Flexural strength and failure of geopolymer concrete beams reinforced with carbon fibre-reinforced polymer bars

Construction and Building Materials 231 (2020) 117185

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Flexural strength and failure of geopolymer concrete beams reinforced with carbon fibre-reinforced polymer bars Hemn Qader Ahmed ⇑, Dilshad Kakasor Jaf, Sinan Abdulkhaleq Yaseen Civil Engineering Department, Salahaddin University – Erbil, Iraq

h i g h l i g h t s
Geopolymer concrete beams with low rebar ratio recorded significant deformation.
Concrete compressive strength had a significant effect on the first crack of GPC beams.
The CFRP-RGPC beams had more deflection than CFRP-ROPC beams.
Number of cracks in the CFRP-RGPC beams was higher than that in the CFRP-ROPC.
CFRP-ROPC beams obtained a higher value of crack width compared with CFRP-RGPC.

a r t i c l e

i n f o

Article history: Received 17 April 2019 Received in revised form 15 September 2019 Accepted 6 October 2019

Keywords: Flexural strength Geopolymer concrete Carbon fibre-reinforced polymer bar Sustainability

a b s t r a c t Geopolymer concrete with carbon fibre-reinforced polymer bars can provide a good construction system with high sustainability, high durability and appropriate strength. Few studies have used a combination of these materials, especially the carbon fibre-reinforced polymer bar. The present investigation obtains the flexural strength and behaviour of geopolymer concrete and ordinary Portland concrete beams reinforced with carbon fibre-reinforced polymer bars. Twelve beams consisting of nine geopolymer concrete and three ordinary Portland concrete beams were cast and investigated by using the four-point bending test over an effective span of 2000 mm. Reinforcement ratio, compressive strength and concrete types were taken as the variables. First cracking load, ultimate load, load-deflection behaviour, load-strain curve, crack width, number of cracks and modes of failure, were studied. Experimental results were compared with the proposed equations provided by ACI 440.1R-15, CSA S806-12 and the proposed method. Results showed the decrease of deflection and increase of first cracking load with the increase of compressive strength, a slight increase in the deflection of geopolymer concrete beams and nearly the same value of ultimate load were observed; geopolymer concrete beams also recorded a lower value of crack width compared with ordinary Portland concrete beams. ACI 440.1R-15 and CSA S806-12 underestimated the flexural strength of the beams, while the proposed method which based on the parabolic stress block predicted more accurately. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Regular ordinary Portland cement has been used as a binder for producing ordinary Portland concrete (OPC) for a long time. Orders for OPC are expected to increase in the future due to the rise of Abbreviations: OPC, Ordinary Portland Concrete; GPC, Geopolymer Concrete; FRP, Fibre-Reinforced Polymer; CFRP, Carbon Fibre-Reinforced Polymer; SROPC, Steel-Reinforced Ordinary Portland Concrete; SRGPC, Steel-Reinforced Geopolymer Concrete; CFRP-RGPC, Carbon Fibre-Reinforced Polymer-Reinforced Geopolymer Concrete; CFRP-ROPC, Carbon Fibre-Reinforced Polymer-Reinforced Ordinary Portland Concrete. ⇑ Corresponding author. E-mail address: [email protected] (H.Q. Ahmed). https://doi.org/10.1016/j.conbuildmat.2019.117185 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

infrastructure requirement of many developing countries and the increasingly old and deteriorated concrete structures requiring urgent repair and rehabilitation. However, the production of Portland cement contributes billions of tons of waste materials and approximately 7% of the world’s greenhouse gases every year [1]. Several environmental issues have been indicated at some stages in the production of Portland cement due to the calcination of limestone and burning of fossil gases, such as the quantity of the carbon dioxide that launches in the atmosphere throughout the manufacture of Portland cement is in the order of 1 ton during the production of 1 ton of Portland cement [2,3]. In 1957 Glukhovsky proposed a working hypothesis in which he established a close relationship between alkalis and cementitious

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materials, that hypothesis has become consolidated and has served as a basis for developing a new type of binders, initially called ‘‘alkaline cements” [4]. In 1988, Davidovits [5] discovered an alkaline activator liquid that could be used to react with silicon and the aluminium in a material of geological origin, source or fly ash and rice husk ash (byproduct materials) and produce binders. The term ‘geopolymer’ is coined to represent those binders. Geopolymer concrete (GPC) is a type of concrete that no longer uses any cement in its manufacturing. GPC has attracted considerable attention from researchers because of its remarkable potential compared with OPC. Researchers are moving their focus on from chemistry to engineering and the commercial production of GPC [6]. Fly ash acts as a partial replacement material for both Portland cement and fine aggregate, it can estimate the compressive strength of concrete containing different levels of sand replacement by fly ash [7]. Geopolymers synthesised from fly ash could provide the basis for long-lasting materials for the building products and construction industries [8]. Geopolymer binders produced by a synthesis of Silica and Alumina rich pozzolanic precursor-like Fly Ash, with alkali solution as an activator through the process of Geopolymerization have become known as luminously promising option to conventional cement [9]. The first scientific attempt of a comparative study of thermal resistance of fly ash-based geopolymer concrete and rubberized geopolymer concrete has been done by Luhar et al., rubberized fly ash-based geopolymer concrete has been prepared using waste rubber tire fibres as a partial substitute for natural river sand, providing an efficient solution to the disposal problems of both fly ash and waste rubber [10]. The mechanical strength of the GPC system depends on several factors. The pH of the activating solution is the major parameter that controls the compressive strength of a GPC. According to Khale and Chaudhary [11], the activating solution with a pH range of 13–14 is the most suitable for the formation of the GPC with better mechanical strength. The properties of source materials also affect the strength of GPC. Source materials with high reactivity produce geopolymer systems with a high compressive strength [12]. The early strength development of geopolymer system, on the other hand, can be enhanced by using NaOH with higher molarity but with low alkali content and by using elevated temperature for curing [11]. Duxson et al. [13] reported that other desirable characteristics of geopolymer concrete such as rapid development of mechanical strength, fire resistance, dimensional stability, acid resistance, excellent adherence to aggregates and reinforcements, and have lower material cost, approximately 10–30% lower than that of OPC. The type of application of geopolymeric materials is determined by the chemical structure in terms of Silica-to-Alumina (Si: Al) Atomic ratio in the polysialate. Davidovits [14] classified the type of application according to the Si:Al ratio as follows: For Si:Al ratio (1): Applications (Bricks, Ceramics and Fire protection). For Si:Al ratio (2): Applications (Low CO2 cements and concretes and Radioactive and toxic waste encapsulation). For Si:Al ratio (3): Applications (Fire protection fibre glass composite, Foundry equipments, Heat resistant composites, 200 °C to 1000 °C and Tooling for aeronautics titanium process). For Si:Al ratio (>3): Applications (Sealants for industry, 200 °C to 600 °C and Tooling for aeronautics SPF aluminium). For Si:Al ratio (20–35): Applications (Fire resistant and heat resistant fibre composites). For many applications in the civil engineering field a low Si:Al ratio is suitable. Moreover, the alkalinity of concrete protects the steel reinforcement (nonprestressed and prestressed) from corrosion, thereby usually resulting in durable and serviceable construction. Structures are subjected to violent environments, including marine systems and bridges and parking garages, which have been exposed to deicing salts. The combination of moisture, temperature and chlorides reduce the concrete alkalinity and cause the corrosion of reinforcing steel. The corrosion action finally causes concrete

