix, typical materials, and properties, such as impact, flexural, shear, and tensile strengths;
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strength-to-weight ratio; rigidity; electrical and thermal conductivity; and fatigue, corrosion, and fire
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resistance. This literature review also provides a comprehensive insight into the integrated applications of FRP composite materials for repair, rehabilitating, retrofitting and strengthening RC structures in the present
M
construction industry.
2
Typical Materials of FRP
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Figure 1: Continuous development of FRP matrix composite from the early 1970s to present [2]
PT
The mutual FRP composite reinforcements utilized in civil engineering are made through a pultrusion
CE
technique from carbon fiber (to produce CFRP), glass fiber (to produce GFRP), basalt fiber (to produce BFRP), and aramid fiber (to produce AFRP) [23, 24]. E-GFRP is the cheapest material of all structural FRPs
AC
and is thus the greatest consumed [25]. Unlike E-GFRP, BFRP costs higher due to lack of manufacturer capacity; however, its cost is reasonable given its superior strength to GFRP, alkalis resistance, and almost infinite resource [4]. Figure 2 illustrates the overall comparison between FRP materials and steel reinforcements based on stress–strain behavior. AFRP is not a popular structural bar because of low compressive strength regardless of fiber alignment direction and high charge [26, 27]. Aramid fiber is the best selection for ballistic-resistant fabrics since the fiber efficiently engrosses effect [28]. CFRP has the
4
ACCEPTED MANUSCRIPT uppermost strength between FRP materials and broadest variety of strengths [29]. The variety is as a result of the source of carbon and production techniques. However, CFRP exhibited the highest resistance to fatigue and creep failure than other FRP materials [30]. The high charge of CFRP is counted by its great strength and resistance to fatigue and cyclic failures [31-33]. FRP materials are reviewed in detail in the subsequent
T
subsections.
CFRP
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2.1
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Figure 2: Comparison of FRP materials with steel [4]
Carbon fibers have diameters limited between 5 and 10 micrometers. The fibers are comprised largely of
US
carbon atoms that bond both in crystals, which are less or more aligned similar to the long axis of the fiber
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given that the crystal arrangement offers high strength-to-volume ratio [21, 34]. CFRP is an enormously light and strong FRP that contains carbon fibers and possesses extremely high tensile strength and strength-to-
M
weight ratio (20% the mass of steel) (Table 1). CFRP also has an ultra-elastic modulus similar to steel, which
ED
is popular in the aerospace and infrastructure industries. The reinforcement of CFRP composite is carbon fiber that affords the strength, and the matrix is generally a polymer resin, for instance epoxy, that attaches
PT
the bars together. Although CFRP can offer 50%–60% mass reduction compared with alike elements in steel,
CE
the cost is 2 to 10 times greater when the costs of materials and processing are considered, as claimed by William [35]. ACI [5, 23, 36] reported that the creep strain for CFRP at 20 °C, and regular humidity rests
AC
under 0.01% after 3000 h at load stages of 80% of comparative ultimate strength. CFRP is developed to strengthen existing RC structures, such as bridges, to avoid replacing constructions that function satisfactorily for many years [21]. Table 1: Typical properties of CFRP [37]
2.2
GFRP
Glass fibers, which are also known as fiberglass and usually added at 0.5%–2.0% by weight to the composite, are referred to as fiberglass reinforced plastic [38]. GFRP is a sort of plastic compound that precisely uses 5
ACCEPTED MANUSCRIPT glass fiber constituents to instinctively increase the stiffness and strength of plastics [39-40] (Table 2). The resin affords a supplemental protection to the fiber thanks to the interaction between different materials [41]. GFRP has become a staple in the building industry since the mid-1930s [42]. Properties of GFRP rely on the features of the type of polymer matrix, reinforcing fiber, fiber content, fiber orientation, and the bonding
T
between fiber and matrix [43]. GFRP also has extremely high strength-to-weight ratio; low weights of 9.67
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kg/m2 to 19.52 kg/m2; and resistance to salt water, chemical effect, and alkaline environment. In addition,
CR
GFRP has a great thermal insulation property, excellent heat resistance, and low cost [44]. The increase in the thickness of GFRP plates with more than 6.35 mm increases the strength associated with anchorages
US
provided at the ends of the plates by 40% to 100% [45]. The creep strain of GFRP is approximately 0.3%–
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1% [46]. Furthermore, GFRP is mostly used in the construction of secondary structures, such as bridges, domes, and building frames or nonstructural elements, such as masonry walls [47].
AFRP
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2.3
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Table 2: Typical properties of GFRP [37]
Aramid fibers are artificial high-performance fibers with molecules branded by moderately stiff polymer
PT
chains and categorized as heat-resistant and strong synthetic fibers [48, 49]. AFRP is one of the most useful
CE
fibers in textiles and fiber reinforced composites (Table 3). It has strong synthetic fibers, great strength and elastic modulus, heat resistance, 40% lesser density than GFRP, and slightly higher cost [50]. AFRP absorbs
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moisture and remains sensitive during manufacturing until impregnated with a polymer matrix [51]. AFRP is a better option given its high resistance to alkaline environments and more economical than CFRP reinforcing bars [51]. The breaking strength of AFRP at high loading rate is comparable and 40% higher than other FRP materials, resulting in only 13% reduction in strength after 100,000 cycles [48]. The creep strain of AFRP is 0.15%–1% [49]. AFRP is often used in concrete structures [56], but the industries continue to restrict the use of AFRP in lightly loaded structures because aramid fibers own extremely low compressive strength and high tensile strength [50]. 6
ACCEPTED MANUSCRIPT Table 3: Typical properties of AFRP [37]
2.4
BFRP
Basalt fibers are materials made from extremely fine fibers with nearly 10 and 20 micrometers in diameter. These materials are composed of minerals such as plagioclase, pyroxene, and olivine [37, 51, 53]. BFRP is a
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new promising technology for the construction industry and an alternative to GFRP bars [54]. BFRP
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composite is one of polymeric matrixes that can assist improving rigidity, strength, matrix interface, thermal
CR
conductivity, and resistance to heat, chemical and physical corrosion [55]. BFRP is also recognized for it’s a great tensile strength, elongation at fracture, and alkalis resistant in ultra-high concrete CFRP and GFRP [53,
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56, 57] (Table 4). The modulus of elasticity of the BFRP is mainly relied on the chemical empathy and
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conformation of the single BFRP fiber. Basalt is plentiful and encompasses equal to 33% of Earth’s crust [55]. The long-standing tie strength-preservation estimates of the bars after 50 years of serviceability in
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moist, dry, and moisture-saturated environs with mean yearly temperatures limited to 5 °C and 35 °C array
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from 71% to 92% [58]. BFRP could reduce automobile body self-weight by 40%–60%. However, the charge of the whole process is presently not economically feasible, and its cost is alike to that of GFRP [32].
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Considering the advantages of basalt fiber, applicable applications exist in the production of basalt–epoxy
CE
compounds, which are also frivolous and feature robust load-bearing characteristics that are valuable in
3
AC
weighty vehicle industries and strengthening materials for structural RC members [37, 55, 58-60]. Table 4: Typical properties of BFRP [44]
Matrix of FRP
The resin is the interaction agent of several composites of FRP and is also recognized as matrix. The most typical resins are thermosetting and thermoplastic polymers [59]. The selection of resins during production process is crucial since the choice impacts the mechanical characteristics of composites. Thermoplastic polymer is not applicable to be used for civil engineering resolutions on account of its low creep and thermal resistances. Though, thermosetting resins, for instance polyesters, epoxies, and vinyl esters (Table 5), which 7
ACCEPTED MANUSCRIPT are the utmost used resins, display suitable thermal constancy and resistance to chemical and endure low creep and stress reduction, as prescribed by ISIS Design Manual 2007 [24, 59] and revealed in Table 5. In addition, FRPs are composites that consist of fibers and matrix (Figure 3) [61]. Fibers are the elements that carry the applied loads, and the matrix guarantees the consistency of the fibers, re-transition of applied loads
T
to the fibers, and defense of fibers from exterior environment [62].
IP
Table 5: Properties of thermosetting resins of FRP matrix [59]
3.1
CR
Figure 3: Typical composite geometry of FRP [61, 62] Epoxy
US
Epoxy is any of the basic adhesive components or cured end products of epoxy resins. It is commonly
AN
applied on the surface of the strengthening area of RC elements with a little 1%–3% addition of a compound [20, 67]. Epoxy typically requires extra remedial agent at a greatly high ratio of mastic to toughened, which
M
is generally 1:1 or 2:1 [67]. The main types of epoxies are non-glycidyl and glycidyl epoxy. Glycidyl epoxy
ED
resins are known as glycidyl ether, glycidyl ester or glycidyl amine, Non-glycidyl epoxy resins are either cycloaliphatic or aliphatic resins [64]. Epoxy is utilized efficiently as a bonding agent, sealing resin for
PT
moistening out mechanical textiles, and coating. Epoxy features tremendous thin-film cure properties and
CE
demonstrates superior micro-cracking resistance to polyester resin. Epoxy also provides 3.5% to 4.5% tensile protraction at failure [20]. For example, Sikadur 330 (1.31 kg/L) comprises two components (A + B),
AC
namely, a liquid epoxy resin (white paste, A) and a hardener (gray paste, B) (Table 6) [63]. The amount of adhesive and room temperature should be carefully monitored during epoxy applications. Epoxy provides excellent fiber bonding (matrix to fiber), improving flexural and compressive strengths, increasing interlaminar shear and impact strength, and enhancing damage tolerance [20, 65]. Table 6: Technical properties of epoxy (Sikadur 330, 1.31 kg/L)
8
ACCEPTED MANUSCRIPT 3.2
Vinyl ester
Vinyl ester is mastic formed by the esterification of an epoxy resin with an insatiate monocarboxylic acid. The retort product is then melted in a retrograde solvent, for instance styrene, to 35%–45% content by mass [71]. This resin is mostly used in bonding the GFRP and basalt fiber reinforced polymer/plastic (BFRP)
T
applications [52, 60]. Vinyl ester additionally delivers enhanced FRP toughness and fatigue resistance over
IP
epoxy and polyester [72]. The material design with FRP decks primarily differs in fiber architecture and resin
CR
type. Thus, vinyl-ester resin is preferred for internal FRP reinforcements due to its excellent environmental resistance [73]. Moreover, the tie strength of GFRP bars by means of vinyl ester is higher than that of BFRP
US
bars using epoxy [35]. Patnaik et al. [52] studied BFRP bars produced with the misty layup method by vinyl-
AN
ester resin and a fiber amount segment of 50%. The elastic modulus of BFRP bars utilizing cross-section characteristics is 43–45 GPa, and the regular rupture strain is higher than 2.5%. On the contrary, Chen et al.
M
[74] used GFRP bars of 9.5 mm diameter with vinyl ester resin and contained a bar surface that was
ED
marginally sand layered and helically giftwrapped with a fiber content of higher than 70% by mass. Vinyl esters cured at ambient temperature display lesser creep resistance than those post-cured at 93 °C [75, 76]. Polyester
PT
3.3
CE
Polyester is a resin that is most widely used in FRP composite industries because it is less expensive, resistant to corrosion, fast curing, convenient, and tolerant of temperature and catalyst extremes [73].
AC
However, it bears drawbacks, such as low modulus of elasticity and enhancement of up to 5%–15% only [77]. Polyester can also cause a creep [78]. Polyester has 1%–2% tensile elongation at failure compared with 3.5%–4.5% for typical epoxy resins [20, 65]. The mechanical and functional properties upsurge with the increase in the content of glass in hybrid GFRP composite with polyester resin [79-81]. Therefore, polyester is a potential candidate for structural composite applications [82]. A polyester resin is primarily infused in GFRP composites, which are used in the face sheets of sandwich-bridge decks [11]. Polyester resin is
9
ACCEPTED MANUSCRIPT preferred for its minimal cost, whereas vinyl-ester resins are favored for saturated environs [73]. The improvement in eventual strength of 115% in tension and consistent 43% upgrade in modulus for incessant filament arbitrary mat glass polyester content are detected in examining for compression and tension by Fernie and Warrior [83]. Mechanical Properties
T
4
IP
Mechanical properties are also utilized to classify and identify materials of FRP bars. The most common
CR
properties considered are impact, flexural, shear, and tensile strengths, creep rupture, and modulus of elasticity. FRP composite bars have been widely used in construction in the last few decades. Table 7 lists
US
the most commercially available FRP bars and their mechanical properties.
4.1
AN
Table 7: Mechanical properties of several classes of FRP materials [6]
Compressive and impact strengths
M
Strengthening the core structural RC elements is crucial since the refurbishment of weakened structures
ED
would impose special techniques, advantages, and technological materials with different features that impact the structures, leading to a huge cost [36, 84, 85] (Table 8). The cost-efficiency factor is realized over a
PT
strength efficacy measure of intended sample (SEff), as shown in Equation (1) [31]. AFRP and CFRP have
CE
the lowest and highest compressive strength, respectively, in comparison with the other typical FRP materials [27]. However, the composite strength is high if basalt fibers are either positioned on the face or
AC
equipped by substitute layers within the composite as a sandwich form [55]. The compressive strength of epoxy-based composites is higher than that of polyester-based composites, indicating that strength of whole the composites with and without fillers with polyester as matrix is less than that of the epoxy laminates [86]. 𝑆𝐸𝑓𝑓 =
𝐶𝑜𝑙𝑢𝑚𝑛𝑠 𝐶𝑜𝑛𝑐𝑟𝑒𝑡𝑒 𝐶𝑜𝑛𝑓𝑖𝑛𝑒𝑑 𝐹𝑅𝑃 𝑜𝑓 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑡ℎ𝑒 𝑖𝑛 𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑃 𝐶𝑜𝑙𝑢𝑚𝑛𝑠 𝐶𝑜𝑛𝑐𝑟𝑒𝑡𝑒 𝐶𝑜𝑛𝑓𝑖𝑛𝑒𝑑 𝐹𝑅𝑃 𝑡ℎ𝑒 𝑜𝑓 𝐶𝑜𝑠𝑡 𝑇
[31]
(1)
However, impact strength is a measure of the sum of energy that a FRP composite material that could be captivated before fracturing under a high rate of deformation at a specific shock loading under impact. The
10
ACCEPTED MANUSCRIPT strength of a GFRP compound is condensed to 9.2% at ambient temperature [87]. For example, Wu and Li [88] showed that the impact strength of samples slowly decreased from 45.0 MPa to 38.8 MPa at ambient temperature and at 300 °C, respectively, exhibiting a 13.8% reduction in strength. It is also reported that the strength of hollow columns wrapped with CFRP is improved by 66% (1 layer) and 123% (3 layers)
T
compared with that of the GFRP material, which only increased by 36% (1 layer) and 105% (3 layers) [84].
IP
However, that of the filled column that comprised of a circular hollow section filled with concrete and
CR
wrapped with CFRP increased by 154% (3 layers) compared with 144% (3 layers) with that of GFRP. The interior moment of the column is increased, causing a reduction in the compressive strength capacity [18].
