Lightweight aggregate concrete fiber reinforcement – A review

Construction and Building Materials 37 (2012) 452–461

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

Review

Lightweight aggregate concrete fiber reinforcement – A review Mahmoud Hassanpour a,⇑, Payam Shafigh b, Hilmi Bin Mahmud b a b

Department of Civil Engineering, Ghaemshahr Branch, Islamic Azad University, Ghaemshahr, Iran Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

h i g h l i g h t s " The addition of fibers on the properties of different types of LWACs was reviewed. " Generally, the inclusion of fibers in LWAC improves its mechanical properties. " Even very low volume fractions of steel fiber prevent brittle failure of LWAC. " The effectiveness of fiber in LWAC is more pronounced than NWC. " A combination of steel fiber and non-metallic fibers results in better toughness.

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 11 February 2012 Received in revised form 8 June 2012 Accepted 21 July 2012 Available online 5 September 2012

The higher brittleness and lower mechanical properties of lightweight aggregate concrete (LWAC) compared to normal weight concrete (NWC) at the same compressive strength has prevented it from being widely used in the construction industry despite its many advantages. Studies have shown that the use of fibers in LWAC is an appropriate solution to resolve such problems. This paper reviews the influence of the addition of fibers on the properties of different types of LWAC. These properties include the workability, compressive strength, stress–strain behavior, tensile strength, modulus of elasticity, and compressive and flexural toughness. Generally, the inclusion of fibers in LWAC, as single or hybrid forms, improve its mechanical properties, and significantly increase its toughness, ductility performance and energy absorption, while decreasing its workability, particularly when steel fiber is used in the concrete mixture. In the case of splitting tensile and flexural strengths, the effectiveness of fiber in LWAC is more pronounced than NWC. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Lightweight concrete Fiber Mechanical properties Toughness Ductility

Contents 1. 2. 3. 4. 5. 6. 7. 8. 9.

Introduction . . . . . . . . . . . Workability. . . . . . . . . . . . Density . . . . . . . . . . . . . . . Compressive strength. . . . Stress–strain diagram. . . . Modulus of elasticity . . . . Splitting tensile strength . Flexural strength . . . . . . . Conclusions. . . . . . . . . . . . References . . . . . . . . . . . .

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⇑ Corresponding author. Tel.: +98 9111130800; fax: +98 1113226777. E-mail addresses: [email protected] (M. Hassanpour), pshafi[email protected] (P. Shafigh), [email protected] (H.B. Mahmud). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.07.071

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M. Hassanpour et al. / Construction and Building Materials 37 (2012) 452–461

1. Introduction Concrete is a widely used material throughout the world. Huge quantities of various types of concrete are used annually. Because of the extensive usage of this material, many researchers are investigating the engineering properties of this material. The enhancement properties of concrete in fresh and hardened states, durability and its environmental impact are very interesting topics for research. One method to increase some engineering properties of concrete is the use of fibers as an additional basic material in the concrete mixture. The fiber can be made from natural material such as asbestos, sisal and cellulose or a manufactured product such as glass, steel, carbon and polymer [1]. The use of fibers to reinforce a brittle material can be traced back to Egyptian times when asbestos fiber was used to reinforce clay pots about 5000 years ago [2]. However, the modern development of fiber reinforced concrete in the concrete industry may have begun around the early 1960s [3]. The most beneficial characteristics of fiber-reinforced systems are those of increased flexural capacity, toughness, post-failure ductility and crack control [4]. In addition, it has been reported [5] that fiber reinforcement in concrete significantly increases the compressive ductility, toughness and energy absorption at early ages. Fibers are categorized as metallic, polymeric or natural [7]. Among the various types of fibers, steel fiber is the most commonly used for most structural and non-structural purposes [2,8]. This is followed by polypropylene (PP), glass and other fibers, however, these are not commonly used for structural concrete applications [2]. The reasons for the greater usage of steel fiber include economics, manufacturing facilities, reinforcing effects and resistance to environmental aggressiveness [9]. Concrete, like glass, is brittle, and hence has a low tensile strength and shear capacity [10–12]. An increase in the strength of concrete causes an increase in its brittleness [13,14] which makes the concrete very susceptible to cracking. This cracking creates easy access routes for deleterious agents leading from early saturation, freeze–thaw damage, scaling, discoloration and steel corrosion [15]. The low cracking potential of concrete in the early stages of hydration and in-service life is desirable for designing a durable structure. It has been reported that in fiber reinforced concrete the crack width and crack spacing reduce, especially at early ages [16,17]. Fiber reinforced concrete shows better durability in service than concrete without fiber as fibers limited spreading of cracks inside the concrete [18–22]. The type of fiber and its volume fraction has a marked effect on the properties of fiber reinforced concrete. The fiber-reinforced composites can be classified as a function of their fiber volume fraction such as low (<1%), moderate (between 1% and 2%) and high volume fraction (greater than 2%) [2]. It has been reported that adding steel fiber into concrete in the amount of 1–1.5% by volume increases its tensile strength by up to 100%, flexural strength by up to 150–200% and the compressive strength increases by 10–25% [8]. The fiber induces a homogeneous stress distribution in the concrete, which causes better exploitation of the high strength matrix [23]. Furthermore, the addition of steel fibers improves the impact strength and toughness [24] and transforms concrete from a brittle to a more ductile material [25,26]. Steel fiber concrete has much higher fracture energy than plain concrete [27]. Dhakal et al. [28] reported that both the compressive strength and the strain corresponding to peak stress increase with the addition of steel fibers. Furthermore, the maximum compressive strain of steel fiber concrete is higher than plain concrete. In the case of tensile strength, it was reported [29] that with the same type and volume of steel fiber, the improvement is much more for LWAC than NWC. This is due to higher brittleness of LWAC than NWC [30].