deterioration and insufficiency of serviceability. To solve this corrosion problem, professionals have used composite substances made of fibres embedded in a polymeric resin called fibrereinforced polymer (FRP) bars as reinforcement [15]. FRP substances are nonmagnetic and noncorrosive; thus, the issues of electromagnetic interference and metal corrosion can be prevented with FRP reinforcement. In addition, FRP materials show considerable residences, such as excessive tensile stress, which make them suitable for use as structural reinforcement [16]. Maranan et al. [17] reported that the GPC reinforced with GFRP bars have a bond strength similar to that of steel-reinforced geopolymer concrete, and suggested that they can be an effective alternative as internal reinforcement for geopolymer concrete structures. The mechanical interlock and friction force resistance provided by the sand coating, bonded on the surface of the GFRP bars, were found to be adequate to secure a composite action between the bars and the geopolymer concrete [18]. Due to the low elastic modulus of GFRP bars, beams reinforced with these bars exhibit low post cracking flexural stiffness compared to the concrete beams conventionally reinforced with steel bars [19], On the other hand, the modulus of elasticity of CFRP bars even lower than the one of the steel bars and is three to four times higher than that of the GFRP bars, moreover, CFRP bars also exhibit substantially higher strength than GFRP bars. Henceforth, although some research has been reported so far on the behaviour of concrete beams reinforced with CFRP bars the problem of the behaviour of these beams under loading as members of reinforced concrete structures is still open to question [20]. Several investigations have been conducted on the flexural strength of steel-reinforced GPC (SRGPC) beams [21–25]. The load-deflection relationship obtained from steel-reinforced OPC (SROPC) and SRGPC beams are almost similar with a slightly higher ultimate load in SRGPC beams. The first cracking load of SRGPC beams is better than that of SROPC beams, which shows better load carrying strength [26]. The failure of SRGPC beams is more ductile than that of SROPC beams, wherein SRGPC beams exhibit a higher number of small cracks compared with SROPC beams [27]. Maranan et al. [18] investigated the flexural strength of glass fibrereinforced polymer-RGPC beams; they concluded that the bar diameter had no significant effect on the flexural strength of the beams, and the serviceability behaviour of a beam was enhanced with the increasing rebar ratio. GPC and carbon fibre-reinforced polymer (CFRP) bars can provide a good construction system by utilising the advantages of both materials. Few studies have combined these materials, especially the CFRP bar, and this is the principal topic of the present study, in which the flexural strength and behaviour of fly ash-based CFRPRGPC beams were investigated. The studied parameters were compressive strength, reinforcement ratio with respect to the balanced reinforcement ratio and concrete types (namely, RGPC and ROPC). The crack pattern, failure modes, load-deflection curves and load– strain curves were presented. Moreover, experimental results were compared with the predicted flexural strength by using the Equations proposed by ACI 440.1R-15 [15] and CSA S806-12 [28], and a prediction method proposed based on the parabolic stress block, strain compatibility and internal force equilibrium. The objectives of the current study can be summarized as follows:
To study the flexural behaviour of GPC beams reinforced with CFRP bar.
To study the effect of compressive strength and reinforcement ratio on the flexural behaviour of CFRP-RGPC and CFRP-ROPC beams.
To study the effect of concrete types namely (GPC and OPC).

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To compare the crack pattern, failure modes, load-deflection curves and load–strain curves of the CFRP-RGPC and CFRPROPC beams.
To compare the experimental results with the proposed equations provided by ACI 440.1R-15 and CSA S806-12.
To develop a proposed prediction method based on the parabolic stress block of concrete. 2. Materials and methods 2.1. Material The alkaline liquid preparation materials used in this study were sodium silicate solution (Na2SiO3), which is available in liquid gel form and locally available with a chemical composition of 13.4% Na2O, 32.5% SiO2 and 54.1%water, and sodium hydroxide (NaOH) pellets with 98% purity, which are presented in flakes and pellets from Don Construction Products (DCP) Company. For the preparation of GPC, the materials were low-calcium fly ash type-F from Don Construction Products (DCP) Company (chemical Composition; Silica (SiO2): 59.07, Calcium Oxide (CaO): 1.189, Alumina (Al2O3): 18.398, Ferric Oxide (Fe2O3): 12.18, Magnesium Oxide: 1.92), coarse aggregates with a maximum size of 9.52 mm and fine aggregates. The grading of aggregates satisfied the limits provided by ASTM C33 [29], and sulphonated naphthalene formaldehyde superplasticizer was used to enhance the workability. 2.2. Mix proportion GPC has only been discovered recently. Hence, no fixed mix design exists for GPC. In this study, 13 trail mixes were conducted to achieve the required compressive strength of RGPC and ROPC beams. The variables considered for the GPC trail mixes were the molarity of sodium hydroxide (Mol), alkaline–fly ash ratio and water–binder ratio, whereas the water-cement ratio was the only variable considered for OPC trails. Table 1 shows the details of the trail mix proportions. Each trail consists of nine 100 mm  200 mm cylinders, which were tested according to ASTM C39 [30] after 28 days. As a result, increasing molarity of sodium hydroxide caused to increase compressive strength up to 12Mol, but when increased to 16Mol compressive strength decreased. When alkaline-fly ash ratio increased caused to increase in the compressive strength, this is because the excess presence of NaOH (hydroxide ions) leads to fast and early precipitation of aluminosilicates gel which hardens rapidly and also inhibits the formation of another geopolymeric precursors [31], and the