US
Al-Sunna [7] found that the service load of beam wrapped with CFRP corresponds to a stress scale in the
AN
upper concrete fiber of about 40% of the compressive strength of concrete compared with the non-fibered sample. Through the ultra-high performance fiber RC showed compressive strengths with values of at least
M
115% larger than that of ultra-high performance concrete [89]. Mastali and Dalvand [90] studied the use of
ED
252 cylinders and cube samples on the reinforcement of plain concrete using jacketing/U-wrapping method and reported an increase of approximately 31.10%, 47.07%, and 65.10% in the impact compressive strength
PT
of samples with recycled carbon fiber of 0.25%, 0.75%, and 1.25%, respectively. In addition, the
CE
compressive strength of sandwich structure strengthened with CFRP is approximately higher by 24.68% than that of none-strengthened samples [91]. Another marked fast upsurge in the strength is obtained for the
AC
material with lengthy bars and high volumes of CFRP/prepreg waste (PW), that is, 83% for PW and 80% for CFRP (long) [92]. The increase in fiber length found the maximum improvement in the strength of nearly 3% in the sample [90]. However, the energy engrossed in breaking the test sample, indicating by the position of sample, is restricted in the vice of the machine of testing. The notch of the sample faces the striker, and the root of the notch is under a similar level with the parallel face of the vice, calibrating dial bounded to the testing machine [93]. Impact strength is equal to energy absorbed (Joule capacity, J) by the pendulum
11
ACCEPTED MANUSCRIPT hammer at the instance of impact over the width of notched face (mm), which is multiplied by the length below the notch in the pendulum hammer after breaking the specimen (mm), as shown in Equation (2) [94]. The thickness of FRP layer considerably affects the compressive strength of the strengthened concrete element zone [29]. 𝐸𝑛𝑒𝑟𝑔𝑦 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑𝑖𝑛 𝑗𝑜𝑢𝑙𝑒𝑠
(2)
T
𝐼𝑚𝑝𝑎𝑐𝑡 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ = 𝑊𝑖𝑑𝑡ℎ𝑜𝑓𝑛𝑜𝑡𝑐ℎ𝑒𝑑𝑓𝑎𝑐𝑒 × 𝐿𝑒𝑛𝑔𝑡ℎ 𝑏𝑒𝑙𝑜𝑤 𝑡ℎ𝑒 𝑛𝑜𝑡𝑐ℎ kJ/m2 [93, 94]
Flexural strength
CR
4.2
IP
Table 8: Qualitative comparison of several fibers used in design of the composites [85]
US
FRP reinforced members are generally over-reinforced, that is, the proportion of FRP bar to concrete is larger than the balanced ratio; hence, concrete crushing of the member controls the failure mode [6, 36]. However,
AN
when the ratio of FRP reinforcement to concrete is less than the balanced ratio, the FRP rupture encounters a failure mode, which is not a preferred ductile failure mode [6, 55]. The reduction factor of flexural strength is
M
limited between 0.55 and 0.65 in the basis of the ratio of proposed reinforcement to the neutral reinforcement
ED
ratio because of the deficiency of ductility in FRP reinforced failure modes [12]. The strength reduction
PT
factor for FRP rupture at failure is 0.55. However, when the failure is by concrete devastating, the reduction factor of flexural strength increased to 0.65, where the ratio of the neutral FRP reinforcement is smaller than
CE
1.4 times the proposed reinforcement ratio [12]. The flexural strength section wrapped with CFRP evidently
AC
decreased with increasing delamination factor [95]. However, the flexural strength of the FRP material is determined using ACI 440 similar to ACI 318 because FRP rebars do not yield similar to steel bars [5, 36, 96]. The flexural aptitude of the reinforced sections is practically supposed to be restricted by the rupture strain of the composite structures [21]. Certain studies investigated the variables and parameters influencing the flexural strength of the compounds, including the length of fiber, thermal treatments, binder content, and pre-activation of the fibers prior creation. For instance, the lengthiest fiber at 10,000 microns has the lengthiest distance between binder interaction points, causing in the feeblest composite [97] (Table 9). Park
12
ACCEPTED MANUSCRIPT and Jang [98] presented carbon fibers, along with fibers of polyethylene (PE) within an epoxy form; to formulate a hybrid layered composite scheme. The hybrid-based composite is powerfully relied on the location of the fiber of reinforcement. Moreover, the placement of CFRP at the marginal sheet delivers a great grade of flexural strength. The increase in recycled CFRP fiber length in self-compacting concrete
T
results in the maximum recorded increase in the flexural strength by almost 6% compared with that found in
IP
the constant fiber length [90]. Mastali and Dalvand [90] claimed that increasing carbon fiber content (1.25%)
CR
improves flexural performance of the beams by 10% and 27.23% more than the mean flexural strengths of 0.75% and 0.25% specimens, respectively. This study also reported that the flexural strength of
US
reinforcement of plain concrete beams increased by around 31.20%, 50.93%, and 66.93% with recycled
AN
carbon fibers of 0.25%, 0.75%, and 1.25%, respectively. Basalt fibers and polyvinyl alcohol (using externally bonded reinforcement (EBR) method) are used to provide substantial backing and resist cracking, thereby
M
increasing the break robustness of matrix and improving the flexural strength by nearly 27% at the yielding
ED
zone compared with that of control specimens [99]. Another study developed flexural beam strengthened using a dubbed carbon skin-basalt core composite and a basalt peel-carbon core compound (EBR method).
PT
The results showed 245% and 32% increase in flexural modulus and flexural strength, respectively, of former
CE
composite compared with the later composite [100]. Using AFRP provides an expressive magnitude of ductility for FRP-reinforced beams [101]. It is found that the silica/polyester composites enriched the flexural
AC
strength from 115 MPa to 156 MPa at dry phase [102]. Besides, using a twin layer carbon–glass fiber composite system (using near surface mounted (NSM) method) to strengthen RC beams increases the strength capacity by 114% on the strengthened beam compared with that of reference control [103]. Carbon fiber exhibited a lower strain than glass fiber, and CFRP samples have less ductility than GFRP samples. The combination of glass fiber and PE fiber shows no destructive impact on the flexural strength of the samples [104]. For flexural strengthening, the plates, tow sheets, and bars, are some of FRP reinforcement products
13
ACCEPTED MANUSCRIPT that used to bond the tension side of a concrete, timber, or even masonry, substrate with epoxy mastic. Strengthening of structural flexural elements improved the load-bearing strength of up to 40%. Thus, Equation (3) is employed to compute the cross breaking strength of flexural strengthened members. Cross breaking strength =
1.5𝑊𝐿 𝐵𝐷 2
, kN, [86] ,
(3)
IP
W = loading, kN B = breadth in mm D = thickness, mm L = span between supports, mm
CR
-
T
where
4.3
US
Table 9: Summary of previous studies on flexural tests
Shear strength
AN
The interpretation of shear strengthening of RC structures by means of outwardly bonded FRP shields significantly relies on the bond performance at interaction between the FRP sheets, FRP jacket thickness,
M
concrete substrates, number of layers, and epoxy materials with fibers [105-112]. The behavior should be
ED
parallel to the principal tensile stresses (Table 10) [11, 113-115]. The increase in the amount of moisture
PT
absorbed on the epoxy leads to loss of desired shear strength of the strengthened concrete element [116, 117]. However, to resist shear forces, an FRP reinforced member must contain ties or stirrups, or, if not practical as
CE
in the situation of RC tanks, rely only on the concrete resistance to shear loading [114]. RC design without
AC
shear reinforcing leads to deep sections where shear is critical [118]. Although a deep member may not initially be desired, it corresponds to the upsurge in the cracking moment of the element [119]. A cracking moment that is 25% larger than the applied service moment allows the use of gross moment of inertia to compute deflections and the full section of the concrete member to resist shear loads [120]. The tensile strengths of FRP rebar can be much greater compared to that of steel. Most FRP bars have significantly small modulus of elasticity or stiffness. Decreased stiffness indicates the necessity of deep members or additional reinforcement to mitigate long-term deflections and limit crack widths [118]. Shear strength design of FRP
14
ACCEPTED MANUSCRIPT uses ACI 318 methods [96], whereas ACI 440 [36] does not allow for dowel action of FRP rebar to resist shear in comparison to ACI 318 that facilitates shear resistance of steel bars. Moreover, the textile is usually formulated to reveal a certain load–strain profile in its 45° directions, permitting the ideal impact to beam shear strength [121]. However, the rosette strain gages exhibited that the sheet led to the beam shear strength
T
after shear cracking. The contribution of shear of the FRP is obtained in the basis of the failure modes, and
IP
the maximum strain is restricted to 0.004 for failure attributable to FRP rupture and 0.002 for attach perilous
CR
applications [119]. The maximum recorded 45° strain reading for the sheet at beam sides is 0.40% compared with its ultimate strain of 1.2% [121]. The ultimate strengthening intervention in hollow concrete bridge
US
column restores its ductility, strength, and flexural shear strength ratio [122]. Certain studies showed that the
AN
fiber shawl had a lower effect on filled columns than on hollow columns (Table 10). This effect is also notable in hollow columns, and GFRP is less effective than CFRP. Another study indicated that shear
M
strength of the reinforced beams, which are supported using CFRP sheets, is increased parallel with eventual
ED
stiffness and strength of the beam associated with that of the control beam and the condensed ductility of the RC beams [86]. It is also reported that the shear strength of GFRP composite decreases to 13% at the case
PT
when the temperature is elevated to 200 °C [87]. Meanwhile, the influence of bucky paper interleaves
CE
produced from carbon nano-fibers on inter-laminar mechanical characteristics of CFRP (using embedded through-section (ETS) method), exhibiting 104% and 31% enhancement in mode II fracture toughness and
AC
interlaminar shear strength, respectively [123]. On the contrary, Han et al. [124] found that the concrete structures strengthened with polyfunctional epoxy resin and CFRP tendons with different diameters (using near surface mounted (NSM) method) increased the shear strength by approximately 30%–40%. Investigation reported that a sudden reduction in the shear strength may be occur when the BFRP composites are engrossed and matured under hot salt water at 40 °C [125]. For shear strengthening, FRP reinforcements are glued to the external of the beams in a vertical U-shaped conformation as an exterior stirrup.
15
ACCEPTED MANUSCRIPT Strengthening of shear walls, such as under- RC walls and unreinforced masonry walls, can be achieved by bonding FRPs to either both or one sides on the wall in either a horizontal, vertical, or X pattern. However, the nominal shear strength is practically calculated using Equation (4) [126]. Vn = Vs + Vc
[126]
(4)
𝐴𝑠 𝑓𝑦 𝑑 𝑠
,
IP
𝑉𝑐 =
T
𝑉𝑐 = 2 √𝑓𝑐′ 𝑏𝑑
US
As = area of flexural reinforcement, mm2 d = distance from extreme compression fiber to centroid of flexural reinforcement, mm b = width of beam, mm s = horizontal spacing of shear reinforcement, mm fy = yield stress of flexural or shear reinforcement, N/mm2 f 'c = concrete compressive strength, N/mm2
AN
-
CR
where
4.4
M
Table 10: Summary of previous studies on RC sections strengthened by CFRP and steel
Tensile strength
ED
FRPs are used for interior bar and strengthening of RC structures that utilize artificial fibers in a polymeric
PT
matrix to afford tremendous tensile strength parallel to the direction of fibers [127]. The fibers are aligned in a parallel, straight, and unceasing configuration within the matrix [43]. However, if radial bursting stresses
CE
become higher than the concrete tensile strength in the element, then cracks develop and the bond between
AC
the concrete and bar is negatively influenced [59]. These FRPs are rarely recognized in the community of civil engineering as ultra-high-strength compounds and can be computed using Equation (6). Tensile strength values of FRP fibers are commonly indicated depending on the matrix, the interface of section, moisture absorption, fiber orientation, and types of fibers [117]. For example, the tensile strength increases with rising weight ratio of fiber by a certain volume [128]. However, CFRP fibers are reported to own greater tensile strength and lesser weight compared with the other typical FRP fibers (Figure 4) [90, 128-154]. This condition leads to significant weight reduction and enlarged span of prestressed structures [124] given that 16
ACCEPTED MANUSCRIPT CFRP is a purely elastic-brittle material [146]. Wu and Victor [81] fabricated a hybrid composite and bonded its interface by CFRP and engineered cementitious composite (ECC). At strain capacity of 1.7%, the tensile strength of ECC at room temperature was resulted of about 4.9 MPa. Thus, the tensile strength of ECC was improved from 3.5 MPa to 4.9 MPa (by about 40%) in comparison with the mortar, meanwhile, the strain
T
capacity enhanced from 0.011% to 1.7% (by 153 times). The adhesive tensile strength of CFRP decreases
IP
with the adding of nanoclay at higher temperatures. For instance, the tensile strength of CFRP is reduced by
CR
13.9% after 744 h of exposure [155]. It is also reported that the decisive temperatures of CFRP tendons are 330 °C, and the residual 88.7% tensile strength is at 100 °C. In addition, the tensile strength shows a drop of
US
approximately 21% from 75 °C to 100 °C, although nearly no change is seen when the temperature elevates
AN
from 25 °C to 75 °C [157]. However, the basalt fibers have first-rate tensile strength and protraction at break [88, 89] (Table 18). Investigations invented and improved the tensile strength of basalt/polypropylene
M
composites (polypropylene-g-maleic anhydride (PP-g-MA) with chopped basalt fibers) [158], basalt/epoxy
ED
composites (tourmaline micro/nano particles (0.5–2 wt.%)) [159] via the vacuum aided resin transform molding method and basalt/vinyl ester composites (by modifying the basalt fiber surface via a silane
PT
precursor as a coupling agent [160]. These laminates of TM/basalt/epoxy display that the tensile strength
CE
upsurges by 16%, while the increase of 153% and 27% is found for the flexural modulus and tensile strength, respectively [166]. Wu et al. [161] revealed that GFRP increased the tensile strength of hybrid FRP
AC
composite by 36% compared with the use of BFRP, which is 2.56% greater than that of polyparaphenylenl benzobisoxazole (PBO) composites. Other experimental outcomes showed that the impregnation of carbon fibers with neat epoxy or nanocomposite system lead to substantial gains in terms of tensile strength by approximately 57% and 58% and ultimate strains by approximately 44% and 42% [162]. Studies on GFRP composites revealed that tensile and short beam strengths of vinyl-ester composites are slightly affected, whereas the tensile strength of polyester composites sharply drops by 80% [163]. However, using MBrace
17
ACCEPTED MANUSCRIPT saturant and Sikadur 30 epoxy reports a 40% reduction and 46% upsurge in the eventual tensile strength of CFRP compound samples, respectively [87]. Besides, the tensile strength of the GFRP polyester-based composites and GFRP epoxy-based composites reduce by nearly 19.71% and 22.8% with wheat husk fillers and rice husk fillers, respectively [87]. This result resolved that the tensile strength of GFRP in hybrid
T
strengthening is more dispersed than that attained with merely CFRP or GFRP strengthening [164]. 𝑊
ultimate tensile strength of FRP, N/mm2
fy =
yielding strength of steel, N/mm2
US
ffu =
(5)
CR
where
IP
𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ, 𝑓𝑓𝑢 = 𝐵𝐷 , kN, [86],
4.5
AN
Figure 4: Factored FRP tensile strength [26, 154, 165] Creep rupture
M
Understanding the limits of a fiber is a crucial feature when choosing the form of RC composite for structural
ED
elements to withstand long-term loading [167]. Cyclic and continuous loading on FRP in exceeding of its ability to endure those loads may inspire fatigue failure, long-term deflection, or creep rupture in the
PT
structural section [168]. In structural elements; the stresses in FRP bars recommended to be less than the
CE
creep-rupture stress range to eradicate the deflections motivated by creep [169]. ACI [62, 96] and other design codes [10, 12, 119, 120, 126] endorse a reduction factor to be applied for the FRP ultimate tensile
AC
strength to decrease fatigue and creep rupture failures (Table 11). The reduction factors prescribed in the ACI codes for carbon, aramid, and glass FRPs are revealed in the following section. A reduction factor for basalt FRP is also guided in the basis of the findings of the research led by Anil [31] and Wang and Wu [168]. Wu et al. [167] reported that CFRP tendon has the greatest and poorest creep rupture performances, exhibiting with its high and low cost, respectively. Ascione et al. [168-170] and Banibayat [171] reported that the creep test only sustains three hours of impact when the BFRP tendons are below a stress rate of less than 70% and
18
ACCEPTED MANUSCRIPT when creep rupture occurs on the tendons. The creep test can withstand 500 h when the stress scale decreases to 65%. Guohua et al. [166] tested carbon GFRP composite tendons and found that the creep rate is 19.05% when the level of stress is 0.8 ffu, which is the ultimate tensile strength. The rate of creep is lower than 10% when the stress scale is lesser than 0.7 fu. Thus, the rate of creep lessens with the reduction in the level of
T
stress.
IP
Table 11: Creep ruptures reduction factors [5, 26]
CR
The creep rupture reduction factors considerably affect the usable strength of the FRP system [26, 171]. Figure 5 displays the tensile strength of several FRPs increased by the suitable creep reduction factors
US
parallel with the limit of standard stress of 80% of the yielding strength of steel. Carbon FRPs have larger
AN
usable strength that equals a condensed volume of FRP for a specified application, which can balance the rise in material and manpower costs [36, 136].
M
Figure 5: Comparison of tensile strength with creep reduction factor [6, 23]
ED
Creep of FRP materials are typically categorised into three different zones, as depicted in the schematic in Figure 6 (b) [24, 168]. The primary creep zone may exist instantly after the first elastic strain, exhibiting that
PT
the creep strain quickly rises with time [168]. The secondary creep zone is mainly significant for analysis due
CE
to that the structure will continue serviceable in this zone [172, 173]. The tertiary zone accords with noticeable material damage in the RC structure. Meanwhile, the typical improvement of strains under
AC
continued loading with time, as revealed in Figure 6 (a). The figure displays two profiles that signify two closely equal load heights of 43% and 45% (a). The primary creep zone, instantly after the first elastic strain, expecting to speedily grow with time, does not occur for BFRP bars. The secondary creep zone of constant strain over a certain period obviously advanced, followed by 8%–10% spear of strain at closely 32%–34% of the whole time to failure. Another region marginally improved, but the tertiary zone for BFRP bars seemed
19
ACCEPTED MANUSCRIPT over a tremendously short time period, and failure is sudden beyond the tertiary zone. This figure illustrates that the creep history of BFRP bars is faintly dissimilar from the ideal creep history shown in Figure 6 (a). Figure 6: Comparison of strain versus time [24] 4.6
Modulus of elasticity
T
The design of the elastic modulus is known by ACI 440.1R as the mean modulus of a manufacturing lot [5,
IP
56, 69]. Compared with conventional materials such as metals, FRP materials mandate distinct amendment in
CR
codes and standards because of their low ductility and modulus [11]. The elastic modulus of the plate material is significantly imperative when the plate is not prestressed before bonding since only stiff plates
US
can release the stresses in the standing interior steel reinforcement [173]. However, due to differences in
AN
engineering properties, this elastic modulus particularly showed marked variances in the extent and magnitude of cracking, extent of deflection, and failure mode [101]. The lowest modulus of elastic is
M
attributed to GFRP and AFRP bars (Table 2), and the largest ones are exhibited by CFRP (Table 1) because
ED
they are sensitive to adhesive thickness of the test, epoxy matrices due to moisture absorption, fiber length, and the percentage of fibers [97, 103, 116, 117]. For example, SikaWrap 230C, a type of CFRP with nominal
PT
thickness equals to 0.131 mm and a modulus of elasticity equals to 234 GPa, equals 0.009 for either CFRP
CE
coupons due to the maximum elongation [175]. Likewise, the modulus and strength of cement mixture are predicted to improve by 39% and 56%, respectively, with the addition of carbon fibers treated by silane
AC
[176]. The glass/epoxy-based composite revealed a substantial improvement in the modulus and strength as the rates of strain are enlarged [177]. However, synthetic fibers, largely polymeric fibers, typically have small modulus of elasticity. Meanwhile, basalt-epoxy-based composites presented alike modulus to glass– epoxy-based composites [178]. The reduction in elastic modulus of FRP rebars indicates to larger cracks, increase in steel cables and higher localized impacts in comparison with those of steel RC beams, thus, this reduction can maintained by the addition of bagasse fiber with glass fiber to the mix design [7, 179-181]. In
20
ACCEPTED MANUSCRIPT addition, BFRP fibers have higher strength-to-weight ratio and modulus of elasticity than GFRP fibers [23, 180, 182]. Hawileh et al. [183] studied that the elastic modulus of hybrid GFRP composite is reduced by nearly 28% when subjected to various temperatures limited between 25 °C to 300 °C. The fatigue loading was found to reduce the bond strength between the high modulus of steel and CFRP by nearly 30% to 20%,
T
respectively [87]. The modulus increases by about 8%–21% nearly linearly as the rate of strain of CFRP and
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epoxy composites increases under dynamic loading condition [139]. Other experimental outcomes show that
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the impregnation of carbon fibers with neat epoxy or nano-composite system led to significant gains in terms of Young’s modulus, which slightly increased by 6% and 8%, respectively, using the new matrices [173].