453

Nevertheless, despite the many advantages of adding steel fiber to concrete, this fiber has certain disadvantages, particularly the reduced workability of fresh concrete [31] and because of its high specific gravity, it can increase the dead load of a composite [32]. In addition, fiber reinforced concrete mixtures need more mixing and placing time than plain concrete [23,33]. The weight of concrete structures is large compared to the imposed load they can carry. With the rapid development of very tall buildings, large-size and long-span concrete structures, structural lightweight concrete (LWAC) with different types of LWA has been widely investigated and successfully developed and used in recent years [34–39]. The application of structural lightweight concrete in the construction industry has many advantages, such as a high strength/weight ratio, savings in dead load for structural design and foundation, reduces the risk of earthquake damage to a structure, good tensile strain capacity, superior heat and sound insulation characteristics, low coefficient of thermal expansion and better durability [33,38,40–42]. Nevertheless, some problems in its engineering properties have prevented it being widely used in the construction industry in load bearing structural members [7,43]. The brittleness of lightweight concrete is higher than normal weight concrete (NWC) for the same mix proportion and compressive strength [29,30]. Furthermore, generally, the mechanical properties of LWAC are lower than NWC [7,44–46]. Recently, high strength lightweight aggregate concrete (HSLWAC) with a compressive strength of 50–100 MPa has been successfully produced with several types of lightweight aggregate [43,47–49] with much better mechanical properties than normal strength LWAC. However, an increase in the concrete strength causes further brittleness of the concrete in compression and tension [13,50,51], especially in the case of LWAC [48]. One way to resolve the brittle texture of LWAC and its low mechanical properties is the use of fibers, such as steel, polymer, glass, carbon and hybrid fibers [7,52–56]. The literature on fiber reinforced LWAC shows that most of the research focused on the use of steel fibers as single or combined with nonmetallic fibers in LWACs. This study provides a brief summary of previous researches on the effect of fibers on the properties of LWAC in its fresh and hardened state. 2. Workability The workability of concrete is chosen with respect to site compaction by vibration. In the case of LWAC the lighter mix causes a lower slump, because of the influence of gravity on the slump value [57]. It has been reported [2] that a 50–75 mm slump may be sufficient for good workability of LWAC. The high slump value of LWAC causes floating of the coarse aggregate and the heavier mortar away from the surface which may lead to finishing problems. Therefore, ACI 213R-87 recommends a maximum of 100 mm slump for achieving a good surface in floors made with LWAC [2]. The inclusion of fibers into concrete has a negative effect on the workability of fresh concrete [58–61]. The effect of fiber on the fresh properties of normal weight self-compacting concrete (SCC) and lightweight self-compacting concrete (SCLC) also shows that the inclusion of fibers into self-compacting concretes has a remarkable negative effect on the fresh properties [6,62]. The degree of decreasing workability depends on the type and content of the fibers used. For achieving a fiber reinforced concrete with good selfcompacting characteristics, it has been suggested that the amount of paste increases with the increasing cement content, thereby increasing the fine aggregate content or using pozzolanic admixtures [6]. Chen and Liu [51] reported that at a fixed volume content of fibers for expanded clay HSLWAC, different types of fibers have different effects on the slump and slump flow. Among the three types

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of fiber, namely polypropylene, steel and carbon, polypropylene fiber showed the lowest effect and steel fiber showed the highest effect on reducing the slump value. Furthermore, when the slump loss was small in the concrete without fibers, concrete with steel fiber showed a significant reduction on slump value with time. Usually, a superplasticizer is used for increasing the workability and reducing the balling effect of fiber in the fiber concrete mixes [61,63]. Campione et al. [64] reported that for pumice and expanded clay LWACs reinforced with steel fiber, good workability of the mixtures was obtained by adding 1.5% of superplasticizer by cement weight. The maximum volume fraction of fiber in their study was 2%, having a length of 30 mm and an aspect ratio of 60. Experimental studies [30] have shown that even for low steel fiber content (<1%), when the steel fiber content is over 0.4%, a higher dosage of superplasticizer is needed in order to ensure the appropriate workability for mixtures and to prevent the concrete mixture from blocking on fibers. In addition, in the case of PP fiber used in expanded slate LWAC, the dosage of superplasticizer increases with increasing amount of fiber in the mixture [65]. For achieving better workability for a mixture containing steel fiber, Olivito and Zuccarello [66] pointed out that an improvement of steel fiber mixtures can be obtained by increasing the fine aggregate content or adding fluidifying additives. In a study [58] it has been shown that in NWC the use of fly ash as a replacement for cement compensates for the decreased workability of fiber reinforced concrete. In this study it was shown that although fibers, such as steel and plastic polypropylene, caused a 2–8% decrease in the workability of the concrete, with the use of a certain percentage of fly ash (FA) in the mixture the workability remained constant. Therefore, similar to that of NWC, the use of FA in fiber reinforced LWAC compensates the reduction in workability. Generally, to ensure good workability for fiber reinforced concrete, the use of low dosages of fibers is recommended [67]. For many fibrous mixes with a low or even zero slump value, the vibration of such concrete is satisfactory. Therefore, the traditional slump test is not generally suitable to evaluate the workability of fiber reinforced concrete, and, therefore, alternative workability test methods should be used [4,58]. Edgington et al. [4] reported that among the three standard workability tests, namely slump, V–B and compacting factor, the V–B test is the best when fiber reinforced concrete is subjected to compaction by vibration. A V–B time in the range of 3–10 s represents adequate workability for placement by vibration [68]. In addition, another test method for assessing the workability of fiber reinforced concrete under vibration is the inverted slump cone test. According to ASTM C 995 [69] with an internal vibration into inverted slump cone, the time of passing of all fiber-reinforced concrete from the slump cone will be recorded. The range of 8–30 s is appropriate for placement by vibration. Times of greater 30 s show that the concrete will be very difficult to place. For inverted cone, flow times of less than 8 (s) using the traditional slump test may be the most practical alternative for such concrete. However, both the inverted slump cone and V–B test results correlate closely [68]. Libre et al. [70] used the inverted slump cone for evaluation of the workability of fiber pumice LWAC. The cement content, the water to cement ratio, the superplasticizer percentage and the weight ratio of coarse lightweight aggregate to natural river sand were 475 kg/m3, 0.3%, 0.7% and 0.56%, respectively. The slump value for this concrete was in the range of 15–20 mm without any fibers. They used end-hooked steel fiber with 35 mm length and 0.55 mm diameter. Steel fibers with volumes of 0%, 0.5% and 1% and PP fibers with volumes of 0%, 0.2% and 0.4% were used individually and combined. The inverted slump test for all fiber-reinforced mixtures showed a flow time in the range of 45–120 s. Inclusion of PP fiber has less effect on workability. Each mixture including steel fiber showed significantly low workability. For achieving better