optimum water-binder ratio was 0.25, Fig. 1 shows the effect of trail mix parameters on compressive strength. 2.3. Preparation of geopolymer concrete The difference between OPC and GPC is related to the binder portion, and the OPC binder consists of cement and water, whereas fly ash-based GPC consists of fly ash, alkaline activator and additional water. The alkaline liquid was prepared a day before casting. Firstly, the sodium hydroxide was dissolved in water on the basis of required molarity (Mol). For example, 480 g was the required amount of sodium hydroxide to be dissolved in water to obtain 1 L of sodium hydroxide solution (12  molecular weight of sodium hydroxide (4) = 480 g), and 1330 g was the experimentally determined weight of 1 L of sodium hydroxide for 12Mol. Secondly, sodium silicate and sodium hydroxide solutions were mixed in a ratio of 2.5:1, which is the best ratio on the basis of a previous study [2]. The water–binder ratio was calculated as follows: the amount of water in sodium silicate solution, sodium hydroxide and additional water was divided by the solid parts in sodium silicate solution, sodium hydroxide solution and fly ash. At casting day, the aggregates were initially placed in an electric mixer. Subsequently, half of the additional water was added and mixed for 5 min, and fly ash was then added and mixed for an additional 5 min. After that, the alkaline liquid was added to the mixture. Finally, the remaining additional water and superplasticizer of 2% of the weight of fly ash were added. The fresh fly ash-based geopolymer concrete was dark in colour and shiny in appearance. Slump test carried out for the fresh GPC concrete according to ASTM C 143 [32]. The obvious factor affected the slump values was the water-binder ratio, in which the lower water-biner ratios recorded lower slump values, even using the superplasticizer. On the other hand, the higher water-binder ratio of 0.35 seemed to like self-compacting GPC. The fresh GPC cast into steel moulds which were coated internally with special tape to avoid the GPC from sticking to the mould. In this study, the oven-dry method was used for curing the GPC. According to Hardjito et al. [2], the suitable curing temp. was in the range of 30 °C to 90 °C, and they found that curing of Geopolymer concrete at higher temperatures up to 60 °C, yielded a higher compressive strength than at a lower temperature. Kirschner et al. [33], obtained the highest strength using alkali-activated metakaolin cured at 75 °C during 4 h. However the selection of curing temp. for the current study based on the information mentioned. The longer curing time, in the range of 4–96 h, produces higher compressive strength of fly ash-based geopolymer concrete according

Table 1 Trail mix proportion details and results. Mix

G (kg/m3)

S (kg/m3)

FA (kg/m3)

Mol

S/S

A/F

EW (kg/m3)

SP %

W/B ratio

ST mm

f ’c (MPa)

M01 M02 M03 M04 M05 M06 M07 M08 M09 M10

1230 1230 1230 1230 1230 1230 1230 1230 1230 1230

660 660 660 660 660 660 660 660 660 660

400 400 400 400 400 400 400 400 400 400

12 12 12 12 16 8 8 8 8 12

2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.4 0.35 0.45

0 32 14 48 36 27 43 50 57 38

3 2 2 2 2 2 2 2 2 2

0.2145 0.3017 0.2511 0.3502 0.3009 0.3019 0.3504 0.3512 0.3521 0.3195

94 197 131 233 210 205 243 235 240 198

45.5 39.5 47.5 26.8 39.4 24.6 19.5 12.9 10.4 34.5

Mix

G (kg/m3)

S (kg/m3)

C (kg/m3)

SP %

W/C ratio

ST mm

f ’c (MPa)

M11 M12 M13

1230 1230 1230

660 660 660

400 400 400

2 2 2

0.3 0.4 0.5

182 196 244

52.2 36.1 25.7

Note/ G: coarse aggregate; S: sand; FA: fly ash; Mol: molarity of sodium hydroxide; S/S: sodium silicate-sodium hydroxide ratio; A/F: alkaline-fly ash ratio; C: cement; EW: extra water; SP: superplasticizer is % of the weight of fly ash; W/B: water-binder ratio; W/C: water-cement ratio; ST: Slump Test.

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Fig. 1. Effect of water–binder ratio, NaOH molarity and alkaline–fly ash ratio on compressive strength.

to Hardjito et al. [2] However, the increase in strength beyond 24 h is not significant. For this reason 24 h curing time was selected. According to Khale and Chaudhary [11], the activating solution with a pH range of 13–14 is the most suitable for the formation of the GPC with better mechanical strength. A rest period of 60 min was considered before placing the casted GPC into an oven and cured for 24 h at a temperature of 70 °C. 3. Experimental program 3.1. Design of beam specimens and program The beam specimens were designed according to ACI 440.1R-15 [15]. The experimental program consisted of nine CFRP-RGPC and three CFRP-ROPC beams with target compressive strengths of 20, 35 and 50 MPa. The beams were divided into four groups, with three beams in each group. Groups 1–4 consisted of CFRP-RGPC (20 MPa), CFRP-RGPC (35 MPa), CFRP-RGPC (50 MPa) and CFRPROPC (35 MPa), respectively. Table 2 lists the properties of CFRP bars which were manufactured by CHONGQING YANGKAI Company, China. All groups share the same height and width of 300 mm and 110 mm, respectively. Moreover, the different reinforcement ratio conditions qf with respect to the balanced reinforcement ratio qfb were obtained in each group (qf < qfb, qfb < qf < 1.4 qfb and qf > 1.4 qfb) due to the limitations of ACI 440.1R-15 [15] in maintaining the three failure modes (that is, tension, tension-compression, and compression failure modes). A clear cover was maintained at 20 mm for all beams. The length of the beams was varied (2200–3600 mm) due to the requirement of development length. Regular deformed steel bars with a diameter of 6 mm were used as a longitudinal reinforcement at the top in shear spans and as vertical stirrups placed in a centre-to-centre spacing of 100 mm to prevent shear failure in the beams. The beams were simply supported over an effective span of 2000 mm. Fig. 2 and Table 3 show the details of the specimens and the test program, respectively. 3.2. Casting beam specimen and test method From the trail mix results, M03, M07 and M10 were selected for casing CFRP-RGPC beams and M12 for CFRP-ROPC beams. Before