US
Moreover, using GFRP fiber is 80 to 160 times higher than those of the matrices, such as polyvinyl chloride
5
AN
(PVC), polypropylene (PP), PE, phenolic, epoxy, and polyester resin [153]. Physical Properties
M
FRP composites kept growing at a remarkable rate because these materials are utilized with naturally
ED
circulating materials or engineered produced from more than one constituent substance with significantly several physical properties, such as density, rigidity, strength-to-weight ratio, and stiffness (Table 12), as
PT
detailed in the following subsections.
5.1
Density
CE
Table 12: Typical properties of different FRP materials [5]
AC
Fiber constituent with low density and high strength, such as carbon, basalt, boron and aramid, were purchased to please the high performance tasks of air travel and space investigation in the 1960s and 1970s [11]. Regardless of the benefits of natural fibers over ordinary materials such as low density and cost, they suffer from low processing temperature [152]. The bulk density of the material is not an important criterion because the density of all fiber composites considered is lower than that of steel [34, 85, 174] (Table 12 and Table 8). Cured composites with the highest modulus and density are the strongest, and thermal treating drones away the binder, causing in a frailer composite with lesser density [97]. However, CFRP and GFRP 21
ACCEPTED MANUSCRIPT are largely used for structural applications in aeronautics thanks to their high strength despite their low density [95]. CFRP has a relatively low density (1.6 g/cm³); thus, X-Ray radiation can easily get through, leading to comparatively high resolution and still providing acceptable contrast between delamination and CFRP [95]. On the contrary, sheet basalt plastic is generally produced with thickness (t) between 1.5 and 2.5
T
mm and density (ρ) between 1360 and 1380 kg/m3 (one-third that of steel) [42]. The density of warp yarn in
IP
basalt-epoxy composites is also greater than that in glass epoxy; thus, basalt-epoxy composites can reinforce
CR
structural concrete element and show even higher tensile strength [184, 185]. Besides, a synthetic thermoplastic fiber of PE terephthalate (PET) with high density PE (HDPE) is used to produce platform
US
mooring ropes [52]. The adhesive has high adhesive paste, recognized as Sikadure-30, with a average
AN
viscosity material and have density of 1.31 kg/L for a resin blend associated with FRP materials for strengthening concrete structures [162]. Another composite was exhibited high paste strength between the
M
HDPE matrix and the basalt fiber [55]. It is also reported that the use of 58 w/t% flax fiber reinforced
ED
polyethylene bio-composites to reinforce HDPE is linearly showed a low density PE [186]. Also, the use of short hemp fibers in HDPE can reduce the tensile modulus of all composites [187]. However, the density of
PT
any designed composite can be computed theoretically using Equation (6) [188].
CE
𝜌𝑐 = 𝑉𝑓 𝜌𝑓 + 𝑉𝑚 𝜌𝑚
[188]
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑏𝑒𝑟
AC
𝑉𝑓 = 𝑇𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒𝑙 𝑎𝑚𝑖𝑛𝑎𝑡𝑒 𝑉𝑚 = 𝑉𝑐 =
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑡𝑟𝑖𝑥 𝑇𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒𝑙 𝑎𝑚𝑖𝑛𝑎𝑡𝑒
𝜌𝑐
−
𝜌𝑒𝑥𝑝
𝜌𝑐
,
where -
ρm = density of matrix Vf = fiber volume fraction Vm = matrix volume fraction Vc = volume fraction of voids ρc = density of composite laminate 22
(6)
ACCEPTED MANUSCRIPT
5.2
-
ρf = density of fibers
-
ρexp = density of a composite laminate (ASTM-D792) [196]
Strength-to-weight ratio
FRP materials are mainly used to repair, overhaul and strengthen aging infrastructures affords a motivating alternative to conventional techniques because these materials have low density, which can provide high
IP
T
strength-to-weight ratio [21]. This characteristic provides important functional and economic wealth,
CR
extending from strength improvement and mass reduction to durability characteristics, apart from its importance in transportation and many structural applications [50, 190]. Also, FRP systems provide mobility
US
to specialists to stratify the strengthening technique on any curved, flat, or geometrically unbalanced surfaces due to their exceptional formability [113]. However, CFRP and GFRP have high strength-to-weight ratio,
AN
leading to three to seven times higher than steel and half that of aluminum; these materials are also 80%
M
lighter than steel. It is also reported that fiberglass is a robust, durable, and lightweight composite material [2,
ED
55]. Composites and fiberglass have the greatest strength-to-weight ratio presented for element production [11]. The increase in the ratio of fiber weight to an optimum value and the bond with resin fabric resulted in
PT
superior mechanical characteristics. However, the extra addition of fiber ratio adversely affects the relative
CE
properties [153]. It is found that the combination of sisal/carbon fiber hybrid composites with divergent fiber weight ratios through chemical resistance test (NaOH treatment) showed that hybrid composite does not
AC
resist the carbon tetra chloride [78]. In general, FRP composite has superior resistance property to chemical attack and chloride ion compared with steel bars [44, 56, 78, 153, 163, 182, 192-198]. Table 12 presents the weights of aramid, carbon, and glass fibers. Aramid has strength-to-weight ratio lower than that found in carbon and glass fibers [5]. Carbon and aramid fibers have high strength-to-weight ratio when examined in the alignment of the fibers direction. Meanwhile, glass has low strength-to-weight ratio and still moderately high, similar to carbon or aramid [2] (Table 13) [188]. By contrast, basalt fibers have greater strength-toweight ratio and elastic modulus than E-glass fibers [23, 180]. The strength-to-weight ratio and density of 23
ACCEPTED MANUSCRIPT FRP are clear features in transportation, handling, and insulation; these features reduce the weight of the concrete structures and any other relevant products. Table 13: Comparative chart of glass, aramid, and carbon fibers
5.3
Rigidity and stiffness
T
FRP composites are considered by specific strength and stiffness that surpasses that of the same metal
IP
structures [173]. Many researchers found that FRP composites have a significant applicable use for several
CR
civil engineering applications, and they generally develop to become the first material between the other alternatives for rehabilitation and retrofit of RC structures given their high stiffness-to-weight ratio. FRP
US
material is preferable to other traditional materials [113]. However, the concrete element stiffness or its yield
AN
load cannot easily be increased unless large cross sections of these materials are used to participate considerably to the element load prior the steel yielding mode [156]. The slope of the load-deflection curve
M
in all zones is governed by the stiffness and rigidity of the samples. Therefore, the increase in deflections
ED
indicates the retraction of stiffness given that damage is accrued with load cycling [21]. The joints of FRP lattices control bond stiffness and rigidity, thus affording a occasionally bonded strengthening system in
PT
cases where nominal connection occurs between the cross-over points [191]. The composite materials with
CE
high rigidity, great stiffness structural fibers, and frivolous, such as boron, carbon, and aramid, have significant mechanical characteristics and durability than the composites alone [11, 114]. A large FRP cross
AC
section may not be economical and may result in a brittle response of the element down to sudden debonding of the strengthening material from the concrete surface [121] (Table 14). The bond strength measures the efficiency of the grip among FRP bars and concrete, and the comparative slip between concrete and FRP is 0.125 mm at unrestricted end of the bar in a shear test on plain bars (Figure 7). Figure 7: Bar-pullout bond test apparatus [192] The pre-compression influence from reduction on the bar does not impact the cracked stiffness, and thus, have no great effect in the overall load-deflection behavior after cracking [193, 194]. For instance, the 24
ACCEPTED MANUSCRIPT toughness of the reference steel-reinforced beam is similar to that of the AFRP-reinforced beams before cracking. Yet, the toughness is numerous times greater than the corresponding reference values after the post-cracking range [101, 175]. It is also reported that the energy ductility of the CFRP-strengthened beam and GFRP is 2.6% and 33%, respectively, since the beam reinforcement that upsurges eventual load capacity
T
significantly increases stiffness and rigidity and decreases deflection [103]. In addition, the stiffness and
IP
strength of the GFRP epoxy-reinforced based composites are 18% higher than that of the BFRP epoxy-
CR
reinforced material [184]. On the contrary, the rigidity and stiffness of carbon fiber are twice that of aramid and five times that of glass fiber [27]. Besides, the stiffness and strength of the natural FRP composites are
US
mainly relied on fiber loading [153]. The composites made of PE fibers along with carbon fibers within an
AN
epoxy matrix resulted superior structural features of the hybrid-based composite relies largely on the alignment of the reinforcing fiber [98]. The strength and stiffness of CFRP cables are close to that of steel
M
[157, 181]. Moreover, the steel-FRC and the FRP grid are effective in increasing the toughness of the steel
ED
deck plate by 47%, 9.45%, and 63.16% with a 4 mm-thick and 60 mm-thick CFRP grid, respectively [188]. Furthermore, the FRP bars stiffness exhibited insignificant changes as a result of freeze-and-thaw (FT)
PT
exposure [23]. The decrease in stiffness is great when the amount of moisture collected by the FRP
CE
composite specimen is large [116, 117]. CFRP/GFPP shows a reduction of 42% in stiffness for the saturated condition when compared with the dry condition [196]. The mechanical properties of GFRP reinforcing bars,
AC
particularly the strength and the stiffness of the composites under elevated temperature, reduced substantially due to the stiffness and strength of the resin lessened hastily when the temperature surpassed its glass transformation temperature [87]. Table 14: Summary of previous studies on FRP material bonding with surface of RC elements
6
Durability Properties
To consider the material durability, existing design codes classify ecological reduction factors for any FRP property that can be used in the design. Therefore, durability properties such as corrosion and fatigue 25
ACCEPTED MANUSCRIPT resistances, brittleness, and moisture content as well as specific gravity, water absorption, and fraction of fibers are scientifically reviewed in the subsections. 6.1
Corrosion resistance
Reinforcements of FRP are initially utilized in RC structures that entail a better corrosion resistance [11].
T
Using FRP composite materials also improves the enactment, condensed drag, used and enriched durability
IP
and resistance of corrosion in RC structures. FRP structure also requires smaller work crews, lighter
CR
equipment, and lighter supporting structures during installation. These advantages translate into better engineered systems such as low stress applications that perform better, last long, cost-effective, and decrease
US
long-term maintenance costs when compared to steel material [50, 157]. Thus, these materials are widely
AN
used in engineering applications [197]. However, FRP is not recommended in dealing with concentrations of corrosion resistance (90 °C to 15% concentration) beyond 50% (Table 15) [50]. Corrosion resistance is
M
controlled by the laminate structure and the resins used. A wide variety of thermoset resins are available to
ED
satisfy a wide range of service requirements, such as polyester or vinyl ester resin, reinforcements (mat and fiberglass roving), and additives (UV inhibitors, pigments) [198, 199]. Yang [37] introduced brominated
PT
epoxy vinyl-ester resins and E-glass that delivered fire retardancy in addition to corrosion resistance as a key
CE
requirement for FRP equipment in many civil, structural, and industrial applications. The results shown that basalt fibers have higher corrosion resistance and greater chemical durability, and could be used in a
AC
chemical environment for long-term service due to these excellent features [182, 198]. In addition, glass fibers under stress are less sensitive to a corrosive environment [200]. However, corrosion resistant fiberglass panels can be manufactured in thicknesses of 3.18 mm to 31.75 mm [199]. Investigations depressed using carbonation fillers in FRP composites envisioned for acid facility since these fillers can increase infusion and diminish resistance to corrosion [182, 199]. The resin-rich veil plies and the mat layers in the corrosion barrier contain almost 90% and 70%–75wt% resins, respectively, creating an efficient boundaries for
26
ACCEPTED MANUSCRIPT corrosion and permeation. A gel coat with UV inhibitors can be applied to the exterior of the panel during manufacturing to improve weathering characteristics. Table 15: Comparison of resistance to corrosion by several composites in corroded environs [76]
6.2
Fatigue resistance
T
Structural element is designed to resist bending and straighten repetitively; thereby it ultimately fails
IP
attributable to fatigue. For instance, CFRP is marginally critical to fatigue and inclines to fail calamitously
CR
without prior signs of distress, thus presenting low fatigue strength values and acceptable damping characteristics compared with epoxy-based composites [117]. Pultruded rods based on carbon CFRP have
US
been progressively utilized in structural applications in several engineering areas down to their outstanding
AN
properties, for example lightweight and great fatigue resistance [208]. Meier [34, 85, 174] soaked CFRP composite in water to nearly 100% saturation. After 12 million cycles, the first steel reinforcement failed due
M
to fretting fatigue. Study exhibited the tensile fatigue performance of the hybrid-BFRP and hybrid-FRP based
ED
composites [161]. A 45% capacity drop is observed due to fatigue load. Similarly, AFRP is more resistant to fatigue. However, this material is susceptible to damage by ultraviolet radiation [23]. GFRP fibers have
PT
higher resistance to fatigue, relying on the setup and the type of glass, thereby indicating higher fatigue
CE
strength than CFRP fibers [188]. Equation (7) is used to reveal the best distribution fits of fibers in FRP
analysis [202].
AC
composites for the life of fatigue and the identical 95% confidence intervals based on previous research
f(x) =
1 0.5987√2𝜋𝑥
[−
(𝐿𝑛𝑥−4.202)2 0.7169
]
[202]
(7)
The use of BFRP based on hybridization for fabrication of a long-span cable-based bridge of a 70 mm-thick steel fiber, resulting that the impact of fiber hybridization improves in the whole fatigue performance [203, 204]. Another research reported no such reduction in bond strength for samples with great modulus CFRP sheets at the case when the ratio of load is larger than 0.55, along with the total 10 million of fatigue cycles
27
ACCEPTED MANUSCRIPT [87]. The isophthalic and bisphenol fumarate resins are classified the greatest fatigue resistance compared with the vinyl-ester resin [161, 205, 206]. The effective design of hybrid-CFRP laminates composite consisting of wave sheets and a tinny stainless steel plate was tested under tension loading [208]. The findings shown that the CFRP thickness and loading conditions are the main parameters influencing the
T
mode of failures and resistance to fatigue as well as the crack propagation control depends on the number of
IP
layer of side bonded plates and its thickness [205]. Zheng X.H. et al. [206] examined the fatigue behavior of
CR
the carbon fiber laminate (CFL) border exposed to humidity and temperature variations. The results exhibited that the accepted temperature and RH negatively influenced the bond performance of CFL and the fatigue
US
life reduced by a bigger stress level. However, the presence of resin can reduce the slip in pultruded FRP
AN
composites under fatigue loading [207]. Studies investigated the damage behavior of BFRP and FRPstrengthened RC composites tested to fatigue loading, and the results shown that the deflection of beam, the
M
strain can be computed with a high degree of accuracy and the cracking pattern and interfacial debonding are
ED
the key damage forms at the low fatigue stress levels for BFRP [209, 210]. Furthermore, the performance of glass fiber based-reinforced GF/epoxy structures entrenched with shape memory alloy (SMA) under cyclic
PT
loadings [211]. However, the findings displayed that the fatigue of SMA structures is twice greater than
6.3
Brittleness
CE
GF/epoxy composites due to the robustness of laminates of SMA composite.