workability for steel fiber reinforced LWAC, use of an air-entraining admixture in addition to a superplasticizer are recommended [55]. One of the main characteristics of a self-compacting concrete is that it has superior performance in the fresh state. Although the addition of fibers to the concrete reduces the workability, the production of fiber reinforced self-compacting concrete was reported as being feasible [71,72]. In the case of LWAC, recently, the successfully production of self-compacting LWAC with different types of lightweight aggregates were reported [73–76]. However, the application of fibers for making a cohesive fiber reinforced self-compacting LWAC needs to be investigated further. 3. Density Among all the types of fibers, steel fiber is the most commonly used for improving the mechanical properties of LWAC [30]. Adding steel fiber to LWAC increases its density. This is because this fiber has high specific gravity. The test results showed that usually a higher dosage of steel fiber causes heavier lightweight concrete [33,50,52]. Because the most general-use concrete has a compressive strength between 20 and 40 MPa [8], achieving compressive strength in this range for LWAC is not difficult and is applicable in the construction industry. Therefore, one of the main disadvantages of using steel fiber in LWAC is increasing the density. This phenomenon should be considered by designers, especially when moderate or high volume (>1%) steel fiber is used in LWAC. Therefore, it is recommended that in the case of steel fiber, low fiber content (1% by volume or less), with or without other types of fibers that do not significantly affect the density (e.g., polypropylene, etc.) should be used in LWAC. The test results of a study conducted by Gao et al. [43] showed that by inclusion of rectangular steel fiber with different aspect ratios (46, 58 and 70) and different volume fraction (0%, 0.6%, 1%, 1.5% and 2%) in an expanded clay HSLWAC, the density was not affected by the aspect ratio of steel fiber, but was mainly affected by the volume fraction. A similar conclusion can be derived from the test results of Khaloo and Sharifian [50] for different aspect ratios (42, 47 and 57). The test results of previous studies [33,43,50,59,70,67,78] showed that the inclusion of steel fiber into structural lightweight aggregate concrete in volume fractions of 0.25%, 0.5%, 0.75%, 1%, 1.5%, 2% and 3% increased the density on average of 1.1%, 3%, 3.3%, 4.6%, 5%, 5.4% and 7.5%, respectively. Gao et al. [43] suggested the following equation for estimating the density of steel fiber concrete:

Dc ¼ Dm ð1  V f Þ þ Ds V f

ð1Þ

where Dc is the density of steel fiber reinforced HSLWAC; Dm is the density of HSLWAC; Ds is the density of steel fiber and Vf is the fiber volume fraction in the range of 0–2%. It was reported that the use of mineral admixtures such as silica fume and fly ash, in LWAC could reduce the density further [79]. Therefore, to compensate for the negative effect on the density of LWAC by adding steel fiber, it is recommended that such additives be used in the LWAC mixture. The results of the tests conducted by Koksal et al. [60] showed that a high strength concrete with 15% silica fume as additive is 2% lighter than the reference concrete (concrete without silica fume). By adding 1% volume fraction of steel fiber (with an aspect ratio of 80 and density of 7.85 g/cm3) to this concrete the density of the concrete was still lighter (about 1%) than the reference concrete while the steel fiber concrete without silica fume resulted in a higher density of about 1%. Furthermore, the use of an air-entraining admixture in LWAC mixtures can be used for reducing the density and also improving the workability [80]. In addition, Swamy and Jojagha [81] suggested the use of water-reducing-plasticizer admixtures, along with PFA to re-

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lease the interlocking friction between the fibers and aggregates in steel fiber lightweight and normal weight concrete with poor workability. Compared to steel fiber, non-metallic fibers are much lighter. For example, the density of carbon and PP fiber is approximately 80% and 88% less than that of steel fiber, respectively [51]. Therefore, in structural lightweight aggregate concrete instead of using a high volume fraction of steel fiber it is suggested that a low volume fraction of steel fiber with a non-metallic fiber (hybrid fiber) be used. The test results showed that a LWAC with 0.5% volume fraction of steel fiber (by length (L) of 35 mm and diameter (D) of 0.55 mm) and 0.2% of PP fiber (L = 12 mm and D = 0.016 mm) is about 3.2% lighter than such LWAC with 0.5% of steel fiber while the engineering properties of both fiber reinforced concretes were comparable [70].