casting, reinforcement was prepared, including the attachment of strain gauges. The gauge locations on the bars were initially smoothened and cleaned. The strain gauges were then bonded to the prepared surface of the FRP bars with an adhesive material. Subsequently, heat resistant wires were soldered to the strain gauges. Finally, the strain gauges were covered by a heat resistant protective tape. The casting moulds, including a steel mould, three 150 mm  300 mm cylinders and three 75 mm  75 mm  400 mm prisms, were used for casting the control specimens of CFRP-RGPC beams. The mould, cylinders and prisms were internally coated with a special tape for the same reason mentioned earlier and then placed on an external vibrator. After casting, the specimens were left for 60 min as a rest period and then placed into the oven and cured for 24 h at 70 °C. The following day, the specimens were demoulded and placed in a laboratory environment until the test day. The classic method was used for casting and curing of the CFRP-ROPC beams. Fig. 3 shows the CFRP-RGPC and CFRP-ROPC beams. A four-point static bending test method was used in this study, as shown in Fig. 4. The beams were loaded at midspan with two concentrated loads spaced at 400 mm. The load was applied using a hydraulic jack with strength and approximate rate of 2500 kN and 2 kN/min, respectively. The midspan deflection was measured using a dial gauge and camera recorder. Furthermore, a data logger was used for recording strains in the electrical strain gauges that were attached on the top surface of the beams and the CFRP bars. A crack measurement sensor was also used for recording the crack width. 4. Results and discussion 4.1. Summary of test results and control specimens Table 4 presents the test results for the first cracking load, ultimate load, deflection and failure modes under the same compressive strength and rebar condition, the ultimate load values of CFRPRGPC and CFRP-ROPC were almost similar. The reinforcement ratio had a significant effect on first cracking load and ultimate load of each group, and the results showed that the enhancement of deflection and first cracking load was obtained by increasing the compressive strength, wherein deflection of CFRP-RGPC was slightly higher. Furthermore, four different failure modes were

Table 2 Properties of CFRP bar. Type

Bar diameter (mm)

Area Af (mm2)

Ultimate tensile stress ffu (MPa)

Elastic Modulus Ef (GPa)

Ultimate Strain %

CFRP bars

6

28

2000

148

1.35

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H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185

Fig. 2. Details of beam specimens with dimensions in mm.

Table 3 Test program. Group

Beam ID

Mix

Concrete type

Compressive strength f ’c (MPa)

Reinforcement ratio condition qf

G1

C01-GPC20 C02-GPC20 C03-GPC20 C04-GPC35 C05-GPC35 C06-GPC35 C07-GPC50 C08-GPC50 C09-GPC50 C10-OPC35 C11-OPC35 C12-OPC35

M07 M07 M07 M10 M10 M10 M03 M03 M03 M12 M12 M12

GPC

20

0.55 1.13 1.56 0.43 1.24 2.31 0.70 1.40 2.15 0.36 1.09 1.89

G2

G3

G4

35

50

OPC

35

qfb qfb qfb qfb qfb qfb qfb qfb qfb qfb qfb qfb

Fig. 3. (a) CFRP-RGPC beams (b) CFRP-ROPC beams.

observed, namely, tension, tension-compression, compression and debonding failure modes. The control specimens of each beam were tested at the same age of the tested beam. Three cylinders were tested according to ASTM C39 [30] for concrete compressive

strengths, three cylinders were tested according to ASTM C 496 for splitting tensile strengths [34], and three prisms were tested for flexural strength of concrete [35]. Furthermore, the elastic modulus of GPC was obtained by testing the three cylinders according

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Fig. 4. Test setup.

Table 4 Results of the tested Beams. Group

Beam ID

First crack load Pcr (kN)

Ultimate Load Pu (kN)

Deflection D (mm)

Failure Mode

G1

C01-GPC20 C02-GPC20 C03-GPC20 C04-GPC35 C05-GPC35 C06-GPC35 C07-GPC50 C08-GPC50 C09-GPC50 C10-OPC35 C11-OPC35 C12-OPC35

9.4 9.5 9.7 8.6 10.1 11.4 14.8 15.7 19.9 14.3 18.5 20.1

52.4 102.8 126.3 55.7 135.4 142.5 112.5 154.1 169.8 54.2 137.4 159.1

23.8 32.3 28.2 26.5 27.9 24.1 27.5 22.5 19.2 21.3 31.1 19.8

Tension Tension-Compression Compression-Debonding Tension Compression Compression Tension Compression Compression Tension Compression-Debonding Compression

