Effect of steel and synthetic fibers on flexural behavior of high-strength concrete beams reinforced with FRP bars

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Abstract

Six high-strength concrete beam specimens reinforced with fiber-reinforced polymer (FRP) bars were constructed and tested. Three of the beams were reinforced with carbon FRP (CFRP) bars and the other three beams were reinforced with glass FRP (GFRP) bars as flexural reinforcements. Steel fibers and polyolefin synthetic fibers were used as reinforcing discrete fibers. An investigation was performed on the influence of the addition of fibers on load-carrying capacity, cracking response, and ductility. In addition, the test results were compared with the predictions for the ultimate flexural moment. The addition of fibers increased the first-cracking load, ultimate flexural strength, and ductility, and also mitigated the large crack width of the FRP bar-reinforced concrete beams.

Introduction

Recently, there has been a rapid increase in the use of fiber-reinforced polymer (FRP) bars substituting for conventional steel bars for concrete structures. Because FRP materials are nonmagnetic and noncorrosive, the problem of electromagnetic interference and steel corrosion can be avoided with FRP bars. In addition, FRP bars have the advantages of high strength and light weight, and a number of design guides and national standards have been published to provide recommendations for the analysis, design, and construction of concrete structures reinforced with FRP bars [1], [2], [3]. However, due to the substantial differences in the physical and mechanical properties between FRP and conventional steel, the use of FRP bars is still a formidable challenge for engineers.

The elastic modulus of FRP bars is much less than that of steel bars. Glass fiber-reinforced polymer (GFRP) bars and aramid fiber-reinforced polymer (AFRP) bars have an elastic modulus of between 35 and 50 GPa, and the elastic modulus of carbon fiber-reinforced polymer (CFRP) bars is between 120 and 150 GPa. This low elastic modulus leads to higher deflection and larger crack width in FRP bar-reinforced concrete beams that have an equivalent reinforcement ratio to steel-reinforced concrete beams; therefore, both deflection and crack width must be checked for the serviceability limit state. In addition, while steel bars behave inelastically after yield strength, FRP bars show perfect elastic behavior up to failure. Since FRP bars are linear elastic to failure and fail in a brittle manner, a ductile steel-like failure does not occur in FRP bar-reinforced concrete beams. To avoid brittle failure, failure by concrete crushing (over-reinforced beams), which is generally avoided in steel-reinforced concrete design, is preferred in FRP bar-reinforced concrete design [1], [2], [3], [4]. However, because concrete itself is a brittle material and high-strength concrete is even more brittle, the ductility of FRP bar-reinforced high-strength concrete beams is less than that of steel-reinforced concrete beams.

In order to overcome the problems in terms of deformability and ductility of concrete beams reinforced with FRP bars, an alternative solution using fiber reinforced concrete (FRC) was proposed. It is now well established that the addition of steel fibers improves the mechanical properties of concrete members. Steel fibers offer increased toughness, durability, and impact resistance, and control the initiation and growth of cracks [5]. Many researchers have proven that the addition of steel fibers increases the ductility of over-reinforced steel reinforced beams and high-strength steel reinforced beams [6], [7]. However, few studies have been carried out on the effects of fibers on the behavior of concrete beams reinforced with FRP bars. It is likely that the efficiency of fibers for the FRP bar-reinforced beams, which have a large crack width and deep crack propagation, can be higher than that for steel-reinforced beams. In particular, because the failure of over-reinforced beams with FRP bars is controlled mainly by the concrete compressive strain, the increased and softened postpeak strain of FRC may considerably improve the ductility of FRP beams.

In this study, six high-strength concrete beam specimens were constructed and tested to investigate the effect of fibers on the behavior of FRP bar-reinforced concrete beams. Carbon FRP (CFRP) bars and glass FRP (GFRP) bars were used as flexural reinforcements, and not only steel fibers but also recently developed polyolefin synthetic fibers [8] were considered as reinforcing discrete fibers. This is because the large crack width and deep crack propagation in FRP bar-reinforced concrete beams have a high potential for the corrosion of steel fibers at cracks, even if crack widths of less than 0.1 mm do not allow the corrosion of steel fibers passing across the crack [9]. This study focused on the flexural behavior of these beams in terms of load-carrying capacity, cracking pattern, and ductility. In addition, the experimental results presented in this paper were compared with the results from flexural strength prediction models proposed by various researchers [10], [11], [12].

Section snippets

Test specimens

Fig. 1 and Table 1 show the details of six beam specimens. All specimens were 2300 mm long with a rectangular cross section of 230 × 250 mm. These were reinforced with two layers of reinforcement, and the effective depths of the outer layer (d1) and the inner layer (d2) were 206 mm and 162 mm, respectively. The main variables were the material of the flexural reinforcement and the fiber. The specimens can be divided into two series: a beam series reinforced with CFRP bars (CC Series) and a beam

General behavior of test specimens

Fig. 5 shows the applied load versus midspan deflection responses, while Table 6 summarizes the loads and midspan deflections at the formation of the first flexural crack and at the peak loads for all beam specimens. All specimens exhibited similar behavior, which was very stiff before first cracking. The specimens with fibers showed higher first cracking loads than those without fibers. In particular, the first cracking loads of specimens CC-ST and GG-ST, which were fabricated with SFRC, were

Flexural strength prediction for FRP bar-reinforced beams with no fibers

Table 7 presents the experimental and theoretical ultimate moment capacities for FRP bar-reinforced beams with no fibers (specimens CC and GG). The depths of the neutral axes, c, were obtained by Eqs. (6), (7) as follows, and the theoretical ultimate moment capacities, Mn, were calculated by Eq. (8) based on the ACI Codes [1], [4]0.85fcβ1bc+Asfs=Ar1fr1+Ar2fr2ɛr1ɛcu=(d1-c)c;ɛr2ɛcu=(d2-c)c;ɛsɛcu=(c-d)cMn=Ar1fr1d1-β1c2+Ar2fr2d2-β1c2+Asfsd-β1c2where Ar1, Ar2, and As = section areas of

Conclusions

The following conclusions were drawn from the flexural tests on six FRP bar-reinforced high-strength concrete beams with steel or synthetic fibers:

  • 1)

    The addition of fibers delayed the initiation of flexural cracks and decreased the crack widths. The first cracking loads of specimens with steel fibers were twice as high as those of specimens CC and GG, respectively.

  • 2)

    The CFRP bar-reinforced beams with steel and synthetic fibers were failed by FRP bar rupture, although all specimens had been designed

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (No. 2007-0056796).

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