4. Compressive strength Among the various types of fibers, the effect of steel and PP fibers has been researched in respect of the properties of LWAC. In most cases, it was reported that although the steel fiber increases the compressive strength of LWAC the increase is not significant and that the PP fiber has no effect on the compressive strength of LWAC. The highest increase in compressive strength for steel fiber sanded LWAC was reported by Libre et al. [70]. They reported that adding 0.5% volume of steel fiber (L = 35 mm and D = 0.55 mm) to natural pumice LWAC decreased its workability significantly and increased its compressive strength by up to 60% while with the addition of 1%, the compressive strength only increased by up to 50%. The decrease in the compressive strength for higher levels of steel fiber may be due to the difficulty in dispersing the fiber and the concrete not being fully compacted [43]. Campione et al. [82] investigated the effect of steel fiber (L = 30 mm and aspect ratio = 60) on the properties of LWAC made of expanded clay (with maximum grain size of 12 mm) in monotonic and cyclic loads. They reported that when this LWAC (of grade 20) was reinforced by steel fiber in volume fractions of 0.5%, 1% and 2%, its compressive strength increased by about 22%, 29% and 38%, respectively in monotonic load and by about 23%, 23% and 41%, respectively, in cyclic load. Furthermore, increases of up to 30% [64], 22% [43], 21% [33], 20% [83] and 14% [78] for compressive strength of steel fiber reinforced LWAC with good workability have also been reported. However, several authors reported that adding steel fiber to HSLWAC does not affect the compressive strength [29,84], or, that it even has a negative effect [77]. The test results reported by Balendran et al. [29] have shown that a sanded LWAC made with sintered pulverized fuel ash (Lytag) LWA with 28-day compressive strength of 90 MPa and density of 2015 kg/m3, when reinforced with 1% volume fraction of straight steel fibers (L = 15 mm and D = 0.25 mm), the compressive strength changed to just 91 MPa while its tensile strength increased significantly. Kayali et al. [52] investigated the effect of steel fiber (L = 18 mm and aspect ratio = 37.5) on a HSLWAC made of coarse and fine Lytag LWAs with a 28-day compressive strength of 65 MPa. To improve the workability and reduce the harshness of this LWAC, they replaced 25% of the fine LWAs with fly ash. The test results showed that steel fibers at 0.56%, 1.13% and 1.7% by volume of the concrete caused a decrease in the compressive strength, on average, of about 5.7%. A reduction in the compressive strength through adding steel fiber was also observed in NWCs [85,86]. For example, adding 2% volume fraction of steel fiber (L = 60 mm and D = 0.8 mm) to a high strength normal weight concrete (HSC) with a 28-day compressive strength of 56 MPa, reduced the

strength by up to 41%. However, by adding 5% and 10% silica fume to this mixture, the reduction decreased to 23% and 85%, respectively [85]. In most cases, the reason for decrease in the compressive strength is that the dispersion of fiber, especially in high volume fractions is very difficult and consequently, causes poor workability and incomplete compaction [43,87]. Domagala [30] pointed out that the type of test specimen has an effect on the compressive strength of steel fiber reinforced concrete. He showed that a LWAC made with sintered fly ash as coarse aggregate, when reinforced by 0.6% volume fraction of a steel fiber (L = 50 mm and D = 0.75 mm) showed three different increases in the compressive strength of about 3.6% for 150-mm cube, 4.7% for 100-mm cube, and 7.0% for 150 mm  300 mm cylinder mold, which reveals that the cylinder specimens have higher compressive strength than the cubic specimens for the same fiber reinforced concrete. However, contrary to this report, Topcu and Canbaz [58] showed that for normal weight, normal strength concrete containing fly ash, a significant increment in the compressive strength of the steel fiber reinforced concrete was observed (up to 95%) for the cube specimens, while the same concrete showed a slight increase (up to 13%) in the compressive strength when it was tested for the cylinder specimens. The test results of Compione et al. [64] showed that the effectiveness of steel fiber on the compressive strength of LWAC strongly depend on the aggregate type. They reported that the incorporation of steel fibers into the matrix of expanded clay LWAC showed an increase in the compressive strength of up to 30%, while in the case of pumice stone LWAC the variation in the strength was negligible. They demonstrated that compared to pumice stone LWA (irregular in shape), the surface of the aggregate in contact with the steel fibers decreases in the case of expanded clay (round and regular in shape). Therefore, a higher strength was observed for expanded clay LWAC. It should be noted that the effectiveness of fibers would be more in composites when they are free from aggregate interference [88], which causes better fiber–mortar interfaces. In addition, data from a study conducted by Duzgun et al. [33] show that the effectiveness of steel fiber on the compressive strength of a LWAC strongly depends on the amount of LWA in the concrete. They substituted 25%, 50%, 75% and 100% of normal coarse aggregate by pumice LWA, which produced four LWACs with densities of approximately 2030, 1840, 1630 and 1450 kg/ m3, respectively. Table 1 shows the increase in the percentage of compressive strength of these LWACs by adding steel fiber (L = 60 mm and D = 0.8 mm) in different volume fractions. As can be seen in this table, it is clear that the effectiveness of fiber on LWAC with higher amounts of LWA (lighter LWAC or LWAC with lower strength) is higher for all percentages of steel fibers. The effect of low volume content of steel fiber (61%) on grade 35 oil palm shell LWAC in different curing conditions showed that in continuous moist curing, the rate of strength gain was greater as the age increased, especially for concretes with higher steel fiber content. This may be due to the better bond between the fiber and the matrix at later ages [78]. Furthermore, it was indicated that