G2

G3

G4

to ASTM C469 [36], and the density of each mix was calculated on the basis of cylinders and prisms. Table 5 shows the results of the control specimens. 4.2. Crack pattern and failure mode Figs. 5 and 6 show the typical failure modes of beams and the failure crack patterns for all the beams, respectively. The difference in crack patterns and failure modes was due to the reinforcement ratio, compressive strength of the concrete and types of concrete (namely, GPC and OPC). The prediction failure modes provided by ACI 440.1R-15 [15] satisfied with the experimental failure modes of the beams in this study by 66.67%, while the difference referred to the criticality of tension–compression failure mode, GPC type of concrete and the ultimate strain assumption of concrete by ACI 440.1R-15 [15]. Before loading, all the beam specimens were initially free of cracks. The first cracking of the CFRPRGPC and CFRP-ROPC beams was observed in the constant moment region exactly under the applied point load. The first cracking load in the CFRP-ROPC beams was slightly higher than that in the CFRP-RGPC beams due to a small difference in concrete compressive strength and concrete types. After the first cracking, new cracks were formed, and the crack width continued to enlarge with the applied load in CFRP-RGPC and CFRP-ROPC beams. At maximum load, more cracks formed in the CFRP-RGPC beams than those in the CFRP-ROPC beams because of the different types of concrete. The crack width in the CFRP-ROPC beams was also more extensive than that in the CFRP-RGPC beams. Fig. 6(a), (d), (g) and (j) respectively show that beams C01GPC20, C04-GPC35, C07-GPC50 and C10-OPC35 failed due to the

rupture of the FRP bars (tension failure). This type of failure occurred for beams with a reinforcement ratio condition of qf < qfb, and the failure location in beams C01-GPC20, C04-GPC35 and C07GPC50 was exactly in the midspan’ whereas that in beam C10OPC35 was under the point load location, and a few cracks and a higher value of crack width were also recorded. These cracks were mainly classified as vertical flexural cracks, which were perpendicular to the longitudinal axis of the beam. Fig. 6(b) present that beam C02-GPC20 failed due to the crushing of concrete under the load, which propagated instantaneously, followed by the rupture of the FRP rebar (tension-compression failure). Similarly, cracking was initiated when the applied moment reached the cracking moment. Cracking consisted of vertical cracks perpendicular to the direction of the principal tensile stress induced by pure moment. As the load increased, flexural cracks reached into the shear span. Fig. 6(e), (f), (h), (i) and (l) respectively illustrate that beams C05-GPC35, C06-GPC35, C08-GPC50, C09-GPC50 and C12-OPC35 failed due to the crushing of concrete on the top surface under the load (compression failure). The beams in this particular type of failure recorded a large number of narrow cracks compared with the other failure mode conditions. Fig. 6(c) and (k) respectively show that beams C03-GPC20 and C011-OPC35 failed due to the debonding of FRP. This failure was due to the debonding of FRP from the concrete at the bottom of the beam (cover part) and caused the propagation of horizontal cracks at the final stage of loading. This behaviour could be attributed to the localized failure of the CFRP bars at the cracks due to the sudden transfer of tensile forces from the fracturing concrete to the CFRP bars.

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H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185 Table 5 Results of control specimens. Group

Beam ID

Compressive strength f’c (Mpa)

Splitting tensile strength fct (Mpa)

Modulus of Rupture fr (Mpa)

Modulus of elasticity Ec (Mpa)

Density kg/m3

G1

C01-GPC20 C02-GPC20 C03-GPC20 C04-GPC35 C05-GPC35 C06-GPC35 C07-GPC50 C08-GPC50 C09-GPC50 C10-OPC35 C11-OPC35 C12-OPC35

23.33 22.66 24.92 32.15 33.87 30.74 44.66 47.49 45.41 34.62 34.62 34.62

1.814 1.984 2.012 2.384 2.431 2.273 3.552 4.456 4.084 2.892 2.892 2.892

3.39 3.65 3.59 4.18 4.22 3.97 4.93 5.33 5.53 3.57 3.57 3.57

25,223 22,669 25,726 23,488 29,894 27,798 29,582 32,272 29,582 28,534 28,534 28,534

2376 2286 2414 2441 2366 2390 2386 2393 2431 2489 2489 2489

G2

G3

G4

Fig. 5. Typical failure modes: (a) tension, (b) tension–compression, (c) compression and (d) debonding.

4.3. Load–deflection characteristics Fig. 7 shows the load-deflection curves at the midspan of the beam specimens. Generally, two important stages of behaviour were observed. First was a linear branch with a steep slope, which is related to the uncracked condition of the beam; this stage occurred when the cracking load was obtained, and a drop in the slope was observed due to the progressive cracking of the beam. The second was the settling of the cracking process, in which a nearly linear segment was observed until failure, and a sudden drop in the load occurred in every crack formation, which was more evident in load-deflection curves, especially for low reinforced beams. Even the number of cracks could be determined from the load-deflection curve instantly. In this work, all the beams were designed to fail in the flexure under the tensile mode, which was distinguished by the crack formation in the tensile stress zone. The FRP then ruptured under the compressive mode, which was characterised by the crushing of concrete.

4.3.1. Effect of reinforcement ratio Fig. 7 shows the load-deflection curves at the midspan of the beam specimens with the same concrete compressive strength and different reinforcement ratios. The reinforcement ratio affected the stiffness of the beam specimens, which appeared on their load-deflection behaviour. As expected, large deformations were obtained at low reinforcement ratios. By increasing qf from qf < qfb to qfb < qf < 1.4 qfb and then to qf > 1.4 qfb, the cracking load Pcr and ultimate load Pu increased compared with the beams with qf < qfb. The cracking load increased by 1.1% and 3.2% and the ultimate load increased by 96.2% and 141.1% for beams with f’c = 20 MPa and the GPC type, respectively (Fig. 7(a)). In addition, the cracking load increased by 17.5% and 32.6% and the ultimate load increased by 143.1% and 155.8% for beams with f’c = 35 MPa and the GPC type, respectively (Fig. 7(b)). Moreover, the cracking load increased by 6.1 and 34.5% and the ultimate load increased by 36.9% and