AC
Concrete is the most brittle material. However, polymers exhibit brittleness and non-ductile properties. Thus, these materials cannot display a continuous rate of strain protraction under the tensile test [177]. The FRP brittleness is deliberated when forecasting the performance of retrofitted elements [212]. This brittleness cannot permit the stress redistribution in RC elements, and hence, the ordinary design philosophies are unacceptable for FRP RC members. The resin and reinforcement are the two main constituents of the FRP structures [155]. For instance, the thermosetting resin is generally very brittle [73, 163]. The properties are
28
ACCEPTED MANUSCRIPT massively enriched by the addition of reinforcing fiber such as glass fiber, carbon, or aramid [46, 48, 59]. The issues of brittleness and weakness in the CFRP composites can be maintained by a hybridization method, that is, by swapping the sheets of the carbon fibers with flexible fibers [213]. Reportedly, the carbon composite laminates can decrease its tensile strength with increasing modulus on account of the material
T
brittleness at higher modulus can cause great stress concentrations [50]. Thereby, the strain at rupture is
IP
highly lower and the carbon fibers exhibited greater brittleness than glass fibers [95]. Study reported that the
CR
upsurge in brittleness of CFRP laminates as the temperature is condensed [87]. UV photons result photooxidative reactions that adjust the vinyl-ester matrix and chemical composite of polymers, causing in
US
material weakening and contributing to extreme brittleness and likely micro-cracking [215]. Tetta et al. [216]
AN
reported that the configuration of two layers FRP-mortar system is more efficient by almost 92% than when of a single FRP were used in U-wrapped formulation, but the breakability of the FRP-mortar system still
Moisture content
ED
6.4
M
persisted. This problem can be maintained by incorporation with other appropriate fibers.
The FRP mechanical and electrical properties are greatly significant; these features largely rely on the
PT
attendance of moisture in the structures [217]. The moisture propagates parallel the GFRP rod, contributing
CE
to thermal failures [218]. The engrossed moisture approval content (Mt) can be computed in line with its weight after exposure (w
after exposure)
and before exposure (w
before exposure),
as presented in Equation (8).
AC
Moisture content is recognized as the function of the square root of time. The moisture saturation level (Mt) is roughly 0.77%, which is consistent with the gravimetric practical results attained in the previous research [219]. 𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (𝑚𝑡 ), % =
(𝑤𝑏𝑒𝑓𝑜𝑟𝑒 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 ) − (𝑤𝑎𝑓𝑡𝑒𝑟 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 ) (𝑤 𝑎𝑓𝑡𝑒𝑟 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 )
× 100 [117]
(8)
The carbon fiber volume fraction is 60% + 1% for all materials reported [116]. The diagrams show that the moisture absorption tends towards an equilibrium value, which depends on the material. The maximum 29
ACCEPTED MANUSCRIPT moisture content of carbon fibers (CF, polyetheretherketone, and PEEK,), epoxy (EP), and resin is approximately 1.6 wt. %, 2.5 wt. %, and 0.3 wt. %, respectively [123]. The specimens absorbed moisture, and the curves reached the saturation level fast when the water temperature is high [123]. For example, the elongation values of the 90% CFRP laminate composite decreased with increasing moisture content [116,
T
117]. Adams and Singh [220] suggested that the temperature of FRPs decrease to 32 °C and 40 °C when the
IP
moisture content is 2% and 1.6%, respectively. Investigations studied the acceptance level of BFRP
CR
composites engrossed in salt water and the impact of moisture captivation BFRP structures aged for 240 days [221, 222]. The results exhibited that the Young’s modulus and tensile strength of the composite reduced
US
marginally. Sergio et al. [21] utilized CFRP composites to improve the flexural capacity of RC beams
AN
subjected to moisture and environmental actions. After eight months of beam exposure, results showed no negative impacts on the interaction between the surface and the composite of the concrete. It is also reported
M
that when the GFRP composites exposed to a moist environment with comparative moisture of 80% at 50 °C;
ED
the result of moisture relies on the type of composite used. For example, the vinyl-ester-based composites and the modified polyester-based composites have the greatest and worst valuable moisture dispersion
PT
characteristics, respectively, and the epoxy-based materials had tolerable absorption rates. However, these
6.5
CE
composites progressively engross higher moisture in a non-Fickian manner [217]. Specific gravity, water absorption, and fraction of fibers
AC
FRP grating has almost two-thirds the specific gravity (SG) of aluminum and nearly one-fourth that of steel [76]. PE fiber has a SG of 0.97. Therefore, it is the merely reinforcing fiber found that is nimbler than water, whereas flame-retardant resins have high SG [199]. The high SG of FRP composites has the tendency to physically reduce permeation. Furthermore, the SG design criterion is one of the main parameters involved in the construction of a filament-wound layer thickness [68]. The SG is usually measured in line with Indian Standard IS: 10192-1982 [223]. The sample should be equipped for the test with accuracy rate of 40 ± 1 mm
30
ACCEPTED MANUSCRIPT with similar thickness to that of the laminate of 4 mm. The sample is first weighed in air by swinging it with the assistance of a filament restricted to the hook of the balance, and then the weight (Wair) at air state is recorded Equation (9). The weight (Wwet) is also noted by gauging the sample in fresh purified water. The sample attains the water temperature by immersing it in water for sufficient time. However, no air foams
𝑊𝑎𝑖𝑟
IP
[217]
𝑎𝑖𝑟 − 𝑊𝑤𝑒𝑡
(9)
CR
Specific gravity = 𝑊
T
should twig to the sample.
The absorption of water test was performed in line with Indian Standard IS: 1998–1962 [152]. A square test
US
sample of 38+0.5 −0.0 mm was established. The sample weight was initially recorded in air (Wwet), and then the
AN
sample was engrossed in purified water for a retro of 24 ±1 hour. The sample was wiped appropriately and weighed within two minutes after removal from water. This weight was then recorded as (Wair), as shown in
M
Equation (10), as follows:
𝑊𝑤𝑒𝑡 − 𝑊𝑎𝑖𝑟 𝑊𝑎𝑖𝑟
ED
Water absorption =
× 100 [159]
(10)
PT
The volume fraction of glass content (glass fibers) was found as stated by ASTM D2584-08 [217]. The test
CE
samples should be prepared. A dried ceramic pot is weighed (W1), and then weighed again together with the sample (W2). Then, the pot with the sample is sited in a soften heater at a temperature of less than 600 °C.
AC
When the carbonaceous material vanished, the crucible is cooled to a room temperature, and then weighed again with glass fibers left alone (W3). Finally, the glass content is computed using Equation (11), as follows: 𝑊 −𝑊
Ignition loss = 𝑊2 − 𝑊3 × 100 2
1
Glass content = 100 — Ignition Loss [226]
31
(11)
ACCEPTED MANUSCRIPT Several studies reported that the whole composites revealed the same volume fractions of fibers with the regular fractions limited between 53%–57% with moderately slight scatter, with the exclusion of the high seed-glass/vinyl ester structure. The more seed-glass/vinyl ester structure samples had almost 11% greater volume fraction of fibers. However, the rate of moisture content depends on the volume fraction and
T
diameters of fibers and hastily increases by increase the sum of FRP laminate layers (Table 16). The ECR
IP
(high seed)-glass/vinyl-ester composite specimens exhibited significantly higher volume fractions of fibers
CR
by approximately 66% than the other seven composite systems, between 53%–57%. The rates of moisture absorption and the high moisture contents found from those samples are highly lower than the corresponding
US
information for the other structure (Table 17).
AN
Table 16: Fiber volume fraction corresponding to the number of laminate layers of CFRP/GFRP [85] Table 17: Volume fractions of fibers (%) and rate of water absorption for different fibers [217]
Functional Properties
M
7
ED
The functional properties of the FRP composites are largely affected by the choice of fiber. The selected fibers are required in the design for composite applications in industries, civil and structural engineering,
PT
compromising with metallic structures. The functional properties of FRP composite materials include
CE
thermal and electrical conductivity, temperature effect, thermal expansion coefficient, and fire resistance, as reviewed in the subsections (Table 18).
7.1
AC
Table 18: Functional properties of different FRP materials
Electrical and thermal conductivity
The thermal conductivity (merely 1/900 that of aluminum and 1/187 that of carbon steel) is obviously benefit at the processes of packing, using, or transferring liquids at higher temperature. Loss of heat develops much smaller, and the hazard that hot apparatus foxes for labors is condensed [83, 206]. CFRP is an electrical insulation and conductive, while, GFRP and AFRP cannot connect electricity [224]. AFRP is utilized for guy lines in broadcast towers. Although it is not transmitting, it can engross water, and the water behaves 32
ACCEPTED MANUSCRIPT electricity. Therefore, in such composite materials, a waterproof layer is used to the AFRP. Galvanic corrosion is an anxiety when it is in interaction with metallic materials because carbon fibers do not conduct electricity [225]. The low thermal conductivity of inorganic polymer coatings also greatly contributes to fire resistance [72]. Studies reported that the use of carbon nano-fiber (CNT)-coated alumina fibers and sic fibers
T
increased the thermal conductivity by 200% and 50%, via the IR microscopy technique, respectively [233,
IP
227]. Liang et al. [228] developed CNT on carbon fibers using the 3-Omega technique and reported
CR
improvements of 33% in thickness. Forty five boaters with carbon fiber poles and masts have educated to isolate their aluminum hooks and networks to reduce corrosion. Veedu et al. [227] reported enhancements in
US
the electrical conductivity on sic fibers with developments of 440% and 360% in the through-thickness and
AN
in the in-plane directions, respectively. Besides, basalt fibers are used to replace asbestos as heat insulators because of their weak thermal conductivity but having a high protection against fire hazards [229, 230]. Temperature effect
M
7.2
ED
In general, the ecological degradation experiments on temperature impacts for FRP materials are restricted to a justly short period, generally not exceeded five years [219]. Though, the predicted service life of
PT
infrastructures, for instance bridges surpasses 50 years where the long-term performance of FRP structures
CE
cannot be effectively anticipated [219]. In terms of the temperature effects, the mechanical performance significantly decreases when the test temperature approaches the glass transformation temperature (Tg) of
AC
FRP composite materials [231]. In particular, the variation of the mechanical performance of CFRP, GFRP, BFRP, and AFRP reinforcing bars subjected to low temperatures ranges from 100 °C to -200 °C, 0 °C to −100 °C , 75 °C to 200 °C, and 25 °C to 180 °C, whereas the elevated temperatures range from 200 °C to 600 °C, 23 °C to 315 °C, 200 °C to 800 °C, and 180 °C to 482 °C [55-60, 63, 74-77, 80-90, 99-103, 163, 177, 190, 196, 264]. Karbhari et al. [232] exposed concrete cylinders reinforced with CFRP sheets to 200 freeze-thaw (FT) cycles limits from 22.5 °C and −20 °C, resulting in a more sudden rupture between the
33
ACCEPTED MANUSCRIPT concrete and reinforcement instead of control. Another research exposed 50 cylinders to 200 and 100 FT cycles between 20°C and −20 °C, respectively, at 70% comparative moistness. The results exhibited closely no impact on the values of connection strength, excluding for the bar of 19.1 mm diameter that revealed 15% reduction after 200 cycles. Davalos et al. [233] contacted up to 30 FT at 60 °C and strengthened with CFRP
T
and GFRP bars. The results showed that the bond strength condensed by around 18% after contact. It is also
IP
reported that the RC beams reinforced with five CFRP and two steel-prestressed bars tested under fatigue
CR
loading at −28 °C [234]. The main findings presented that at a low temperature resulted a slip between the concrete and the bars at load with up to 90% of the eventual strength capacity of the RC beams. It is also
US
found that the concrete cubes reinforced with FRP bars at different temperature (20, 70, 50, 100 °C)
AN
increased the micro-cracking in concrete and decreased the ultimate tensile load with up to 16% in comparison with the control as a result of the weakness of the FRP matrix under elevated temperature that
M
instigated the bond failure [235, 236]. Similarly, another experimental result exhibited a reduction in the
ED
bond strength of FRP bar between 20% and 40% at 100 °C, 75% at 150 °C, and 90% at 220 °C [237]. Besides, the cylinders fabricated with GFRP bars and exposed to 40 °C and 60 °C and left humid for 4
PT
months [238]. The findings showed 18% and 20% reduction in the bond strength when exposed to 40 °C and
CE
60 °C, respectively, attributable to the increase in the diameter of GFRP bars. Accordingly, the contact between temperature and moisture speeded the environmental deprivation on the FRP composites [219].
AC
Moreover, it indicated that glass fibers have thermal insulating properties that are 10 times lower than that of basalt fibers. Temperatures beyond zero may cause alterations in mechanical characteristics and generate further micro-cracks in FRP materials. Micro-cracking at low temperatures promotes the increase of water absorption at elevated temperatures while the transverse micro-crack increases in the expansion of freezing water found in voids and cracks [88, 124, 144-150]. Such freezing exposures contribute to a material deprivation through increasing brittleness and reducing debonding [183,144-151, 190, 196]. The E-
34
ACCEPTED MANUSCRIPT glass/vinyl ester and carbon-vinyl ester composites at low temperature thermal cycling can cause a significant reduction of the structural properties FRP [156, 157, 161, 183, 190, 196, 220, 236]. However, the possible contact to fire of the RC structures must also be deliberated during the stage of design [157, 161, 183, 236, 237]. RC generally provides a great resistance to fire at low cost. The use of flammable FRP
T
materials is, consequently, problem that recommended to be considered extremely [33, 88, 126]. All FRP
IP
composite materials are disposed to deprivation of mechanical characteristics at elevated temperatures [146,
7.3
CR
150, 183, 220, 134, 237]. Coefficient of thermal expansion
US
The coefficient of thermal expansion (CTE) may be detrimental when an FRP liner is used to a steel
AN
substrate and CTE value is twice that of steel/carbon [199]. FRP bars have higher CTE than concrete in the transverse direction (Table 19). In unidirectional structures, the CTE in the transverse direction is larger than
M
that in the longitudinal directions (Table 20) [126, 220]. Gaitonde [239] reported that the thermal expansion
ED
coefficients of carbon fiber strengthened polyether ether ketone (PEEK) exhibited that the worst errors for modulus values are -1.4% at -100 °C and -2.3% for the PEEK material. Similarly, the CTE of epoxy
PT
composites perpendicular to the fibers is approximately (60 ×10-6) per °C; hence, the shear modulus is
CE
underestimated by 2.9% at −100 °C. At high temperatures, the FRP bar entrenched in concrete expands, causing higher radial bursting stresses than tensile strength and thereby initiating cracks and negatively
AC
affecting the interaction between the concrete and the bar [59]. The crack mechanism arising from the limited tensile deformation capacity of concrete directly influences aesthetics, stress transfer, and structure durability [5, 6, 30, 36, 238]. Differences in surface configurations and mechanical properties exist when traditional steel reinforcement is compared with FRP reinforcement. Therefore, differences in the cracking behavior such as crack pattern, crack width, and crack spacing are expected [198, 119]. However, after an initial linear elastic behavior of RC ties and once the load causing first cracking is attained, cracks appear randomly due to
35
ACCEPTED MANUSCRIPT non-homogeneity of concrete [101, 240]. At cracked sections, concrete stress drops to zero and strain compatibility is lost [59, 238]. Stress is transferred from reinforcement to concrete and, at some distance, the compatibility condition is recovered due to the action of bond forces. The required strain compatibility recovering distance is short when the bond behavior between the two materials is excellent [125, 219, 233].
T
During this crack creation stage, fresh cracks seem as the load is enlarged, thereby reducing the average
IP
crack spacing [11, 21, 114, 174]. This behavior remains valid until the cracking stabilization load is attained
CR
[174]. From this point, no cracks can appear, and the average crack spacing remains constant. An increase in load is obtained during the opening of the existing cracks [238]. AFRP and CRFP are resistant to high
US
temperatures have no dissolving points and utilized for protecting fabrics and clothing near fire [27, 48, 51].
AN
Although GFRP is extremely resistant to elevated temperatures, it ultimately melts [84, 177]. Furthermore, CFRP and AFRP are used to make protecting welding blankets, firefighting and hands while using a knife
M
[26, 27, 45, 101, 241].
ED
Table 19: CTE of different FRP materials Table 20: Transverse and longitudinal CTE of prestressing steel tendons and CFRP tendons [124]
Fire resistance
PT
7.4
CE
Fire resistance is a major issue that results in the limited application of FRP materials. Thus, rejoinder under fire contact is a main anxiety as a consequence of the extensive applications of FRP strengthening in RC
AC
structures, particularly in buildings, where fire resistance is one of the RC design requirements [242]. FRP is utilized as a material system that must gratify the requirements of the local design code to prove its fire safety [243]. The FRPs fire resistance can be enriched by means of fire-resistant polymers [242, 244-246]. FRP materials must still exhibit sound behavior associated to high temperature, particularly predicted to the fire design given these considerable features, thereby leading to the inactive protection of buildings in case of fire [244]. Reportedly, the fiberglass bars with thermoset resins are generally utilized in corrosion resistant coats, owning a low rate of flame when fire is sustained by outdoor source [242-244]. Fire retardant 36
ACCEPTED MANUSCRIPT thermoset resins classically comprise either bromine or halogens molecules [244-247]. All matrix resins, for instance polyesters, epoxies, and vinyl esters, used for FRP composites are classified as non-fire resistant materials. It is reported that the FRP-strengthened RC beam can stand more than 3 h when expose to fire, exhibiting a low rate of fire resistance than that of an un-strengthened RC beam [242, 245]. Another study
T
revealed that when the FRP reinforcing system is secured with fire insulation of 25 mm diameter, the FRP-
IP
strengthened beam can stand more than 1 h 60 min compared with that of an un-strengthened beam [28].