Table 1 Increase in compressive strength of pumice LWAC by volume fraction of steel fiber for different densities [33]. LWA quantity (kg/m3)

Density (kg/m3)

28-day compressive strength of plain LWAC (MPa)

215 415 606 790

2030 1840 1630 1450

19.5 16.0 13.6 10.9

Increase in compressive strength by volume fraction of steel fiber (%) 0.5

1

1.5

2.9 4 4.7 9.3

7.1 8.8 9.7 15.8

10.4 12.9 13.5 21.1

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M. Hassanpour et al. / Construction and Building Materials 37 (2012) 452–461

35

Table 2 Properties of three types of fibers [51]. Steel

Polypropylene

5 7 Straight 1.6 240 1.4 2500

25 500 Crimped 7.8 200 3.2 1500

15 100 Straight, round 0.9 8 8.1 800

Stress (MPa)

Length (mm) Diameter (mm) Shape Density (g/cm3) Modulus (GPa) Elongation at break (%) Tensile strength (MPa)

30

Carbon

25 20 15 10

L0

5

L0.5

L1.0

0 0

oil palm shell LWAC containing steel fiber appears to be less sensitive to poor curing. The sensitivity decreases when the fiber volume is more than 0.5%. Therefore, it can be concluded that steel fiber can be used as an alternative material for reducing the sensitivity of LWACs in poor curing conditions. Among the several types of non-metallic fibers, the effects of PP fiber on the properties of LWAC have been investigated. The test results of some researchers [51,52,62,65,70,89] have shown that, generally, if the PP fibers are used in single form in the mixture of a LWAC mixture, they have a relatively low effect on the improvement of the compressive strength and may even reduce it. Chen and Liu [51] investigated the effect of three types of fibers on the properties of expanded clay LWAC. The properties of these fibers are shown in Table 2. Among these fibers, the PP fiber does not affect the compressive strength, while carbon fiber has the highest effect, up to 15%, and steel fiber up to 10%. 5. Stress–strain diagram The stress–strain curves of most LWACs for both normal and high strength levels is typically linear to levels approaching 90% or higher of the failure strength [30,45,90]. While for normal strength NWC, the curves are roughly straight to about one-third to one-half of the concrete’s ultimate strength [91]. For high strength NWC it has been found to be 85% or more of the peak stress [92]. Such high linearity of the stress–strain relationship is attributed to the absence of micro-cracks at low load levels [93], which causes sudden failure and high brittleness behavior of LWAC. HSLWAC is more brittle than high strength NWC [48]. The steel fiber reinforced LWAC shows different behavior to plain LWAC. Because of arresting of cracks by steel fibers, concrete can be subjected to very large deformations before total uncontrollable collapse [52]. It was reported that the addition of steel fibers to LWAC has little effect or no effect on the ascending part of the stress–strain relationship while it has a significant effect on the descending part of the curve [33,70,78]. Similar effects were reported [86,95] on the ascending and descending portion of the stress–strain curve of high strength NWC reinforced by steel fibers. However Ding and Kusterle [5] found that at early age, fiber reinforcement has a significant effect on the ascending portion of the stress–strain curve. In addition, it should be noted that the descending portion of the stress–strain curve is an essential key element in the non-linear analysis and design of reinforced concrete members under compression loads [94]. As can be seen in Figs. 1 [78] and 2 [33], by increasing the fiber volume fraction at a constant aspect ratio for LWAC, the slope of the descending part of the stress–strain curve decreases. Fig. 3 [82] also shows a complete stress–strain curve of a LWAC made of expanded clay LWA and different volume fractions of steel fiber (L = 30 mm and D = 0.5 mm). This figure clearly shows that the steel fiber reinforced LWACs have greater energy absorption (total area under compressive stress–strain curve) under compression than plain LWAC.

0.002

0.004

0.006

0.008

0.01

0.012

Strain (mm/mm) Fig. 1. Typical stress–strain relationship of oil palm shell lightweight aggregate concrete containing 0%, 0.5% and 1% steel fiber (after Shafigh et al. [78]).

Fig. 2. Typical stress–strain relationship of pumice lightweight aggregate concrete containing 0%, 0.5%, 1% and 1.5% steel fiber (after Duzgun et al. [33]).

Fig. 3. Typical stress–strain relationship of expanded clay lightweight aggregate concrete containing 0%, 0.5%, 1% and 2% steel fiber (after Campione et al. [82]).