50.93% for beams with f’c = 50 MPa and the GPC type, respectively (Fig. 7(c)). Furthermore, the cracking load increased by 29.4% and 40.6% and ultimate load increased by 153.5% and 193.5% for beams with f’c = 35 MPa and the OPC type, respectively (Fig. 7(d)). Regarding the increasing percent in cracking and the ultimate load for both concrete types of beam specimens, CFRP-ROPC beams were more affected by the reinforcement ratio than CFRP-RGPC beams as noticed from the mentioned increasing percent earlier. 4.3.2. Effect of concrete compressive strength Fig. 8 shows that the load-deflection curves at midspan of the tested beam specimens with the same cross-section, bar number and concrete type would only vary in compressive strength, C01GPC20 vs C04-GPC35, C02-GPC20 vs C07-GPC50 and C03-GPC20 vs C05-GPC35. By increasing the concrete compressive strength, the deflection was decreased in the same corresponding load levels. The percentages of decrease varied on the basis of the reinforcement ratio. The ultimate load increased by 6.3% for beams with one bar (Fig. 8(a)), whereas the ultimate load increased by 9.4% and 7.2% for beams with two and three bars, respectively (Fig. 8(b) and (c)). The compressive strength had a significant effect on the first crack, especially when the compressive strength increased from 20 MPa to 50 MPa (Fig. 8(a)) wherein the first cracking load increased by 55.8%, while increasing the compressive strength from 20 MPa to 35 Mpa did not much affect the first cracking load. Beam C02-GPC20 failure mode was tensioncompression failure, and when the compressive strength increased to 50 MPa (Beam C07-GPC50), the failure mode became pure tension-failure, indicating that the higher compressive strength prevented the beam to fail in compression. 4.3.3. Effect of concrete type Fig. 9 shows the effect of concrete types (namely, GPC and OPC) for specimens with the same cross-section, compressive strength and reinforcement ratio, C04-GPC35 vs C10-OPC35, C05-GPC35 vs C11-OPC35 and C06-GPC35 vs C12-OPC35. The recorded

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H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185

(a) C01-GPC20

(b) C02-GPC20

(c) C03-GPC20

(d) C04-GPC35

(e) C05-GPC35

(f) C06-GPC35

(g) C07-GPC50

(h) C08-GPC50

(i) C09-GPC50

(j) C10-OPC35

Fig. 6. Crack pattern and failure mode.

deflection of GPC was slightly higher than that of the same corresponding load levels. However, the ultimate load-carrying strength of OPC was slightly higher (10.4%), especially for beams with qf > 1.4 qfb than that of beams with qf < qfb and qfb < qf < 1.4 qfb, wherein the load-carrying strength was approximately the same value at 2.7% with a 1.4% variance. 4.4. Crack width, number of cracks, concrete strain and FRP strain Fig. 10 shows the relation between crack width and number of cracks, and Table 6 lists the number of cracks, crack width, concrete strain and FRP strain. Higher crack width values were obtained for beams with low compressive strength, and CFRPROPC obtained higher crack width values than CFRP-RGPC beams. The relationship between the crack width and number of cracks

indicated that beams with a large number of cracks obtained a low value of crack width. Beams with a high reinforcement ratio obtained a large number of cracks and low values of crack width. As the ultimate loads reach the maximum limit for both CFRPRGPC and CFRP-ROPC beams, the differences in max crack width were observed. The number of cracks in the CFRP-RGPC beams C04-GPC35, C05-GPC35 and C06-GPC35 were 4, 9 and 13 respectively which were higher than that in the CFRP-ROPC beams C10OPC35, C11-OPC35 and C12-OPC35 with number of cracks 2, 6 and 9 respectively, indicating a good interaction between materials in one side and sufficient bond between CFRP bar and GPC in another side, same as reported by Maranan et al. [17]. Fig. 11 shows the load–strain curves for different failure modes, wherein the first cracking and crack propagation could be visualised on the curves easily. For beam C01-GPC20, the strain in

H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185

C01-GPC20 C02-GPC20 C03-GPC20

Load (kn)

180 160 140 120 100 80 60 40 20 0 0

5 10 15 20 25 30 35

C04-GPC35 C05-GPC35 C06-GPC35

180 160 140 120 100 80 60 40 20 0

In this study, the theoretical flexural strength Mu of CFRP-RGPC and CFRP-ROPC beams were calculated based on the equations provided by ACI 440.1R-15 [15], CSA S806-12 [28] and proposed method, then compared with the experimental flexural strength Mu, Exp. 0

Load (kn) 0

5 10 15 20 25 30 35

Deflection (mm)

5 10 15 20 25 30

Deflection (mm) (b) Group G2

C07-GPC50 C08-GPC50 C09-GPC50

For beam C03-GPC20, neither strains reached the ultimate strain, strains were (0.0096 and 0.0031) in FRP bar and GPC respectively, thereby indicating failure by debonding of the FRP bar. 5. Theoretical prediction

Deflection (mm) (a) Group G1 200 180 160 140 120 100 80 60 40 20 0

9

200 180 160 140 120 100 80 60 40 20 0

C10-OPC35 C11-OPC35 C12-OPC35

5.1. Equations provided by ACI 44.1R-15 and CSA S806-12 On the basis of ACI 440.1R-15 [15], the flexural strength of a concrete beam reinforced with FRP bars can be calculated based on strain compatibility, internal force equilibrium and control of the failure mode (tension or compression failure). The predicted failure modes can be determined by comparing the following reinforcement ratio qf (Eq. (1)) to the balanced reinforcement ratio qfb (Eq. (2)) which indicates the rate of concrete crushing and FRP rupture.

qf ¼ 0

5 10 15 20 25 30

Deflection (mm)

Af bd

ð1Þ

where Af is the area of the FRP bar, b is the width of the rectangular cross-section and d is the distance measured from the extreme compression fibre to the centroid of FRP bars. 0

Fig. 7. Effect of the reinforcement ratio on load–deflection curve.

the CFRP bar (0.013) nearly reached the ultimate strain whereas that in the GPC (0.0023) did not reach the ultimate strain, thereby indicating tension failure. For beam C02-GPC20, both strains in CFRP bar and GPC were (0.0133 and 0.0027) respectively, and both were near the ultimate strain, thereby indicating tensioncompression failure. For beam C05-GPC35, the strain in the GPC (0.0034) nearly reached the ultimate strain, whereas that in the FRP bar (0.0085) did not, thereby indicating compression failure.

qfb ¼ 0:85b1

Ef ecu fc f fu Ef ecu þ f fu

ð2Þ

where f’c is the specified compressive strength of concrete, ffu is the ultimate tensile stress of FRP bars, Ef is the elastic modulus of the FRP and єcu is the ultimate strain in concrete equal to 3%. Factor b1 can be calculated as follows: 0

b1 ¼ 0:85  0:05

f c  28  0:65 7

ð3Þ

Firstly, if qf > qfb, then the beam is considered over-reinforced, the controlling limit state is the crushing of concrete, and the flexural strength Mu can be calculated as follows:

Fig. 8. Load–deflection curve: Effect of compressive strength on beams with (a) one, (b) two and (c) three bar(s).