CR
Qinghua et al. [126] reported the fire resistance of CFRP tendon with the matrix epoxy resin (transition temperature ranges from 90 °C to 110 °C). Therefore, the disintegration of epoxy resin at elevated
US
temperature leads to the reduction of at least 50% of the ultimate strength followed by the failure of the
AN
CFRP tendon when the temperature exceeded 250 °C. Basalt fibers do not produce toxic substances when subjected to fire, thereby overcoming a serious drawback of conventional fibers [23]. The BFRP bars with
M
different types of heat-resistant epoxy and vinyl resins can resist up to 100 ºC is because of the impregnation
ED
and adequate cohesion between the basalt fibers and the heat-resisting resins [244]. The mechanical characteristics of FRPs weaken with elevating temperature. The maximum temperature is the glass transition
PT
temperature Tg of the matrix of polymer, presenting typically in the array of 65 °C to 120 °C for matrices
CE
utilized in infrastructure applications [37, 54, 242]. The longitudinal properties are considerably influenced by low temperatures compared with the transverse properties due to the anisotropy of unidirectional FRP
AC
structures; therefore, the shear and transverse stiffness and strength reduce quickly above Tg [242, 243]. The deprivation of the mechanical properties of FRPs at elevated temperature is naturally ruled by the characteristics of the matrix polymer [242, 243]. Figure 8 displays the temperature-dependent strength of carbon, aramid and glass fibers in the basis of the data reviewed from previous studies [197]. This figure advocates that carbon fibers are moderately unresponsive to high temperatures, while aramid and glass fibers experience substantial weakening of strength at elevated temperature [124, 197, 246]. The relatively robust
37
ACCEPTED MANUSCRIPT fire resistance of any RC beams strengthened with glass FRP can possibly accredited to the clear concrete cover of 70 mm that was afforted for the FRP bars. Several literature reviews and theoretical studies have also been presented [156, 157, 161, 183, 190, 196, 220, 236, 242-247]. Table 21 presents the summary of imperative comparison and rankings for steel and FRP, particularly their practicality and durability
IP
RC members in current codes and standards are extremely limited.
T
properties. Furthermore, a review of the literature indicates that fire design guidelines for FRP-strengthened
CR
Figure 8: Modification in FRP bond strength with temperature [197] Table 21: Imperative comparison and rankings for steel and FRP [188] Serviceability of FRP
US
8
AN
Serviceability frequently governs the design of concrete elements that are internally strengthened with FRP reinforcements by reason of the hardened properties of FRP composite materials. Three main serviceability
M
criteria, namely, crack width, deflection, and fatigue, should be satisfied. These factors should be considered
ED
when the shear strengthening application is being designed. The reasons are given in the following subsections. Crack width
PT
8.1
CE
The cracking phenomenon in FRP composites mainly depends on the fiber type, fiber breaking, fiber surfaces, matrix debonding, matrix interface, moisture content, and thermal cycles, which all contribute to
AC
mechanical degradation [198, 212]. Corrosion crack propagation is also associated to the toughness of the resin, fiber, and matrix used to bind the FRP composites [198]. The toughness (Gc) of a composite is the amount the energy captivated per unit area of crack under Equation (12) [50]. If the crack is solely 𝑓
promulgated straight over the toughness of matrix (𝐺𝑐𝑚 ) and toughness of fibers (𝐺𝑐 ), then a simple rule-ofmixtures may be expected. However, if the length of the fibers is less than lc, then the system will not fracture.
38
ACCEPTED MANUSCRIPT 𝑓
𝐺𝑐 = 𝑓𝑓 𝐺𝑐 + 𝑓𝑚 𝐺𝑐𝑚
[50]
(12)
where m = matrix f = fiber lc = critical fiber length, mm ff = fraction volume of fibers fm = fraction volume of fibers = 1 − ff
T
-
IP
Crack width is computed based on the same perception for FRP rebar because this parameter is for steel-
CR
reinforced members. However, the parameter is modified by a bond quality coefficient (kb) [5, 6, 23, 36], as
US
expressed in Equation (15). The bond between FRP bars and concrete is generally lower than that of steel bars because of less prominent deformations and long-term effect on crack widths, particularly in GFRP
AN
composite structures. The design recommends that the crack width should be increased by the bond quality coefficient (kb) equal to 1.4, except when a FRP bar manufacturer can prove by testing that the bond with
M
concrete results in a decreased bond quality coefficient [5]. The maximum probable crack width (w) in mm is
PT
ED
given by Equation (13).
𝑓
𝑠
𝐶𝑟𝑎𝑐𝑘 𝑤𝑖𝑑𝑡ℎ, 𝑤 = 2 𝐸𝑓 𝛽𝑘𝑏 √𝑑𝑐2 + (2)2
-
β
(13)
= is the ratio of the distance from the neutral axis to the extreme tension fiber to the distance from the neutral axis to the centroid of the tensile reinforcement dc = the concrete cover thickness measured from the center of the closest reinforcing bar to the extreme tension fiber, mm s = longitudinal bars spacing, mm
AC
-
[96]
CE
where
𝑓
High aging temperatures that approach the glass transformation temperature of samples can increase the structural performance by depreciate the materials by prompting thermal cracks, which do not appear in the actual use of FRP structures [125, 219, 133]. Width of crack is usually smaller steel-RC beams than in in glass FRP-RC beams on account of the small modulus of elasticity of FRP bars [11, 114]. Reportedly, the 39
ACCEPTED MANUSCRIPT short-fiber press shaped flanges can be disposed to cracking and an inappropriate for tough services [199]. It is also reported that the largest crack width in CFRP-reinforced and AFRP beam tested under cycling load, is greater than the reference values for steel-reinforced beams due to the decrease of their strength at a postcracking variety and the crack width should not exceeding 0.051 mm [21, 101]. Meier [174] reinforced the
T
flexural beam by using CFRP epoxy structures with laminate of 2 mm thickness, therefore, the finding
IP
shown a reduction from 3.85 mm to 2.58 mm at nonlinear range. Salakawy and Benmokran [247] fabricated
CR
bridge deck slabs strengthened with FRP reinforcements. The results showed that the FRP reinforcement ratio increased from 100% to 200% and the crack spacing decreased by 4% to 10% and 44% to 49% for slabs
US
reinforced with carbon and glass FRP reinforcements, respectively. The measured crack widths for FRP-RC
AN
slabs and steel-RC structures are recorded within the permissible code as 0.5 mm and (0.4 and 0.3 mm for interior and exterior exposures), respectively (ACI 440) [5]. Deflection
M
8.2
ED
Serviceability problems, for instance deflections, mostly govern the design due to the low elastic modulus of FRP bars [51]. The deflection control of cracked RC elements relies on the effective moment of inertia of the
PT
section (Ie), as prescribed in most the design codes [247]. This technique is in the basis of the hypothesis that
CE
the moment–curvature profile of cracked FRP RC element residues elastic under the increment of applied load, with a rigidity of flexural (Ec Icr), and the tension hardening is insignificant [119]. Computation of the
AC
long-term deflection is in the line ACI 318 [96]. In this approach, the applied unfactored moment surpasses the cracking moment (Mcr), and the moment of inertia is condensed by the diminished tension stiffening in FRP-reinforced sections relative to that in steel-reinforced sections. The degree of tension stiffening in the FRP-reinforced sections decreased with the volume of reinforcing associated with the balanced reinforcing ratio [254]. For a one-way slab under flexural loading, the greatest deflection is provided in Equation (14).
40
ACCEPTED MANUSCRIPT 𝛿𝑚𝑎𝑥 =
24𝐸𝑐 𝐿𝑐𝑟
𝑎 3
𝑎
𝐿𝑔 3
[3 (𝐿 ) − 4 (𝐿 ) − 8𝜂 ( 𝐿 ) ] , and 𝜂 = (1 −
𝐿𝑐𝑟 𝐼𝑔
)
[119]
(14)
P = applied load, kN L = span of the slab, mm a = shear span Lg = distance, mm; support to point where Ma = Mcr in simply supported slabs Lcr = Cracking length of the section, mm
T
where -
𝑃𝐿3
IP
This section presents the observation of common inclines in the load–deflection profile thru the testing of the
CR
reinforced samples. In Figure 9 and Table 22, Sergio et al. [21] analyzed that the samples in zone A remain
US
uncracked, whereas zone B parallels to the cracked concrete and linear performance of the FRP laminates and reinforcement. The deflection is governed by the volume of FRP composites and reinforcement on the
AN
cross section [21]. However, zone C is measured by the stiffness and strength of the FRP structures and the straw- hardening strength of the bars. The deflection curve in region C is large for the strengthened element
M
because of the stiffness of CFRP composites. Furthermore, Olofin and Liu and Nor et al. [181, 248] found
ED
that when the tensile strength (3500–7000 MPa) and elastic modulus (230–650 GPa) of CFRP are higher than those of steel, then the deflection of structural members and the elongation at failure is improved in the
PT
range between 0.6% and 2.4%. For clarity, the restrictions in deflection of serviceability are forced on RC
CE
members to confirm their structural reliability under service load conditions. Under the same condition, RC members strengthened with FRP reinforcements are resulted larger deformations than of those of steel-
AC
reinforced member attributable to the low elastic modulus of the FRP rebars [249-252]. Figure 9: Qualitative depiction of the load–deflection profile of the RC members [21] Table 22: Typical behavior of specimens strengthened with different material properties [21]
8.3
Fatigue
FRP is an excellent-performing material with high strength fatigue resistance, which is extensively and widely used in engineering applications [197]. The material FRP composite elements experience progressive deformation subjected to a constant load over a retro in a process known as creep, followed by fatigue failure 41
ACCEPTED MANUSCRIPT mode [24]. The knowledge obtained from investigation regarding the CFRP steel composite system may not be appropriate to the CFRP concrete composite as a result of the distinguished variation between the fatigue failure mode and debonding mechanisms for connections and steel members [94]. Studies on the influence of fatigue on FRP-RC elements specify that the fatigue failure mode restricts the volume of stress permitted on
T
FRP rebar [30, 87, 135, 138-143, 150] (Table 23). Another study examined the effect of fatigue loading at
IP
several load ratios that array from 0.15 to 0.55 on the interaction between CFRP and steel sheets with
CR
nominal modulus (240 GPa–640 GPa). The findings exhibited that the decrease in bond strength for the nominal modulus CFRP sheet is almost 20% to 30% (Figure 10) [138, 222]. The impact of fatigue loading on
US
joint rigidity decreases less than 10% of the rigidity as a result of the accrued damage resulted by the
AN
increasing of fatigue loads.
Figure 10: Fatigue damage zone [138]
M
Investigations of BFRP and GFRP-RC composites under fatigue loads, thus, the results showed 36% increase
ED
in modulus/tensile strength of BFRP compared with the GFRP [161, 203]. The BFRP rupture strain is also found 2.56% greater than that of PBO composites. It is also reported that the AFRP composites exhibited
PT
higher resistance to fatigue, and isophthalic and bisphenol fumarate resins are known the lowest fatigue
CE
resistance compared to with those of the vinyl-ester resin [27, 199]. Moreover, samples made from Araldite 420 and Sikadur 30 showed no reduction in bond strength at temperatures less than −40 °C [30, 87, 130].
AC
Meanwhile, the bond strength of MBrace saturant is reported deceasing by 40% when the temperatures released from 20 °C to −40 °C Table 23: Summary of tests of beams under fatigue loading
9
Design of FRP
Design loads on a RC structure are determined using the same methods, whether reinforced using steel or high-strength FRP reinforcing bars [249]. Steel and FRP- RC members are analyzed using similar methods to satisfy the strength and serviceability criteria, namely, factored moment, factored shear, crack width, and 42
ACCEPTED MANUSCRIPT long-term deflection [5, 12, 250]. FRP is used mainly to strengthen existing RC structures, and this process involves multiple assessments and requires a good sympathetic of the current RC structural circumstances along with the use materials to overhaul the building before to FRP installation [23, 251-254]. The applicability of FRP for a reinforcing project could be measured by sympathetic FRP, its features, and more
T
significantly, its restrictions. For instance, GFRP is the toughest and greatest resistant to twisting forces when
IP
polymer fibers are equivalent to the applied force and vice versa [8]. Figure 11 illustrates the analysis and
CR
design of FRP-strengthened RC members, for example beam, deck-slab, and plate-girder-bridge [255]. In FRP design, the moment–curvature profile of a RC section reinforced with FRP composites is computed by
US
dividing the section into horizontal portions and assigning precise material properties to each portion [21].
AN
The techniques utilized to measure the strain and stress in each portion and compute the identical curvature and moment are labeled in Figure 11. Interior force equilibrium is developed for sequences of extreme
M
concrete compressive strains. Curvature and moment can be computed at each point identical to the extreme
ED
strain of concrete given to the extreme fiber under compression, as revealed in Equation (1). The method utilized to calculate curvature and moment of a reinforced section is noticeable in the following steps [12,
PT
21]: (1) set the supreme compression strain in the concrete (εc max) to a value between the greatest usable
CE
concrete strain (εcu) and zero, (2) approximate the primal neutral axis location (h/2), (3) compute the strain shape based on the maximum compression fiber strain and the location of the neutral axis and then calculate
AC
the identical interior stress elements by means of the material prototypes approved, (4) inspect equilibrium in the horizontal direction via the interior stresses, (5) regulate the depth of neutral axis (c) until force equilibrium is attained, (6) measure the interior curvature and moment; and (7) upsurge (εc
max)
for the
purpose of accuracy. However, the interior stress in each portion is computed at mid-thickness and presumed constant throughout the portion. The distribution of stress is estimated by sequences of squares with a depth equivalent to the
43
ACCEPTED MANUSCRIPT potion thickness and height identical to the stress computed from the stress–strain profile [Equation (2)]. The force envelopment from each portion is calculated with the cross section width at the portion mid-plane (bi) and the portion thickness (t-portion). This envelopment is showed for non-rectangular cross sections in Figure 11. The interior force elements are timed by their distance to the neutral axis zi to compute for the
T
interior moment. Meanwhile, the curvature is computed by dividing the extreme compressive strain by the
IP
neutral axis depth. In the last step, the model primarily estimates that CFRP composites residue connected to
CR
the surface of concrete [23]. Under this assumption, the supreme stress that possibly established in CFRP is identical to the stress of rupture fpu. The envelopment of CFRP composite to the complete tensile force
𝜀𝑐 𝑚𝑎𝑥 𝑐
-
(17)
PT
h = overall depth of beam; ds = effective depth; b = the beam width; c = the neutral axis depth εcu = concrete crushing stain; Acs = effective area of compression steel under compression fc = concrete compressive strength; fsc = stress in compressive steel; fy = steel yield strength As = effective area of steel under tension; Efrp =tensile elasticity modulus of CFRP bar; Internal moment, kip-in; Ø = Curvature, 1/mm; Fi = Internal tensile, kip zi = distance from internal force component to neutral axis, Fi, mm. εc max = strain at the extreme
CE
-
[21],
(16)
ED
where
AN
Ø=
[21]
M
𝑀 = 𝛴𝑖 𝐹𝑖 𝑍𝑖
US
develops zero when stress of rupture is reached [21].
AC
compression fiber; c = neutral axis position, mm; Afrp= effective area of CFRP rebars; εfrp = strain in CFRP bar; α1 and β1 = compressive stress block parameters in compression zone of concrete Figure 11: Internal stresses and forces in strengthened section [12, 21, 249, 255] Furthermore, Table 24 shows the guidelines strategies of the FRP as proposed by various codes for strengthening RC structures at different situations such as flexure, shear, and column confinement. The table is basically briefly presented the related parameters controlled the design strategy of the FRP such the concrete strength, the quality of the concrete surface, the glue line thickness, and the stiffness, the effective 44
ACCEPTED MANUSCRIPT bond length and width of the FRP sheets. It can also be used as a sample guideline for engineers, researchers, and material producers and ease them to develop their understanding of FRP strengthening technology. Table 24: Guidelines strategies of the FRP sheets as proposed by various standards for strengthening RC structures
T
10 Strengthening techniques
IP
Externally bonded reinforcement (EBR) and Near-surface mounted (NSM) strengthening approaches are the
CR
most common and lately utilized encouraging strengthening approaches for RC structures [223, 256-258]. The NSM approach is performed through the following steps: (1) splits of 4-5 mm width and 12-15 mm
US
depth are cut by a diamond knife cutter over the surfaces of concrete of the members to be reinforced, (2) the
AN
splits are gutted by compressed air, (3) CFRP layers are gutted by acetone, (4) epoxy resin is formed in line with provider commendations, (5) the splits are filled with the epoxy resin, (6) epoxy resin is used on the
M
laminates surface, and (7) laminates are presented into the splits and the extra epoxy resin is removed. To use
ED
wet lay-up slices of a CFRP laminate by the EBR approach, research achieved the subsequent processes [223, 259-263]: (1) On the regions of the surfaces of beam where the slices of the laminate should be stuck,
PT
emery was used to eliminate the superficial paste; (2) the remains were apart by compressed air, (3) a layer of
CE
primer was used to normalize the surface of concrete and improve the strength capacity of the concrete
266].