Domagala [30] reported that a plain HSLWAC made with sintered fly ash Pollytag as a coarse LWA does not have a descending part in the stress–strain relationship. Consequently, explosive fracture was observed for this concrete. However, he demonstrated that even with a small addition of steel fiber (0.4% by volume fraction) completely eliminates the sudden failure of such concrete. The PP fibers can also be used in LWAC to prevent brittle behavior. Libre et al. [70] reported that low strength pumice LWAC is brittle. As can be seen in Fig. 4, they concluded that the minimum amount of PP fibers (L = 12 mm and D = 0.016 mm) to prevent brittle behavior of such LWAC is about 0.4% by volume of concrete. In addition, they reported that PP fibers slightly enhance the energy absorption while steel fibers have a great effect on the energy absorption of concrete under compression.

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Fig. 4. Typical stress–strain relationship of pumice lightweight aggregate concrete containing 0%, 0.2% and 0.4% polypropylene fiber (after Libre et al. [70]).

The addition of fibers to concrete matrices increases the compressive strain at the peak stress (e0) of concrete [33,82,86,95]. The test results of steel fiber reinforced LWAC showed that the strain at peak stress increases with increasing volume fraction [30,64,78]. The increasing percentage of e0 value depends on the type of LWA. For example, two types of LWA, namely, expanded clay and pumice stone of similar compressive strength showed an approximately 13% and 9% increase in e0 value by incorporating 1% volume of steel fiber (L = 30 mm and aspect ratio = 60 mm) [64], while oil palm shell LWAC with 1% volume of steel fiber (L = 35 mm and aspect ratio = 65 mm) showed 66% higher e0 value than the plain ones. For this type of LWAC, the following equation was suggested to estimate the strain at peak stress [78]: 2

e0 ¼ 1:18fc½0:29þ0:0002VðL=D Þ

ð2Þ

where e0 is strain at peak stress and expressed in parts per thousand, fc is the compressive strength in MPa, V is the fiber volume as a percentage, L is fiber length in mm and D is the fiber diameter in mm. 6. Modulus of elasticity The modulus of elasticity of LWAC is 25–50% lower than NWC at the same compressive strength [1]. Increasing the LWA content in LWAC causes a lower modulus of elasticity [33,35,59]. The experimental test results reveal that, generally, the reinforcement of LWACs by fibers is not a suitable solution to enhance the modulus of elasticity. It does not have a significant effect on the modulus of elasticity, especially, when the volume content of fibers is low [2,4]. In the case of steel fiber, in destructive and standard test of modulus of elasticity, reports [30,52,65,64,78] showed that steel fiber can increase modulus of elasticity of LWAC between 6% and 30%. However, depending on the type of LWA and dosage of steel fiber, it may decrease modulus of elasticity up to 12% [52,64]. For PP fibers, the maximum increase in modulus of elasticity was reported up to about 4% [52,62]. However, with high volume of PP fiber, modulus of elasticity decreases up to 12% [52]. In a non-destructive digital ultrasound test on prismatic specimens of a normal weight concrete incorporating 0.75% steel fiber (L = 50 mm and aspect ratio = 100) conducted by Kurugol et al. [59], the modulus of elasticity increased by up to about 36%. The same increase in the modulus of elasticity for a pumice stone LWAC with similar mix proportions was also observed.

457

Domagala [30] demonstrated that the most important factor for the effect of fibers on the modulus of elasticity of LWAC is the adhesion between the aggregate and the cement matrix. For example, the modulus of elasticity of steel fiber (2% by volume) reinforced LWAC made with expanded clay LWA revealed approximately 18% higher than plain concrete. While for pumice stone it was approximately 12% lower. In the case of pumice, because of high water absorption and the rough texture, the bond is very strong. Whereas, expanded clay may have much more regular and smooth grains and lower water absorption. In addition, this LWA may be less homogenous. Therefore, concrete with expanded clay LWA may crack earlier during loading, even in the range of stresses applied for the static modulus of elasticity testing. Currently, steel fiber as one of the components of concrete with a high modulus of elasticity, plays its role by bridging mechanism. In respect of the reason for reduction in the modulus of elasticity of steel fiber pumice LWAC, Campione et al. [64] explained that this is probably due to the reduced compaction of the concrete. They demonstrated that such a reduction in compaction is due to the size and shape of the aggregate, which are not appropriate in respect of the size of the fiber used. Gao et al. [43] reported the following equation for the prediction of modulus of elasticity of steel fiber reinforced expanded clay HSLWAC:

Ec ¼ Em ð1 þ 0:173V f Lf =Df Þ

ð3Þ

where Ec is the modulus of elasticity of steel fiber reinforced LWAC; Em is the modulus of elasticity of plain LWAC; Vf is fiber volume fraction and Lf/Df is the aspect ratio. 7. Splitting tensile strength The tensile strength of concrete is much lower than the compressive strength, which is normally assumed to be equal to zero and is not considered directly in design [96]. Li [32] demonstrated that under tensile loading, cracks propagate rapidly at much lower stress levels, which cause brittle failure in concrete in tension. Even for reinforced concrete structural members, due to the low tensile strength, the concrete cover cracks. For design purposes, BS 8110: Part 2 [97] specified that for grade 25 LWAC and above, the design shear strength should not exceed 80% of the value for NWC. The tensile strength and the modulus of deformation are two important properties that influence the safety, serviceability of concrete elements and durability [98,99]. As specified by ASTM C 330 [100] structural lightweight aggregate concretes need to have a minimum splitting tensile strength of 2.0 MPa. The splitting tensile strength of LWAC can be considerably lower (up to 30%) than that of ordinary concrete of the same compressive strength [101]. To improve the tensile strength of concrete, fiber reinforced concrete and polymer concrete have been developed [32]. Fiber reinforced concrete has superior tensile properties over plain concrete, particularly ductility [102]. In the case of LWAC and semi-LWAC it has been reported that the addition of fibers provide a significant increase in the splitting tensile strength [80,103]. By increasing the tensile strength of LWACs with fiber reinforcement, this improvement may be enough to avoid shrinkage even in LWAC with large drying shrinkage [104]. Previous studies [29,30,33,43,51,52,64,70,77,78,82] have shown that the addition of steel fiber in very low ratios (Vf 6 0.5%) to LWAC, when compared to concrete without fiber, provided a maximum increase in the splitting tensile strength in the range of 16– 61%. For low volume fractions (0.5% < Vf 6 1%) and higher volume fractions (1% < Vf 6 2%) a maximum increase in the splitting tensile strength of about 19–116% and 61–118%, respectively, have been reported.