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H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185

Fig. 9. Load–deflection curve: Effect of concrete type.

compression failure). Finally, if qf < qfb, then the beam is considered under-reinforced, wherein the controlling limit is the FRP rupture, and Mu can be estimated using as follows:

  b c Mu;ACI ACI ¼ Af f fu d  1 2 b1 c ¼

Fig. 10. Relationship between the crack width and number of cracks.

M u;ACI ¼ qf f f bd

2

1  0:59

qf f f f

! ð4Þ

0

ð6Þ

Af f f 0:85f 0 c b

ð7Þ

where c is the distance measured from the extreme compression fibre to the neutral axis. On the basis of CSA S806-12 [28], the FRP-reinforced concrete beams should be designed, such that the controlling limit state is the crushing of concrete. єcu is equal to 3.5% and also depends on strain compatibility and internal force equilibrium. The balanced reinforcement ratio qfb and factors a1 and b1 can be calculated by using the following equations, respectively:

c

0

where ff is the tensile strength of the FRP bars, which can be calculated as follows:

qfb ¼ a1 b1

Ef ecu fc f fu Ef ecu þ f fu

ð8Þ

0

a1 ¼ 0:85  0:0015f c  0:67

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðEf cu Þ2 0:85b1 f 0 c ff ¼ Ef ecu  0:5Ef ecu  f fu þ 4 qf

ð5Þ

Secondly, if qfb < qf < 1.4 qfb, then this particular condition is described by using a transition zone, wherein the crashing of concrete is followed by the rupture of the FRP bar (tension-

b1 ¼ 0:85  0:0025f

0

c

ð9Þ

 0:67

ð10Þ

The flexural strength Mu, stress in the FRP bar ff and the neutral axis c can be calculated by using the following equations, respectively:

Table 6 Crack width, number of cracks, Concrete strain and CFRP strain. Beam ID

Crack Width (mm)

Number of Cracks

Concrete Strain (mm/mm)

FRP Strain (mm/mm)

C01-GPC20 C02-GPC20 C03-GPC20 C04-GPC35 C05-GPC35 C06-GPC35 C07-GPC50 C08-GPC50 C09-GPC50 C10-OPC35 C11-OPC35 C12-OPC35

4.44 2.78 1.83 4.05 2.22 1.15 3.59 1.64 1.23 5.25 2.64 1.54

5 7 9 4 9 13 6 11 13 2 6 9

0.0023 0.0027 0.0031 0.0023 0.0034 0.0032 0.0019 0.0029 0.0031 0.0015 0.0026 0.0029

0.0130 0.0133 0.0096 0.0121 0.0085 0.0070 0.0141 0.0090 0.0059 0.0138 0.0100 0.0061

11

H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185

Fig. 11. Load–strain curves for different failure modes.

M u;CSA ¼ qf f f bd

2

f f ¼ Af Ef

1

ecu ðd  cÞ c

0

a1 b1 f c bc  Af Ef

qf f f 0 2a1 f c

!

< f fu

ecu ðd  cÞ c

¼0

2 ¼ ecu 1  3

ð11Þ

ec

ð12Þ

ct ¼

ð13Þ

ct ¼ 

fc ¼ It is established fact that the rising part of the stress-strain data for concrete material closely approximates a parabola [37], set of equations derived for predicting the flexural strength (Mu, Proposed) of CFRP-RGPC and CFRP-ROPC beams based on the parabolic stress block, strain compatibility and internal force equilibrium. Fig. 12(a) shows the balanced condition of compressive and tensile forces, the balanced reinforcement ratio qfb can be found based on the following derived equation:

qfb ¼

0:75cb f 0 c df fu

ð14Þ

where cb is the depth of the neutral axis when the maximum compressive strength achieved and can be calculated using the linear distribution of strains with respect to the neutral axis as follows:

cb ¼

ecu d ; ecu þ efu

ð15Þ

where єfu is the ultimate strain of CFRP bar. Fig. 12(b) shows the stress-strain distributions of the proposed approach in a section at tension failure. The criteria of tension failure assume FRP ruptures before concrete crushing, such a section is said to be under-reinforced, and this means that the strain in the FRP will be єfu and the corresponding strain єc at the extreme compressive fibre will be less than the ultimate compressive strain єcu. Strain at top єc, depth of the neutral axis ct and compressive strength fc corresponding to the top concrete strain єc can be calculated based on the following derived equations:

ð16Þ

ec d ; efu þ ec 3f 0 c ec 2 4ecu 2

5.2. Proposed method

sffiffiffiffiffiffiffiffiffiffiffiffiffiffi! f 1  0c ; fc

ð17Þ

q dffu hf   i; 2 eecuc  1

ð18Þ

 0 2     3f c ec ecu 4  3 ; 4ecu 2 ec

ð19Þ

The distance between the centroid of parabolic stress block and the neutral axis defined by x’, which can be determined based on the derived equation as follows:



 3  2 4 3 4ðeecu Þ 0 i; x ¼  0 c 2 h   3f c ct ec ecu 2 ec  1 4e 2 3f 0 c ct 2



4ðeecu c Þ

ð20Þ

cu

By taking the moment about the centroid of the compressive force, the proposed flexural strength Mu, Proposed for tension failure of CFRP-RGPC and CFRP-ROPC beams can be calculated based on the following derived equation:

0

Mu;Proposed ; ¼ Af f fu d  ct  x

ð21Þ

Fig. 12(c) shows the stress-strain distributions of the proposed approach in a section at compression failure. The criteria of compression failure assume that concrete is crushed before FRP rapture, which means that the ratio of FRP in compression failure is higher than the balanced ratio (over-reinforced). The same philosophy as the balanced case is used in compression failure for calculating the FRP tensile strength ff which is less than the ultimate strength ffu and the distance from the extreme concrete fibre of maximum strain to the neutral axis at compression cc, respectively:

ff ¼

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 qf dEf ecu þ qf dEf ecu þ 3qf d2 f 0 c Ef ecu 2qf d

 f fu ;