AC
substrate. Lastly, (4) the slices of the laminate were pasted using epoxy resin onto the beam surfaces [264-
11 The bond characteristics of FRP Bond FRP plate or sheet for strengthening and rehabilitation concrete members is became more popular in civil and structural engineering fields due to the remarkable characteristics, such as anti-corrosion, and light weight, compared with the traditional strengthening techniques [16-27, 119, 120, 304, 305]. However, the bonding with the concrete is essential to the exterior reinforcement technique for either repairing or
45
ACCEPTED MANUSCRIPT strengthening reinforced concrete (RC) elements. In most of strengthening cases, the bond of interface is critical in transforming stresses from the existing RC structures to the externally bonded FRP composites [223, 256, 257]. In structural RC system, sheet bonding has a superior potential for construction flaws, due to the mixing of resins and the curing of FRP composites. The Japan Concrete Institute (JCI), Canada, and the
T
Great Britain, recognized a technical committee on retrofitting technology that concerned on the bond
IP
properties between the existing RC structures and retrofitting materials for both glue bonding and
CR
overlapping retrofitting technologies [217]. The current applications of the sheet bonding system include shear strengthening, flexural strengthening, and column wrapping [113, 121, 219, 271, 293-295, 306, 308,
US
311-315]. In the case of column wrapping, the failure of bond interface is not a main concern compared to
AN
the fracture of FRP sheets is the major mode of failure [306, 311, 312, 315]. Thus, it has been recently recommended that FRP materials should be used with a high fracture strain capacity [3-5]. However, for RC
M
members strengthened with FRP sheets for shear and flexure; debonding of the FRP from concrete is
ED
governed the overall structural failures [121, 271]. In the flexure strengthening cases, the debonding of FRP sheets from the concrete substrates are having more complex corresponded with several failure mechanisms
PT
unlike with the shear strengthening cases that are found in line with results obtained in pull-out shear bond
CE
tests and; respectively [273, 277, 278]. To evade the mid-span debonding failure, the most design guidelines endorse limits on the strains in the FRP sheets because the mid-span debonding is always occurred due to the
AC
interaction of the concrete cover, the FRP sheets and the steel reinforcement. Through the results of the pullout tests, it is reported that the concrete strength, the quality of the concrete surface, the glue line thickness, and the stiffness, the bond length and width of the FRP sheets are having a major impact on the bound strength of FRP composite system despite the diversity of reinforcing materials used (i.e., carbon fiber, steel, and glass fiber composites), the different concrete compressive strengths [217]. Several studies reported that the stiffness of the FRP sheets (elastic modulus × thickness) can lead to influence the bond strength [39-40,
46
ACCEPTED MANUSCRIPT 103, 151, 181]. For instance, it has been obtained that bond layers with lower elastic moduli, but good stiffness, could contribute to greater interface bond strengths [97, 116, 117]. It is also found that the sheet width does not affect the moderate bond strength of interfaces when the width of sheets limited between 50 mm to 200 mm. It is reported that the bond strength increases as the sheet bond length is increased.
T
Nevertheless, the effective bond lengths are varying considerably between 45 mm and 275 mm [217].
IP
Moreover, the design guidelines of FRP for strengthening RC members with respect to local bond
CR
characteristics and the role of the interface bond characteristics on RC member behavior will be informative to engineers, researchers, and material producers and motivate them to advance their understanding of FRP
US
retrofitting and strengthening technology. Moreover, a summary of the bond strength equations as pre
AN
various design standards and the commendations afforded by several scientists are given in Table 25. Table 25: Bond provisions as proposed by various standards for strengthening RC structures
M
12 Applications of FRP
ED
Large-scale research projects undertaken in 1993 in the USA and Canada involving CFRP/GFRP composites confirmed the decision to use such materials in construction (Table 26) [256-267]. The USA is not only the
PT
largest producer and user of FRP composites but also leads the world’s composite technology development
CE
and implementation. Nearly half of the world claim for FRPs exist in the US and account for 4.2 billion in 2002 (Table 26 and Figure 12). The US composite industry is expanding despite the overall slow-down in the
AC
US economy in the past couple of years, and the FRP manufacturing is predicted to improve at yearly rate of 4% to 5% over the next five years (Figure 12) [267]. The US has higher than 13,000 services that process composites, hiring 236,000 persons, and backing over 524 billion to the state’s economy (Figure 12). Reportedly, about half of the global demand for FRPs resides in the United States, accounting for 4.2 billion pounds in year 2002, but the main market share comprises almost 21% in construction, 32% in transportation, 12% in corrosion-resistant application, 10% in marine businesses, 10% in electronic
47
ACCEPTED MANUSCRIPT industries, and the residual 0.6% is utilized in aerospace and aircraft industries, as indicated by SPI composite association (Figure 12) [268]. In civil engineering sector, the advanced application of composite materials has grown gradually because of economic constraints, complex technique involves at substituting the advanced composite systems instead of conventional structural systems to [16, 32, 33, 38]. The
T
developing field of regenerated engineering, articulating the character of FRP structures in civil engineering,
IP
is separated into 1) rehabilitation, comprising the applications in the direction of overhaul RC structures and
CR
(2) new building with the entire FRP resolutions or novel composite FRP/concrete composites [269]. The operational efficiency of FRPs in the reintegration of existing RC systems is also proved with large-scale
US
structural investigation to retrofit and strengthen unreinforced and reinforced brickwork walls for seismic
AN
loads [19, 148, 169]. However, FRP composites have been efficaciously utilized for the seismic upgrade of RC structures, including their interior and exterior structural elements due to its high resistance to corrosion
M
[13, 17, 18, 122, 270]. These composites comprise justifying brittle failure modes, such as shear failure of
ED
columns/beams; withstand buckling of the longitudinal steel bars of confined column, shear failure of unconfined column–beam joints, and lap joint failure [106, 271]. These FRP systems upsurge the energy
PT
dissipation capacities and global displacement of the RC structure and increase its global performance [189].
CE
FRP composites are typically used for external strengthening and internal bars (dowels, rebar, and posttensioning tendons) [20, 63, 85, 103, 108, 109, 113-115, 134]. Externally bonded or close-surface-mounted
AC
FRPs are commonly used for the structural rehabilitating and overhaul of RC, timber, masonry, and steel structures [107, 111, 112, 114, 120, 256, 263]. FRPs utilized as internal reinforcement are included in roads, bridges, slopes, tunnels, and marine environments [15, 19, 126, 201, 229]. Internal bar with FRPs grasps a specific benefit in channel diaphragm walls, where steel bar damages the surface of a channel boring machine. FRP bars are utilized in hospitals as medical scanning device, such as magnetic resonance imaging, is applied in Maglev railway bonds and constructions and based on a large electric motors since such bars are
48
ACCEPTED MANUSCRIPT charismatically transparent [2, 32, 57, 81, 153, 154, 185, 238]. Furthermore, structural repair with exteriorly bonded FRP bar, particularly with high strength carbon FRP, has been proved by codes for seismic upgrades of RC structures for many years [10, 12, 19, 23, 36, 119]. The eccentric and axial loading of columns could be improved by wrapping columns with FRP bars [115, 121, 197, 268, 269]. Besides, FRP composite system
T
afford a useful tool to strengthen, repair and retrofit RC structures and are suitable for shear strengthening,
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flexural strengthening, confinement of column, and improvement of ductility, as all are reviewed in
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following subsections.
Figure 12: Current markets and applications of FRP materials
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Table 26: FRP merits and suitability of applications [195]
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For a practical case study on the utilization of FRP in practice, Kaitbay fence [267] is deemed one of the ancient places in Cairo constructed from stones and utilized as small shops. The lintels utilized in the front
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entrances of the shops were made of stones with shear keys and without mortar. Weakening of the joints
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between the stones and excessive deflection of the lintels was reported and as a result one of the stones fell off. However, CFRP laminates were utilized as tension bars at the bottom surface of the lintels. Holes were
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penetrated at the two ends of the lintel. Therefore, the CFRP laminates were strained by means of a rotating
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device at the penetrated holes, while connected to the bottom face of the lintel and the area of CFRP laminates was formed to preserve the self-weight of the stone lintels. Another example is the bridge deck
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slab [214], the system was required strengthening to prevent cathodic corrosion protection (Figure 13). Smart-deck slab (carbon textile-reinforced mortar, CTRM) consists of two layers of an epoxy-resin permeated carbon grid in combination with a high-performance mortar and textile material for the strengthening. CTRM layer was installed between the surface of the road and the reinforced concrete bridge deck slab surface (Figure 13 a). It was also mounted in segments to gain known sectors in the longitudinal direction in order to leak in the surface of the road and to prevent the damage of the CTRM layer (Figure 13 b). However, the tendency of the damage depends on the traffic volume. The two CTRM layers were made 49
ACCEPTED MANUSCRIPT with carbon reinforcement of 35 mm thick, installed at a distance of 15 mm, and fitted with electrical attachments for the monitoring. The system was also strengthened with a carbon grid with a 38 mm mesh opening as well as epoxy-resin incorporated with carbon nanotubes, aiming to increase the electrical conductivity. However, on the basis of the test, the fatigue and ultimate capacity of the strengthened bridge
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deck slabs and beams were increased, due to the use of the CTRM layers that offered an advanced method
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applied for strengthening evaluates by integrating the benefits of light, glued CFRP strips and the better bond
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features of an additional concrete layer.
Figure 13: Smart-deck Bridge; and (b) the location of sawn segment in a supporter slab [214]
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12.1 Flexural strengthening
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For flexural strengthening, FRP bar products, for instance plates, tow sheets, and rebars, are attached to the tension side of a masonry, concrete, and timber substrate with epoxy resin/ fibers parallel to the principal
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stress alignment [11, 103]. Consequently, numerous researchers studied the use of diverse methods to
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eliminate debonding as a mode of failure due to the methods explained in Figure 14. The use of CFRP composites in the type of strips can provide a cost-effective method for the flexural strengthening of deficient
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RC slabs [270]. In flexural strengthening, these methods principally used 0° FRP fibers and increased the
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load bearing strength of slabs up to 40% [21, 121]. Besides, beams strengthened with FRP showed that the flexural strength increases by 36% to 57%, the flexural stiffness upsurges in the range of 45 to 53%, and the
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flexural ductility and rigidity reduce as a result of the elastic behavior of FRP composite to tensile rupture and the damaged degree [103, 280]. The RC beams strengthened with CFRP sheet is identified to be liable to the premature, fragile, and enormously adverse failure [277, 278], since the process avoids the entire application of the strength characteristics to the tensile of polymer. Some beams strengthened with CFRP sheet can fail by localized debonding of strengthening from its fastening region or regions with extreme concentration of shearing and/or flexural cracks or by premature forms of failure [279]. Lately, the attention
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ACCEPTED MANUSCRIPT on the RC beams strengthening via exteriorly bonded FRP composites was largely increased; this approach improves the shear capabilities of RC beams [280-281]. Meanwhile, shear failure was reported to occur due to the influences of the ratio of longitudinal tensile reinforcement [6], effective span to depth ratio [281-283290], and the volume, orientation, spacing of CFRP strips [283]. Although considerable investigations have
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been directed on flexural strengthening of RC beams by using FRP materials, these studies focused on the
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effect of the size and depth of the beam and thickness of concrete cover, the FRP thickness, width, and type
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on the modes of failure [293-295]. However, it is reported that the beam size has no such effect on the ductility and rigidity of the RC beams strengthened with CFRP sheets significantly and the incorporation of
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GFRP and CFRP is improved by the ductility and rigidity through adjusting the modulus ratio [171, 296].
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Meanwhile, the CFRP tensile strength is less circulated in hybrid strengthening than in GFRP [103]. Attari et al. [103] revealed the use of twin-layer CFRP–GFRP composite materials to RC beams increased the
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strength capacity by 114% compared with the control beam, but increases by 84% and 72% compared with
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the reference beam as reported by [91] Mehmet and Zarringol [63], respectively. Similarly, the upsurge on flexural strength of RC beams was reported by 38.86% for three CFRP layers, 46.6% for two layers, and
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15.5% for one layer compared to the control beam [84]. But, the increase on flexural strength of the RC
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beams strengthened with GFRP sheets was by 45% and by 27% when the RC beams strengthened with BFRP sheets relying on the number of layers used and the reduction of displacement is 53.6% [21, 70].
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However, the reduction on the flexural strength capacity can be improved by the addition of carbon fibers, PE fibers, fiber orientation and epoxy resins [98]. Figure 14: Flexural strengthening techniques (e.g., CFRP composites) [26, 121, 167] 12.2 Shear strengthening For shear strengthening, the design procedures of RC structures with exteriorly bounded FRP are found in quite a lot of documents [5, 12, 24, 119, 120]. For shear strengthening, FRP bars are attached to the external
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ACCEPTED MANUSCRIPT of beams in a vertical U-shape formation as an exterior stirrup [113, 121, 219, 271]. The walls shear strengthening, for example under-RC, walls, and unreinforced masonry walls, could be achieved by warping FRPs to both or one sides on the wall in a vertical, horizontal, or x pattern (45°) [216, 279]. Shear strengthening is completed as very tinny edges with merely two or one sheets that are 0.5–1.0 mm thick and
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attains substantial seismic enhancements, particularly for a response of in-plane shear wall [297]. Existing
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epoxy resins are robust; hence, surface failures commonly happen in the concrete, in particular, at a weak
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joint in RC member that requires being shear strengthened [269]. The EBR method is applied as continuous jacketing or strips. Three main formations of FRP strengthening, namely, U-wrapping, complete wrapping
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and side bonding (Figure 15) [273]. The complete wrapping of structural RC elements is recognized as the
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most effective method for FRP shear strengthening due to its feasibility at the presence of some geometric restrictions [113]. Some studies were used different methods on the strengthening of RC beams in shear
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under, for instance exterior prestressed, bonded steel plate and fiber materials [282–291, 298-300]. Externally
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applied FRP, comprising aramid, carbon, and glass fibers, have been widely used for shear and flexural strengthening of RC beams/ columns [283-300]. Reportedly, the use of CFRP for shear strengthening of RC
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beams showed an increase in the shear strength by 19%-122% attributed to the orientation of the FRP at 45°
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and CFRP sheet, compared to the control beams [301, 302]. The shear strength of FRP-strengthened beams is generally computed by the addition an individual elements of shear resistance from the concrete, FRP, and
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steel stirrups. It is reported that the use of U-wrap CFRP shear strengthening system in RC beam is increased the shear capacity by 50% for one CFRP layer [303] and 92% for two CFRP layers [268], accredited to the shear-span-to-depth ratio that limited to equal to 3 or greater than 2. For example, RC beams of ratio of 1.5; CFRP shear strengthening showed no such increasing in the shear strength. Another study examined bridge desk slabs strengthened with CFRP bars with ratios higher than the balanced reinforcement ratio, the shear strength capacity was increased by 81% to 111% compared to the control slabs [247]. But the study on the
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ACCEPTED MANUSCRIPT influence of bucky paper interleaves formed from carbon nanofibers on the interlaminar hardened characteristics of CFRP exhibited 31% and 104% enhancement in interlaminar shear strength [123]. Liu et al. [222] studied the allowable level of BFRP composites absorbed in salt water for 240 days. The finding displays no reduction in shear strength of the BFRP composites even after 199 FT. However, a unexpected
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12.3 Column confinement and ductility improvement
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Figure 15: Shear strengthening [11]
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reduction in shear strength happened after being immersed in hot salt water at 40 °C.
FRP composites are a progressively suitable substitutive to steel reinforcement for RC structures, comprising
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cast in-situ and pre- and post-tensioned bridges, columns, beams, precast concrete pipes, and other elements
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[18]. The FRP and steel reinforcement’s performance, comprising resistance to corrosion, is illuminated on Table 15. Column structures also benefit from FRP reinforcement [18, 84, 197, 255, 274, 275] (Table 27).
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The use of structures as original reinforcement to strengthen other structures is specified increasingly by
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structural design engineers in private and public construction industries [14, 38, 91]. Presently, column wrapping by FRP is a mutual solution in seismic retrofitting. This method, related with other conventional
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jacketing choices, provides several advantages, for instance reversibility, ease of application, and great
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corrosion resistance. Besides, this tactic is approved in the retrofit of RC elements with numerous purposes, for example providing ductility, preventing bar buckling, increasing shear capacity, and enhancing hollow
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elements [257, 268]. The efficiency of FRP wrapping in lap splice areas for rectangular and circular cross sections has been verified in numerous studies [110, 257, 268, 269]. Moreover, many authors afforded analytical clarifications concerning the mechanisms involved the improvement in the bond between the concrete and the lap-spliced bars attributable to wrapping, mainly to FRP wrapping [256, 269] and provided further confinement to the concrete by using FRP [1-15]. The main advantage of FRP is its orthotropic performance that restrains the bound between the concrete and jacket in the axial direction; this performance
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ACCEPTED MANUSCRIPT contributes to the initially activation of its confinement [16-27, 304, 305]. Investigations revealed that the ductility and strength of columns are enhanced by allocating the longitudinal bars around the core perimeter and reserving these bars with laterals, for instance ties and the smaller volumetric ratio of ties decreases the concrete core confinement and [306, 307]. The larger volumetric ratio of tie flaws in concrete steadiness
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generates a weak zone between the concrete cover and the core [308]. Saatcioglu and Grim et al. [309] study
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the column strengthened with welded reinforcement grids; the results showed a shortage of confinement
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caused by ties and recommended to be improved by expanded welded wire mesh, metal mesh, ferrocement and FRP to restrain the concrete core [310]. Thus, ferrocement is widely utilized in the repair, overhaul and
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rehabilitation of current concrete columns [306, 308, 311-315]. Ho et al. [312] strengthened RC columns
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with upgraded ferrocement jackets, including wire mesh, and rendering material. However, few studies considered hollow RC with CFRP confinement compared with any section of solid column [269]. CFRP
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confinement has been verified to improve the concrete ductility and strength and delays the buckling of
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reinforcing bars and concrete cover spalling [268]. The uniform distribution of bars in mortar enhances several engineering properties, for instance durability, ductility, in-plane strength, and the crack resistance.