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Compared to steel fiber, the addition of PP fibers in volume fractions lower than 1% does not significantly increase the splitting tensile strength of sanded-LWAC with only a maximum increase of about 25% being reported [51,62,70]. However, a report conducted by Kayali et al. [52], showed that the splitting tensile strength of LWAC made with sintered fly ash (Lytag), and LWA as coarse and fine aggregate, was significantly increased by adding PP fibers. They reported that adding the PP fiber at 0.285%, 0.56% and 1% by volume of the concrete caused an increase of 59%, 94% and 71% in the splitting tensile strength, respectively. Chen and Liu [51] investigated the effect of three types of fibers (Table 2) and their blended form on the properties of expanded clay HSLWAC. The amount of fibers individually and in blended form, was 1%. They reported that steel fiber reinforced LWAC showed the highest splitting tensile strength by increasing it by about 24%. Carbon fiber increased it by 16%, whereas the PP fiber resulted in a slight reduction of about 2%. However, they found that hybrid fibers have a significantly higher positive effect on the compressive and splitting tensile strength. Among all the combinations of these fibers, a combination of carbon and steel fibers provides the best effects. This combination increased the splitting tensile strength by up to 39%. They demonstrated that this may be due to the fact that a combination of fibers with different sizes and types control different scales of cracking. Also, in NWC, as reported by Yao et al. [105], the carbon–steel combination gave the highest splitting tensile strength. The benefits of using a combination of fibers in concrete were also reported by other researchers [67,72,106,107]. Generally, the splitting tensile strength of plain concrete is 8– 14% of the compressive strength [8]. Although this ratio for LWACs is usually lower than NWCs at the same compressive strength [44], even with the inclusion of a low volume of fiber (especially steel fiber) in LWAC, this ratio increases and falls in the range of normal weight plain concrete. The test results of the splitting tensile strength of different types of LWACs with normal strength concrete [30,33,62,64,70,77,78] showed that the splitting tensile to compressive strength ratio of plain LWAC is in the range of 6–11% while this ratio for concretes reinforced with low volume fraction of fibers (individually or as hybrid) ranged between 8% and 14.2%, which shows that the splitting tensile strength to compressive strength ratio of fiber-reinforced LWAC is in the range of NWCs. Studies have shown that the effectiveness of steel fiber on the splitting tensile strength of LWAC depends on the amount of LWA in the concrete. Balaguru and Foden [55] investigated the effect of steel fibers at three different dosages on the properties of a LWAC made with expanded shale as coarse and fine aggregate. The fine aggregate consisted of either all LWA or a combination of LWA and natural sand. They reported that the positive effect of fiber addition in concrete with a higher volume of LWA is more significant. Such effectiveness was also observed for expanded polystyrene LWAC [77]. Furthermore, the use of hooked end fiber and the addition of silica fume created a better fiber/concrete bond, and, consequently a higher increment in tensile strength [29]. In this regard, the usefulness of combined steel fiber and pozzolanic materials, because of the improved bonding strength, was recommended by Tsai et al. [61]. It should be noted that increased value of splitting tensile strength of steel fiber concrete has been shown to be dependent on the specimen size. As Domagala [30] and Balendran et al. [29] reported, smaller size samples show greater improvement. Gao et al. [43] suggested the following equation for predicting of the splitting tensile strength of steel fiber reinforced HSLWAC made with expanded clay LWA:

F st ¼ 0:94F t ð1  V f Þ þ 3:02V f Lf =Df

ð4Þ

where Fst is the splitting tensile strength of steel fiber HSLWAC, Ft is the splitting tensile strength of plain HSLWAC, Vf is the fiber volume fraction and Lf/Df is the aspect ratio.