ð22Þ

12

H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185

(a) Balanced failure

(b) Tension failure

Fig. 12. Stress and strain distribution for the proposed method.

cc ¼

Ef ecu d ; Ef ecu þ f f

ð23Þ

By taking the moment about the centroid of the parabolic compressive force, the proposed flexural strength Mu, Proposed for compression failure of CFRP-RGPC and CFRP-ROPC beams can be determined based on the following derived equation:

M u;Proposed ; ¼ Af f f ðd  0:417cc Þ

ð24Þ

5.3. Comparison of experimental results with theoretical predictions The predicted flexural strength was calculated on the basis of (Mu, ACI) of ACI 440.1R-15 [15], (Mu, CSA) of CSA S806-12 [28] and

(Mu, Proposed) of the proposed method. Table 7 compares the experimental flexural strength (Mu-Exp) results. The prediction formulas provided by ACI 440.1R-15 [15] and CSA S806-12 [28] underestimated the flexural strength of beams by 64.3% and 65.8%, respectively, while the proposed method predicted more accurately 76.9%. CSA S806-12 [28] predicted high compressive strength values more accurately by 72.9% in comparison with ACI 440.1R-15 [15] (66.3%), while the proposed method predicted more accurately 83.9%, whereas the prediction values were significantly affected by the low reinforcement ratio, especially for qf < qfb. Beams C01-GPC20, C04-GPC35, C07-GPC50 and C10-OPC35 did not satisfy the limit state condition (concrete crushing) provided by CSA S806-12 [28].

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H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185 Table 7 Comparison of theoretical prediction and experimental results. Flexural Capacity

Comparisons %

Group

Beam ID

Mu-Exp (kN.m)

Mu, ACI (kN.m)

Mu, CSA (kN.m)

Mu, Proposed (kN.m)

Mu,

G1

C01-GPC20 C02-GPC20 C03-GPC20 C04-GPC35 C05-GPC35 C06-GPC35 C07-GPC50 C08-GPC50 C09-GPC50 C10-OPC35 C11-OPC35 C12-OPC35

20.96 41.12 50.52 22.28 54.16 57.00 45.00 61.64 67.92 21.68 54.96 63.64

14.46 23.50 29.00 14.66 33.51 36.64 28.85 40.55 46.89 14.69 35.85 40.92

N/C 24.43 30.09 N/C 35.45 38.11 N/C 44.12 50.35 N/C 35.85 40.67

14.34 27.75 38.27 14.50 41.62 50.83 28.69 53.45 68.69 14.55 41.62 54.22 Average

69.0 57.1 57.4 65.8 61.9 64.3 64.1 65.8 69.0 67.8 65.2 64.3 64.3

G2

G3

G4

ACI/Mu-Exp

61.2

64.0

66.3

65.8

Mu,

CSA/Mu-Exp

N/C 59.4 59.6 N/C 65.5 66.9 N/C 71.6 74.1 N/C 65.2 63.9 65.8

59.5

66.2

72.9

64.6

Mu,

Proposed/Mu-Exp

68.4 67.5 75.8 65.1 76.9 89.2 63.8 86.7 101 67.1 75.7 85.2 76.9

70.5

77.0

83.9

76.0

Note: N/C: not-calculated because CSA S806-12 [28] considers (concrete crushing) as the only controlling limit state.

Flexural Strength, kN.m

ACI 440.1R-15

CSA S806-12

Proposed

Experimental

70.00 60.00 50.00 40.00 30.00 20.00 10.00 0.00

Fig. 13. Graphical comparison of predicted and experimental flexural strength of beams.

Fig. 13 shows a graphical comparison of the flexural capacities provided by ACI 440.1R-15 [15], CSA S806-12 [28], the proposed method and the experimental results. 6. Conclusions The flexural strength and behaviour of CFRP-RGPC and CFRPROPC beams were investigated by using a four-point bending test. On the basis of the experimental outcomes, the following conclusions can be drawn.
Four different types of failure were observed, namely, tension failure for beams reinforced with qf < qfb, tension–compression failure, compression failure for beams reinforced with qf > qfb and debonding failure. Furthermore, the prediction failure modes provided by ACI 440.1R-15 satisfied with the experimental failure modes of the beams in this study by 66.67%.
The reinforcement ratio affected the stiffness of the beam specimens. Consequently, the beams with low reinforcement ratio recorded significant deformation and the ultimate load enhancement (17.5–155.8%) was recorded with increasing reinforcement ratio with respect to their load-deflection behaviour, as expected.
The compressive strength had a significant effect on the first crack. The first cracking load increased with the compressive strength (1.1–40.6%), and the lower compressive strength had more deflections compared with higher compressive strength.


The CFRP-RGPC beams had more deflection than CFRP-ROPC beams at the same corresponding load level. The ultimate load-carrying capacity of CFRP-ROPC beams was slightly higher (10.4%) especially for beams with qf > 1.4 qfb; however, for beams with qf < qfb and qfb < qf < 1.4 qfb, the load carrying capacity was approximately the same value at 2.7% with a 1.4% variance.
The crack width recorded high values for beams with low compressive strength, and the beams with a large number of cracks recorded a low value of crack width.
The number of cracks in the CFRP-RGPC beams was higher than that in the CFRP-ROPC beams, and the beams with higher reinforcement ratios obtained a larger number of cracks and lower crack width values. Thus, the CFRP-ROPC beams obtained a higher value of crack width compared with CFRP-RGPC beams.
The prediction equations provided by ACI 440.1R-15 and CSA S806-12 underestimated the flexural strength of the beams by 62.5% and 66.1%, respectively, while the proposed method which based on parabolic stress block predicted more accurately 76.9%.
Based on the findings of this study, the following suggestions and recommendations may be considered for future studies: studding the flexural behaviour of geopolymer concrete beams with different types of FRP bars (GFRP and AFRP), shear strength and behaviour of GPC beams reinforced with FRP bars and because of the difference in property of GPC and OPC, more comparison could be established in future studies.

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H.Q. Ahmed et al. / Construction and Building Materials 231 (2020) 117185

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