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The application of CFRP confinement does not increase the ductility of the hollow column but also avoids
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any slippage caused by insufficient overlapping. Kus and Hadi [274] compared the confinement of circular hollow RC column with CFRP sheets. The results found significant findings where reinforced columns
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without CFRP confinement show brittle failure mechanism. By contrast, a more sustained, higher ultimate load and a larger axial deflection are observed in CFRD. Another study led by Lignola [276] showed the initial point of buckling in compressive reinforcement with 15% confinement of CFRP strength compared with the column without confinement. Yeh and Mo [277] reported that the ductility factors obtained from their experiment ranged from 3.3 to 5.5, which is sufficient to sustain under seismic load. Pavese [13] discovered that CFRP strengthening permitted the completion of 4.8% drift for concrete that is crushed and
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ACCEPTED MANUSCRIPT bars that are buckled at the column base. This finding was also proven by another experimental study regarding the seismic performance of CFRP confinement that efficiently enhanced the strength capacity of hollow RC columns [270, 278]. Table 27 shows the extensive summary of the researcher’s parameter. One, three, and four CFRP layers were used to the hollow column section in previous research [14, 262, 267, 270].
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Ductility increases with the thickness of FRP jacket, restrains all shear cracks, and modifies the mode of
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failure of the sample from shear to flexure [14, 275]. Lignola [275] conducted experiments on seven hollow
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prismatic columns, considering the effects of FRP confinement. Results showed that using additional layers of FRP can increase the ductility of columns and prevent shear cracking. The performance of circular hollow
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reinforce concrete columns is not jeopardized with a minimum two layers of wrapped CFRP [267].
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Moreover, a minimum of one layer of wrapped CFRP for hollow column section was used by several studies [16, 17, 40]. All such works reported that the performance of the CFRP-wrapped hollow section is enhanced
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even when only one layer of CFRP is utilized. Kus and Hadi [274] also stated that CFRP confinement in the
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loop direction can increase the ductility and strength of a circular hollow RC column relative to those of the reference control. Hassan et al. [267] studied the effectiveness of FRP laminates in enhancing the strength of
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uniaxially loaded high-strength concrete columns by increasing the flexural capacity by up to 23% and 59%
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for small and large eccentricity-loaded specimens, respectively. Moreover, the use of ferrocement jackets including steel bars, in strengthening of circular and square RC columns were reported to effectively improve
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the seismic performance [307, 313-316]. Table 27: Summary of parameter studies on CFRP strengthening confinement of hollow RC bridge columns
13 Conclusion FRP is a composite system composed of a matrix of polymer strengthened with fibers, which are generally carbon, glass, basalt, or aramid. FRP composites have protracted as a substitute material to produce reinforcing bars for RC structures because FRP reinforcement bars provide benefits, such as noncorrosive and nonconductive properties, over steel reinforcement. Unique guidance on the construction and 55
ACCEPTED MANUSCRIPT engineering of RC structures strengthened with FRP bars is needed as a result of other changes in the mechanical and physical behaviors of FRP composites against steel. This comprehensive literature review shows that the majority of investigations are limited to the evaluation of composite characteristics of FRP rather than the characteristics of its materials and their influence on the strength and ductility of FRP
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composite matrix. The selection of FRP materials to strengthen any concrete structural element should
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consider its influences in terms of properties and long-term service and its reaction with the ecosystem.
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However, FRP composites obtained a great reputation worldwide; thus, global construction agencies have authored design and construction codes, in particular, for the use of FRP reinforcements as concrete rebar.
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This literature review aims to provide fundamental information reference on the basis of knowledge reviewed
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from global conducted studies, simulation work, and practical applications of FRP reinforcement. This review guides researchers, designers, and manufacturers about the latest developments in scientific research,
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particularly, in FRP composites, and the level of its performance for practical use. Moreover, this review is a
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continuation of research on resolving their limitations. However, the differences between the current review papers on FRP in strengthening RC structures comprise basically of superficial discussions handling only
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few functions or properties with no depth in any aspect. No existing study has touched the applications and
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properties of FRP materials in a wide range and provided meaningful guidelines for researchers, designers, and manufacturers. Meanwhile, this review mainly covers an extensive study on FRP design, matrix,
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material properties, applications, and serviceability performance. These literature reviews provide a comprehensive insight into the integrated applications of FRP composite materials for improving the rehabilitation techniques, comprising the applications for the repair, overhaul, strengthening, and retrofit of RC in the construction industry at present. The aforementioned review mainly intended to evaluate the current material properties of FRP. However, this review is expanded to enhance the FRP design matrix and combine two FRP materials to improve the
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ACCEPTED MANUSCRIPT techniques of rehabilitation, including strengthening, repair, and retrofit of structures. Moreover, the achievement of structural reintegration measures with innovative composite materials contributes to the advance of novel lightweight structural theories that utilize FRP composite materials to create novel structural RC composite systems. The use of FRP composites in the industries remains significantly
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advanced over time. Thus, many methods remain under investigation because of their application to
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strengthen current RC structures. The subsequent are the recommendations for further studies. 1)
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Investigations on the fatigue performance of strengthened RC elements are limited. 2) A literature review indicates that the fire design codes for FRP-strengthened RC members in available standards and codes are
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limited. 3) The effect of creep performance of the tendon of FRP on its long-standing work capability should
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be studied. Therefore, the durability and safety of the tendon in its typical service life can be confirmed. The affiliation between stress level and creep rupture time is also constructed based on the test data to expect the
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CFRP tendon capacity in long-standing work. A wide image from previous study findings on the application
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schemes of advanced composites in civil engineering is afforded in this paper to illustrate the scope of current growths and discover the practicality of upcoming applications in civil engineering sector. A
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prediction for the advanced composites application in civil engineering industries is produced by delineating
engineering society.
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14 Acknowledgment
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critical technical execution problems that should be determined prior to a comprehensive acceptance of civil
The authors gratefully acknowledge the financial support from the Department of Civil Engineering, Faculty of Engineering, Amran University, Yemen (RG: 2018110) and the Department of Civil Engineering, College of Engineering, Prince Sattam Bin Abdulaziz University, KSA (RG: 2017889), for this research.
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ACCEPTED MANUSCRIPT Table 1: Typical properties of CFRP [37] Trade Name
Tensile Strength, (MPa) 1596 2068 2250 1200
Ultimate Tensile Strain 0.013 0.017 0.015 0.012
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V-rod Aslan Leadline Nefmac
Modulus of Elasticity, (GPa) CFRP 120 124 147 100
Tensile Strength, (MPa)
Ultimate Tensile Strain
710 690 600
0.015 0.017 0.020
US
V-rod Aslan Nefmac
Modulus of Elasticity, (GPa) GFRP 46.4 40.8 30
CR
Trade Name
IP
Table 2: Typical properties of GFRP [37]
Tensile strength, (MPa) 2.3-3.4 3.3 3 2.8
M
Modulus of elasticity, (GPa) 70-43 70 79 123
ED
Trade Name Kevlar Technora Twaron Heracron
AN
Table 3: Typical properties of AFRP [37]
Extension to break, (%) 1.4-4 4.3 3.3 2
Density, (g/cm3) 1.44-1.47 1.39 1.44 1.44
PT
Table 4: Typical properties of BFRP [44]
AC
CE
Trade Tensile strength, Modulus of Coefficient of Thermal Expansion Elongation (%) Name (MPa) elasticity, (GPa) (x 10-6/°C) Rockbar 1000 50 2.24 2.0 BCR 1100 70 2.20 0.35-0.592 Annotation: Coefficient of thermal expansion of concrete = 10x10-6 /°C reliant on the concrete mixtures Table 5: Properties of thermosetting resins of FRP matrix [59]
Resin Epoxy Vinyl Ester Polyster
Specific Gravity 1.2-1.3 73-81 1.1-1.4
Tensile Strength (MPa) 55-130 1.12-1.32 34.5-103.5
Tensile Modulus (GPa) 2.75-4.1 3-3.35 2.1-3.45
85
Cure Shrinkage (%) 1-5 5.4-10.3 5-12
ACCEPTED MANUSCRIPT Table 6: Technical properties of epoxy (Sikadur 330, 1.31 kg/L)
Technical values Design code 2 7 days (at +23 °C) ~ 3 800 N/mm ` (DIN EN 1465) [73] Component A+B mixed 1.30 ± 0.1 kg/L (ISO 527) [74] (at +23 °C) Tensile strength 7 days (at +23°C) ~ 30 N/mm2 ⸗ 2 Modulus of elasticity in tension 7 days (at +23 °C) ~ 4 500 N/mm ⸗ Elongation at break 7 days (at +23 °C) 0.9 % ⸗ Tensile adhesion strength > 2 N/mm2, Concrete fracture on sand (EN ISO 4624) [75] blasted substrate Coefficient of thermal expansion at −10 °C to +40 °C 4.5 × 10−5 1/K (EN 1770) [76] Glass transition curing Temperature Curing time 30 days Temperature +30 °C (EN12614) [77]
US
CR
IP
T
Properties Modulus of elasticity in flexure Density
Table 7: Mechanical properties of several classes of FRP materials [6] Density, (g/cm3)
Steel Glass FRP Basalt FRP
500-500 600-1400 1000-1600
Aramid FRP
1700-2500
Carbon FRP
1755-3600
7.75 - 8.05 2.11-2.70 2.15-2.70 1.28- 2.6 1.39-1.45 1.55-1.76
Tensile Strength (MPa)
Specific Gravity
Elastic Modulus (GPa)
Strain at Break, %
480-1,600 1,035-1,650
7.8 1.5-2.5 2.7-2.89
(200) 35-51 45-59
1.2-3.1 1.6-3.0
1,720-2,540
1.38-1.39
41-125
1.9-4.4
1,720-3,690
1.0-1.1
120-580
0.5-1.9
AN
Yield Strength (MPa)
ED
M
Reinforcing Material
CE
PT
Annotation: - Ultimate tensile strength, ffu - Tensile Modulus of Elasticity, Ef - Elongation at Break, εfu - The values of fiber volume fraction of FRPs are limited between 0.5 and 0.7. CFRP and GFRP have a tensile elastic modulus of at least 124 GPA and 39.3 GPa, respectively
AC
Table 8: Qualitative comparison of several fibers used in design of the composites [85] Criterion
Tensile strength Compressive strength Young’s modulus Long-term behavior Fatigue behavior Bulk density Alkaline resistance Price
Type of Fiber used in Composite Carbon Fibers Glass Fibers Aramid Fibers Very good Very good Very good Very good Inadequate Good Very good Good Adequate Very good Good Adequate Excellent Good Adequate Good Excellent Adequate Very good Good Inadequate $7.11-18.11/m2 $0.13-0.27/m2 $8-12/m2
86
ACCEPTED MANUSCRIPT Table 9: Summary of previous studies on flexural tests
-
[108] [109] [110]
[111] [1]
-
[112]
-
AC
CE
PT
ED
M
[26]
T
[107]
IP
[21]
CR
-
US
[106]
Experimental Parameters • Composite type • Anchorage at end of plate Composite type Shear span/depth ratio Effect of pre-cracking Surface preparation • Configuration of CFRP system • Fiber orientation Number of composite plates Anchorage at end of plates • Anchorage technique at ends of plates • Composite type Thickness and/or number of plies • Number of plies • Effect of pre-cracking • Anchorage by wrapping with CFRP sheets External anchorage for CFRP plates (to control slip) • Shear span to depth ratio • Plate end anchorage Existing reinforcement ratio Effect of composite area to steel ratio • Placement of CFRP system • Anchorage with vertical sheets
AN
Refs. [105]
87
ACCEPTED MANUSCRIPT Table 10: Summary of previous studies on RC sections strengthened by CFRP and steel
Single lap joint with circular hollow section Single lap joint with circular hollow section Single shear pull test
Static
No. of CFRP layer 5
Static
1-5
Sheet
40–85
640
Static
1
Plate
350
165
-
Single shear pull test
Static
1
Plate
300–380
150
-
Static Static Static Static Static Static
1 3 1 1 1 1 1
250 40–80 50–200 300 30–250 500 25–250
160 240 338 197 479 195 151
-
Static
Plate Sheet Plate Plate Plate Plate Plate
[137]
Single shear pull test Double shear pull test Double shear pull test Double shear pull test Double shear pull test Double shear pull test Single lap, double shear, and T-peel test Pure bending test
Static
1
Plate
203
[19]
Double shear pull test
Fatigue
1
Plate
200
[30] [138] [20] [139141] [142]
Double shear pull test Double shear pull test Full-scale bridge girders
Fatigue Fatigue Fatigue
3 1 1
Sheet Plate Plate
40–60 60 457
155 166 and 640 479 112 205
Double shear pull test
Impact
1 or 3
Sheet
Pull-off test
Sheet
[143]
Double shear pull test
Impact 1 or 3 Large 3 deformation
E C
Ref. [127] [128] [129] [20, 130] [131] [132] [133] [134] [135] [136] [25]
[144, 145] [144, 146] [145] [147] [148] [15, 194] [150] [151]
Type of test
Type of loading
Double shear pull test
cyclic
Double shear pull test
Static
Double shear pull test Double shear pull test
C A Static Static
CFRP (plate or sheet) Sheet
Bond length (mm) 23–126
CFRP modulus (GPa) 230
Eco-condition
Araldite 420 , (1,901)
-
T P
Three types, (4,013 - 10,793) Araldite 420, Araldite 2015, Sika 30, Sika 330 (1,830 - 11,250) Two types (4,013 - 10,793) Araldite 420 (1,901MPa) SPSpa bond, (3,007) Sikadur 30, (4,500) and Sikadur 330, (3,800) Araldite 420 (1,901) and Sikadur 30 (9,282) Sikadur 30, (4,500) Tw types of epoxy ,(1,000)
I R
C S
U N
A
M
Araldite 420, (1,901)
-
-
Adhesive (modulus), MPa
-
Two parts epoxy system (1,240)
-
Sikadur 30, (2,689) Araldite 420, (1,901) Araldite 420 ,(1,901) Plexus MA555, (107) MBrace saturant, (2,229)
10–100
205
-
T P
100 and 150
205
Subzero
Sheet
D E
205
Temperature
Araldite 420, (1,901)