8. Flexural strength It was reported that the flexural strength of plain LWAC is lower than NWC of the same compressive strength [30]. The inclusion of fiber in LWAC increases its flexural strength. The reason is that, after matrix cracking, the fibers will carry the load that the concrete sustained until cracking by the interfacial bond between the fibers and the matrix [43]. Therefore, the fibers resist the propagation of cracks and do not fail suddenly, which causes an increase in the load carrying capacity. The increase in flexural strength due to the addition of fiber in LWAC is higher than in NWC [29,55]. As reported by Balendran et al. [29] the flexural strength of concrete is size dependent. It decreases as the specimen size becomes larger. The size effect is more prominent for materials with higher brittleness. Therefore, the size effect is expected to be less for fiber reinforced LWAC due to the fibers improving the ductility of LWAC. For example, when the flexural strength of HSLWAC was 5.9 MPa for a specimen size of 50 mm (height)  100 mm (width)  200 mm (span) this concrete showed 44% and 54% lower flexural strength for this specimen than bigger specimen sizes of 100 mm  100 mm  400 mm  500 mm and 200 mm  100 mm  800 mm  840 mm, respectively. While such a reduction for steel fiber reinforced concrete was 18% and 30%, respectively. This shows that fiber reinforced concrete is much less sensitive to the size effect. Previous research [29,30,33,43,52,64,70,78] have shown that by adding steel fiber to LWAC, the flexural strength of standard specimens increased by about 6–38% for Vf 6 0.5%, 14–182% for 0.5% < Vf 6 1% and 42–120% for 1% < Vf 6 2%. In the case of PP fiber, the highest increase reported in the flexural strength, of about 20%, was by incorporating 0.4% [70] and 0.56% [52] volume fraction. The test results for the study by Tanyildizi [40] showed that carbon fiber (with an average length of 5 mm) in volume fraction of 0.5%, 1% and 2% increased the flexural strength of a whole pumice LWAC by about 13%, 32% and 7%, respectively. It can be seen that by incorporating 2% of carbon fiber, the increase in flexural strength is significantly lower than 1%. This may be due to the absence of good fiber dispersion with high volumes of carbon fiber in concrete. Mirza and Soroushian [16] investigated the effect of glass fibers (L = 12 mm and D = 135 lm) on the flexural strength of totally perlite LWAC. The flexural strength of prismatic small specimens (38  38  160 mm) showed that increasing the glass fiber in volume fractions of 0.125–0.75% significantly increases the flexural strength of plain LWAC in the range of about 64–120%. Mehta and Monteiro [2] demonstrated that the greatest advantage in fiber reinforcement of concrete is the improvement in flexural toughness. They showed that for a conventional aggregate concrete, with the inclusion of 1.25% volume fraction of steel fiber increases the flexural strength by about two times, the increase in toughness was as much as 20 times, which clearly shows that the improvement in the toughness is much higher than the improvement in flexural strength. The test results of Libre et al. [70] showed that the effectiveness of fiber for improving the toughness of LWAC is much higher than NWC. This may be due to the brittleness of LWAC being higher than NWC. Fig. 5 shows that increasing the volume fraction of steel and PP fibers individually or in combined form to pumice LWAC enhanced both the flexural and strength and toughness. Whereas, the increase in toughness for all concretes containing steel fiber is much higher than the increase in flexural strength. For example, 1% volume fraction of steel

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9. Conclusions

Fig. 5. Influence of increasing fiber volume (steel and polypropylene) on toughness and flexural strength of pumice lightweight aggregate concrete (after Libre et al. [70]).

The inclusion of fibers, particularly steel fibers, into structural lightweight aggregate concrete decreases its workability. For compensating reduction of workability, the use of higher dosage of superplasticizer and fine aggregate and also the use of fly ash in concrete mixture are recommended. Steel fiber increases the density of LWAC. The use of mineral admixtures, air-entraining admixtures and small steel fiber content may compensate for the increase in the density of LWAC. Generally the inclusion of steel fiber to LWAC increases the compressive strength. But, inclusion of steel fiber more than 2% volume fraction may reduce it. However, a fiber reinforced LWAC have significantly higher splitting tensile strength than plain LWAC even at low volume of fibers (especially steel fiber). The use of hooked-end steel fiber and also cementitious materials in fiber reinforced LWAC, result in a higher improvement of tensile strength. The positive effect of the addition of fiber on the splitting tensile strength of LWAC is more significant in LWAC with a higher volume of LWA. In addition, the inclusion of fiber to LWAC increases its flexural strength. The increase in flexural strength due to the addition of fiber in LWAC is higher than NWC. The effectiveness of steel fiber in flexural strength seems to be significantly more pronounced than other types of fibers. However, a combination of steel fiber and non-metallic fibers results in higher flexural strength than the usage of individual types of fibers. The inclusion of steel fibers in LWAC significantly affects the descending part of the stress–strain curve. Even very low volume fractions of steel fiber help to prevent brittle failure of LWAC. However, the addition of steel fibers to LWAC has little or no effect on the ascending part of the stress–strain relationship. Therefore, in most cases the modulus of elasticity of fiber reinforced LWAC is not much different than plain LWAC. The effectiveness of fiber for improving the toughness of LWAC is much higher than NWC. A combination of steel fiber and non-metallic fibers results in better toughness.

References

Fig. 6. Effect of steel fiber volume fraction and aspect ratio on flexural toughness of expanded clay lightweight aggregate concrete (after Gao et al. [43]).

fiber increases the toughness of concrete by about 78 times while the increase in flexural strength was just three times. The combination of 1% volume fraction of steel fiber and 0.4% of PP fiber gave the highest increase in the both toughness and flexural strength. However, the effectiveness of 0.2% of PP fiber by steel fiber does not significantly affect the flexural strength and toughness. They demonstrated that this might be due to the lower tensile strength of PP fiber and also the weaker bond between PP fibers and the cement matrix. Chen and Liu [51] also reported that the combination of carbon and steel fibers (each one of 0.5% volume fraction) gave the highest value for toughness index, as compared to 1% volume fraction of a single type of each fiber. Gao et al. [43] reported that increasing the steel fiber in expanded clay LWAC increased its fracture toughness. In addition, as can be seen in Fig. 6, they showed that a higher steel fiber aspect ratio resulted in higher flexural toughness.

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