Sheet
100
205
Elevated Temperature
Araldite 420, (2,012); MBrace saturant (1,482) and Sikadur 30, (9,515)
1 or 3
Sheet
20–150
205
Elevated
3
Sheet
100
1 or 3
Sheet
20–100
640
Elevated Temperature
3
Sheet
1800
230
Seawater
Sikadur Hex 330, (50,350), SikadurHex 306 (44,900), and Tyfo SW-1, (62,500)
3
205
Temperature
Araldite 420, (1,901) and MBrace saturant (2,229)
Araldite420(1,901MPa) Araldite 420, (2,012); MBrace saturant (1,482) and Sikadur 30, (9,515) Araldite 420, (1,901)
Four-point bending test
Static
Double shear pull test
Static
1
Plate
200
418
Seawater
SP Spabond (2,980)
Double shear pull test Double shear pull test
Static Static
3 1 or 3
Sheet Sheet
100 30–100
205 205
Seawater UV ight
Araldite 420, (1,901) Araldite 420, (1,901)
88
ACCEPTED MANUSCRIPT Table 11: Creep ruptures reduction factors [5, 26] Property Creep-Rupture Stress Limit, Ff,s
GFRP 0.20
BFRP 0.20
AFRP 0.30
CFRP 0.55
Table 12: Typical properties of different FRP materials [5]
T
Strength to weight ratio 564 1013 993 1000 28
IP
Density of laminate g/cc 2.4-2.5 1.9-2.1 1.44 2.6-2.8 1-1.15
CR
Laminated strength , MPa 1500 1600 1430 1100 12-40
US
Table 13: Comparative chart of glass, aramid, and carbon fibers
PT
ED
M
AN
Fibers Glass Aramid Carbon Properties 2 2 Cost $0.13-0.27/m $8-12/m $7.11-18.11/m2 Weight to strength ratio P E E Tensile strength E E E Compressive strength G P E Stiffness F G E Fatigue resistance G-E E G Abrasion resistance F E F Sanding/Machining E P E Conductivity P P E Heat resistance E F E Moisture resistance G F G Resin adhesion E F E Chemical resistance E F E Annotation; E=Excellent, G=Good, P=Poor, F=fair
CE
GFRP CFRP AFRP (Kevlar) BFRP Epoxy
Fibre strength , MPa 3450 4127 2757 3792 -
AC
Material
89
Material Young's Modulus, GPa 30-40 125-181 70.5-112.4 70-90 3
ACCEPTED MANUSCRIPT Table 14: Summary of previous studies on FRP material bonding with surface of RC elements
[109] [110]
-
[111] [1]
-
[112] [26]
-
ED
-
T
[108]
• Concrete crushing • FRP composite debonding • Shear at ends of plates Peeling-off at end of plates Shear/peeling-off • Peeling-off along concrete cover Debonding Crushing (fully wrapped beams) Debonding along concrete cover • Debonding if not anchored • Gradual slip if anchored Shear/Peeling along concrete cover Debonding • Concrete crushing (high, ρ) • FRP debonding (low, ρ) FRP rupture if transverse sheets are used along entire length Debonding when plates are placed on bottom of beams
-
Peeling-off related to thickness and stiffness of plates Unless anchored, plates peel off • Use anchoring system to avoid brittle mode of failure Full wrap required to achieve maximum strength without debonding
IP
[106, 107]
-
CR
-
• Debonding at adhesive-concrete interface • Shear-peeling at ends of plates Debonding of FRP composite
Main conclusions Stress concentration at end of plates needs more study Selection of bonding agent is critical • Improve concrete-FRP adhesion • Wrapping entire length effective as anchorage Fiber orientation has large effect on maximum strength Pre-cracking has negligible effect • Brittle failure modes need to be considered in design • Need to improve knowledge on adhesion performance
-
•
US
Observed modes of failure Debonding after yielding of reinforcing steel
AN
[106, 107] [21]
-
M
Ref. [105]
-
Anchorage required for adequate performance
Failure mode depends on shear span/depth ratio Anchorage required at ends especially for low a/d ratios • Unable to develop full FRP strength without anchorage Wrapping along full length of CFRP increases maximum load Bonding plates on bottom and sides improves performance
Table 15: Comparison of resistance to corrosion by several composites in corroded environs [76] Conc. HCI N Y Y N
Dilute HNO3 N N N N
Chloride Salts N Y Y N
Dilute NaOH N N N N
AC
CE
PT
Material Dilute H2SO4 Conc. H2SO4 Dilute HCI FRP (laminate) N N N Carbon steel N N Y Stain less N N Y Hastelloy N N N Annotation; Y (yes) =Corroded; N (no) = Unaffected
Table 16: Fiber volume fraction corresponding to the number of laminate layers of CFRP/GFRP [85] Property fiber type (Laminate type number) 1 2 3 4 5
Fiber volume fraction (%)
Footnote
70 51 66 70
Fiber type 1 is a hybrid composite of carbon/glass fibers Fiber type 2 is a hybrid carbon fiber/resin composites Fiber type 3 is a hybrid glass fiber/epoxy composites Fiber type 4 is a hybrid carbon fiber/vinyl ester composites Fiber type 5 is a hybrid glass/carbon fiber composites
90
ACCEPTED MANUSCRIPT Table 17: Volume fractions of fibers (%) and rate of water absorption for different fibers [217]
55.4 ± 3.0 66.2 ± 2.5 55.9 ± 4.8 53.1 ± 4.8
3.45 2.29 3.20 4.12
T
E-glass/modified polyester E-glass/epoxy E-glass/vinyl ester ECR (high seed)-glass/modified polyester ECR (high seed)-glass/epoxy ECR (high seed)-glass/vinylester ECR (low seed)-glass/epoxy ECR (low seed) -glass/vinylester
Rate of water absorption, Average value 1 mm 2 mm 4 mm 15.1 4.54 8.65 4.07 1.59 2.02 3.74 1.18 1.95 11.96 11.9 7.34
Volume fraction of fibers (%) 55.8 ± 5.4 53.1 ± 4.4 56.9 ± 3.5 56.4 ± 5.4
IP
Material
2.27 1.38 1.79 2.33
1.42 0.86 0.85 0.94
CR
Annotation: ECR-glass = E-glass with higher acid Corrosion Resistance Table 18: Function properties of different FRP materials
M
AN
US
BFRP GFRP CFRP 8.0 2.9-4.5 0.05 3.15 4.7-5.6 1.2 982° 650° 1100° 820° 480° 1000° 260° -60 -170° 0.031-0.03 0.034-0.04 0.035-0.04 1450° 1120° 1550° 1050° 600° 1300°-1670° 1.91 0.32 1.75 9-23 9-13 9-15 <0.1 <0.1 <0.1 100 100 100 95 92 94 82 52 80 0.2 0.7 0.05 5.0 6.0 5.0 2.2 38.9 15.7 Annotation: Part per million/degree Celsius of temperature (ppm/°C); Watt per meter per kelvin (W/m K)
Direction Longitudinal Transverse
AC
CE
PT
ED
Properties Thermal Linear Expansion Coefficient, ppm/°C Elongation at break, % Maximum application temperature, (°C) Sustained operating temperature, (°C) Minimum operating temperature, (°C) Thermal conductivity, (W/m K) Melting temperature, (°C) Vitrification conductivity, (°C) Glow loss, (%) Filament diameter, (microns) Absorption of humidity (65%RAH), (%) Stability at tension (20 C°), (%) Stability at tension (200 C°), (%) Stability at tension (400 C°), (%) H2O, (%) 2n NaOH (Sodium Hydroxide), (%) 2n HCI (Hydrochloric acid), (%)
Table 19: CTE of different FRP materials
Coefficient of thermal expansion (x 10-6/°C) Steel GFRP CFRP 11.7 6 to 10 -1 to 0 11.7 21 to 23 22 to 23
AFRP -6 to -2 60 to 80
Table 20: Transverse and longitudinal CTE of prestressing steel tendons and CFRP tendons [124] Property Units Longitudinal thermal expansion coefficient ̊C Transverse thermal expansion coefficient ̊C Relaxation ratio at room temperature % Annotation: Carbon fiber composite cable (CFCC)
Leadline -0.9 × 10-6 27 × 10-6 2-3
91
CFCC -0.5 × 10-6 21 × 10-6 0.5–1
Prestressing steel 11.7 × 10-6 11.7 × 10-6 8
ACCEPTED MANUSCRIPT Table 21: Imperative comparison and rankings for steel and FRP [188]
IP
Steel 4 2 3 3-4 3-5 4 3 4 5 3
CR
FRP Strength/stiffness 4-5 Weight 5 Corrosion resistance/ Environmental durability 4-5 Ease of field construction 5 Ease of repair 4-5 Fire 3-5 Transportation/handling 5 Toughness 4 Acceptance 2-3 Maintenance 5 Annotation: (1) Very Low, (2) Low, (3) Medium, (4) High, and (5) Very High
T
Merit/Advantage (Rating)
Property (Parameter)
Region
US
Table 22: Typical behavior of specimens strengthened with different material properties [21] Material Behaviour Concrete Elastic Elastic Yielding
AC
CE
PT
ED
M
A B C
AN
FRP Elastic Elastic Elastic
92
Reinforcement Un-cracked Cracked Cracked
ACCEPTED MANUSCRIPT Table 23: Summary of tests of beams under fatigue loading Failure Parameters Ref.
Number of Specimens
Cross Sectional Shape
[21]
1
[243]
1
[244]
Composite Type
Mode of Failure
Rectangular
Number of Cycles 480,000
Tee
12,000,000
Steel Fracture
Rectangular
20,000 732,600 508,500
Steel Yield Steel Yield Steel Fracture
1,889,087
Steel Fracture
>11,968,200 150,000
No Failure Steel Fracture
2,000,000 1,800,000 1,756,000 3,000,000 3,215,000
CFRP Fabric Rupture CFRP Fabric Rupture CFRP Fabric Rupture CFRP Fabric Rupture CFRP Fabric Rupture
Steel Fracture
Hybrid (33% Carbon/ 67% E-glass) Sheet CFRP Sheet
5
[245]
6
Tee
T P
D E
E C
C A
93
Number of cycles indicates fracture of first reinforcing bar although cycling was continued
U N
CFRP Fabric
T P
After 10.7 x 106 cycles, testing was continued in an environmental chamber at 40ºC and 95% relative humidity Control Control Maximum load represented the same percentage of the ultimate load as for specimen 2 Stress range in the reinforcement approximately equal to the stress range of specimen 1 Stress range in the reinforcement approximately equal to the stress range of specimen 1 Strengthened after applying 150,000 cycles
I R
C S
CFRP Pultruded Plates
A
M
Annotation
Accumulation of damage was characerised by a loss of stiffness in the load-deflection plots after different numbers of cycles
ACCEPTED MANUSCRIPT Table 24: Guidelines strategies of the FRP sheets as proposed by various standards for strengthening RC structures Country Name of code Year of publishing
Title of the FRP design code/standard/guideline
Shear
Strengthening of RC structures
Technique
Flexural Column confinement
Europe countries Eurocode 8-3 2004 Currently no Eurocode for FRP Design of structures for earthquake resistance – Part 3: Assessment and retrofitting of buildings) -
(𝐸𝑓 𝑡
4𝑓𝑐𝑡𝑚
le,=
Should be between 40 -240 mm 𝑤𝑓
√
E C
1.5(2− (1+
The concrete strength, MPa
C A
𝑠𝑓
𝑤𝑓
)
)
D E
(𝑛𝐸𝑓 𝑡 0.58
Should not be less than 200 mm
Af = 2ntfwf
Canada
Australian
Japan
Italian
CSA S806-02 2004 Design and construction of building components with fibrereinforced polymers
HB 305 2008 Design guideline for RC structures retrofitted with FRP and metal plates: beams and slabs
JSCE -E 541 2002 Japanese Design and construction guidelines for seismic retrofit of existing reinforced concrete buildings
CNR-DT 200 2004 Guide for the design and construction of externally bonded FRP systems for strengthening existing structures.
T P
I R
C S
U N
A
23300
T P
Kp = Width of the FRP sheets, wf
China
ACI 440.2R CECS 146 2002 2003 Guide for the Technical design and specification for construction of strengthening externally concrete bonded FRP structures with systems carbon fiber for reinforced strengthening polymer laminate concrete structures Embedded Through‐Section Near Surface Mounted (NSM) Near Surface Mounted (NSM) Steel Fiber-Reinforced Concrete (SFRC overlay) Externally Bonded Reinforcement (EBR) Genetic Algorithm (GA) Jacketing (U-wrapping) Steel wire reinforced polymer (SWRP) le, = √
Effective bond length of the FRP sheets, le
American
M
𝐸𝑓 𝑡𝑓
le, = √
𝑐2 𝑓𝑐′
le, =0.7 √ le,=
25350 (𝑛𝐸𝑓 𝑡𝑓)
0.58
𝐸𝑓 𝑡
𝐸𝑓 𝑡𝑓
le, = √
𝑓𝑐′
200 mm (not including section cut away from edge)
𝐸𝑓 𝑡𝑓
le, = √
𝑓𝑐𝑡𝑚
2𝑓𝑐𝑡𝑚
Should not be less than 200 mm Kp =1.06
50mm ≤ wf≤ 250mm
Af = nf tf wf
wf = df –2Le
100 𝑚𝑚
Structural grade ≥ 30 MPa Should be at least 30 MPa Required a high quality surface Up to 2 mm 2-3 mm 1-2 mm 0.17 – 0.83 mm
Pmax = 𝑏𝑓 √2𝐸𝑓 𝑡𝑓 𝐺𝑓′′
√
2−
(1+
𝑤𝑓
)
𝑏 𝑤𝑓
≥1.0
)
400 𝑚𝑚
Quality of the concrete surface The glue line thickness, mm Up to 2 mm 2.0 -2.3 mm Up to 2 mm Thickness of FRP, tf Ratio of strength/stiffness of the 4-5 FRP sheets Annotation: Ef = elastic modulus of FRP, Le = effective bond length, f ´c =concrete strength, fck = characteristic strength of concrete, fctm = mean tensile strength of concrete, n = number of layers of FRP, tf = thickness of FRP, df and wf = effective depth and width of FR, respectively. sf is the spacing between the centreline of the FRP strips, and Af is the effective area of the FRP, nf is the number of FRP layer, 𝐺𝑓′′ Young's modulus of FRP, Kp Factor related to the width of the bond of FRP sheet, Pmax maximum load of the section, b = width of section soffit in mm.
94
ACCEPTED MANUSCRIPT Table 25: Bond provisions as proposed by various standards for strengthening RC structures Name of design code
Related equation (Bond strength) ɛf =0.41 √
ACI 440.2R-07
Footnotes
𝑓𝑐′
-
(𝑛𝐸𝑓 𝑡𝑓)
ɛf =0.484 𝑘𝑝 √
CNR DT 200
𝑓𝑐′
𝑓𝑐𝑡
𝑤𝑓
(𝑛𝐸𝑓 𝑡𝑓)
ɛf =0.5 𝑘𝑝 √
Concrete Society TR 55
Kp =1.06 √
𝑓𝑐𝑡
2− (1+
(𝐸𝑓 𝑡𝑓)
(𝐸𝑓 𝑡𝑓) 0.2 𝑙𝑏
)
Kp =1.06 √
IP
√(𝑛𝐸𝑓 𝑡𝑓)
−
T
1
ɛf = kp fct (
CECS-146
)
≥ 1.0
400 𝑚𝑚
Gf = interfacial fracture energy taken as 0.5 N/mm in absence of test values
𝐺𝑓
ɛf = √
JSCE -E 541
)
𝑏 𝑤𝑓
𝑤𝑓
2−
𝑏
(1.25 +
)
𝑤𝑓 𝑏
)
CR
Annotation: kp retrofit geometry parameter (factor accounting for wf/w in design), Gf interfacial fracture energy, fct concrete tensile strength, lb provided anchorage bond length
Strength/stiffness Weight Corrosion resistance/ Environmental Durability Ease of field construct Ease of repair
Aerospace Marine, construction, pipes, bridges, reinforcing bars, automotive Aerospace, marine, construction, pipes, bridges, reinforcing bars, automotive Marine, boat industry, construction industry, aerospace Automotive, leisurely applications Buildings, bridges, pavements, kiln linings, wind mill blades, radomes Bridges, tunnels, underwater piles. Aerospace, marine, automotive, blast resistant FRP construction. Bridge decks, leisure products, marine boats Shapes, bridge decks, components and assembled FRP systems Bullet proof vests, vandalism and graffiti proof walls. Construction and aerospace industries Offshore and fire resistant applications.
ED
Fire
FRP applications Application
AN
FRP Very high High Very high Very high High High High Very high Medium Very high High low Low
M
Parameter
US
Table 26: FRP merits and suitability of applications [195]
Transportation/handling Toughness and impact
PT
Acceptance
CE
-
AC
Actual projects used FRP
-
Egyptian Museum in Cairo and Kaitbay Fence (Egypt) Bridge deck slab (Germany) Brisbane City Council - Bowen Bridge (Queensland) Melbourne Water – Maribyrnong and River Pipe Bridge - Melbourne Airport – Service Culverts, – Kororoit Creek Rd Bridge & Arden St Bridge (Victoria) Main Roads WA - Greenough River Bridge (Western Australia) Ouse River Bridge & Emu River Bridge (Tasmania) Superstructure strengthening of Alaskan Way Viaduct bridges and WSDOT Evaluation Project (United State)
95
ACCEPTED MANUSCRIPT Table 27: Summary of parameter studies on CFRP strengthening confinement of hollow RC bridge columns Dimensions, mm
[33]
[17]
Rectangu lar
Square
Rectangu lar
160 0
160 0
140 0
450 x 900
450x45 0
550 x 350
550x35 0
[26 1]
Circular & Square
300
150 x300
[19 7]
Square
100 0
120×80 ×8
300x750
300x300
340x140
AC
4
2
2
Tensile Streng th (MPa)
Wrapping Position
` Axial Load
0.165
1820
3000
Longitudina lly wrapped
250 & 500
3480
Unidirectio n al 3m up the column
3600 & 3900
Wrapped spaced at 100mm along height
3800
Wrapped 200mm & 100mm spaced
3750
Confined 500mm to the bottom column
0.137 5
0.117
0.117
-
-
11
0.111
3800
-
200
2& 3
0.111
300
3972
106 x106
2
0.165
200
1102
140×80× 8
3
-
-
3769
330x130
CE
[18]
Rectangu lar
140 0& 280 0
900x900
Densi ty (g/m3 )
96
Confined 500mm to the bottom column Wrapping Position
Materials properties (MPa)
Load
Concre te
Long. Reinf.
-
33
670
no data
-
30
418 & 420 (64ϕ2 2)
413 & 420 (10ϕ @ 200m
700
500
35
450
450
700
500
28
435 (40ϕ8 )
440 (2.6ϕ)
Lateral
T
IP
1500 x 1500
2
Thickne ss (mm)
CR
[16]
350 0
300x300 900 &
Loading (kN)
US
[14]
Circular & Square
450x45 0 1500 &
No. of Lay er
AN
[13]
900 & 135 0
Inner Dia (d)/Dimens ion
M
Square
Heig ht
ED
Section
CFRP
PT
Ref.
Outer Dia. (D)/ Dimensio n
480
72.32 (min) , 202.3 5
22.4
Transv. Reinf.
500
500
(20ϕ10)
(6ϕ@10)
413, 420 (6ϕ@10 0)
(max)
909.6
1000
30.4
418, 420 (20ϕ1 0)
7511563
-
40
8 mm steel rod
-
3436. 1
-
40
235
-
ACCEPTED MANUSCRIPT Highlights
ED
M
AN
US
CR
IP
T
FRP composite is highly resistant to chloride ion and chemical attack compared to steel bars.
PT
CE
FRP has a distinctive tensile strength property of being greater than that of steel, yet, it weighs only one quarter as much. FRP composites were accepted as a mainstream construction material for repair, strengthening and retrofit of concrete structures. Rehabilitation function is mainly influenced by the direction of polymers fibres used.
AC
97
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Figure 15