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Composite Structures

Experimental response and code modelling of continuous concrete slabs reinforced with BFRP bars

Abstract

This paper presents test results and code predictions of four continuously and two simply supported concrete slabs reinforced with basalt fibre reinforced polymer (BFRP) bars. One continuously supported steel reinforced concrete slab was also tested for comparison purposes. All slabs tested were 500   mm in width and 150   mm in depth. The simply supported slabs had a span of 2000   mm, whereas the continuous slabs had two equal spans, each of 2000   mm. Different combinations of under and over BFRP reinforcement at the top and bottom layers of slabs were investigated.

The continuously supported BFRP reinforced concrete slabs exhibited larger deflections and wider cracks than the counterpart reinforced with steel. Furthermore, the over reinforced BFRP reinforced concrete slab at the top and bottom layers showed the highest load capacity and the least deflection of all BFRP slabs tested. All continuous BFRP reinforced concrete slabs failed owing to combined shear and flexure at the middle support region. ISIS-M03-07 and CSA S806-06 design guidelines reasonably predicted the deflection of the BFRP slabs tested. However, ACI 440-1R-06 underestimated the BFRP slab deflections and overestimated the moment capacities at mid-span and over support sections.

Introduction

The use of fibre reinforced polymer (FRP) reinforcement in corrosion-prone reinforced concrete structures has rapidly increased due to their excellent corrosion resistance, high tensile strength to weight ratio and good non-magnetisation properties. However, FRP reinforced concrete members behave differently from these reinforced with steel owing to the linear elastic stress–strain relationship of FRP bars up to rupture. In addition, the lower modulus of elasticity of FRP causes a substantial decrease in the flexural stiffness of FRP reinforced concrete members after cracking and, consequently, larger deformations under service conditions. As a result, the design of FRP reinforced concrete members is often governed by the serviceability limit state. Therefore, the use of FRP reinforcement requires a better understanding of the behaviour of FRP reinforced concrete members.

Many studies investigated the flexural behaviour of simply supported beams and one way concrete slabs reinforced with different types of FRP reinforcing bars [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] or reinforced with plastic fibres or long strips made from waste polyethylene terephthalate bottles [11], [12]. However, little experimental investigations on continuously supported FRP reinforced concrete beams have been reported [13], [14], [15], [16], [17] and no investigations on continuous FRP reinforced concrete slabs. Grace et al. [13] tested seven two-span concrete T-beams reinforced with different arrangements of longitudinal and shear reinforcement of carbon and glass fibre reinforced polymer (CFRP and GFRP), and steel bars. Their work concluded that beams with different FRP combinations showed the same load capacity as these reinforced with steel but failure modes and ductility were different. Likewise, Razaqpur and Mostofinejad [14] presented experimental results of four continuously supported CFRP reinforced concrete beams with steel stirrups or CFRP grid as shear reinforcement. It was shown that continuous FRP reinforced concrete beams with over-reinforcement ratio demonstrated a semi-ductile behaviour. It was also observed that CFRP grid in the specimens tested had a similar performance to steel stirrups. Furthermore, Ashour and Habeeb [15], [16] tested four simply and six continuously supported concrete beams with different arrangements of CFRP and GFRP bars. They concluded that continuous GFRP reinforced concrete beams developed earlier and wider cracks compared with the counterpart steel reinforced concrete slab. In addition, continuously supported GFRP and CFRP reinforced concrete beams did not demonstrate any significant load redistribution. The study also indicated that ACI 440 1R-06 equations can reasonably predict load capacity and deflection of simply supported GFRP beams but progressively underestimate deflections of continuously supported FRP reinforced concrete beams after first cracking. Recently, El-Mogy et al. [17] reported experimental results of seven GFRP and two CFRP reinforced concrete continuous beams. This study showed that increasing the GFRP reinforcement at mid-span sections had a more positive effect on reducing mid-span deflections and improving load capacity than over the middle support regions, consistent with that reported by Habeeb and Ashour [16].

This research presents the testing of four continuously and two simply supported concrete slabs reinforced with BFRP bars. The influence of different reinforcement configurations on the flexural behaviour of continuous slabs was investigated. Modes of failure, crack widths, end-support reaction, moment capacity and deflections were measured and compared against existing design guidance for concrete elements reinforced with FRP bars. The test results would contribute to future development of design guidelines for continuous concrete slabs reinforced with FRP bars.

Section snippets

Test specimens

Two simply and four continuously supported BFRP reinforced concrete slabs were tested in flexure. In addition, a continuously supported slab reinforced with conventional steel rebars was also tested as a reference slab. All slabs tested were 500   mm in width and 150   mm in depth. The continuous slabs comprised of two equal spans, each of 2000   mm, while the simply supported slabs had a span of 2000   mm, as shown in Fig. 1. A concrete cover of 25   mm thickness was kept constant for all reinforcement.

The

Material properties

The BFRP bars used in this research were manufactured by the pultrusion process where tightly packed tows of basalt fibres, impregnated with epoxy resins, are pulled through a shaped die to form highly aligned, continuous sections of BFRP bars. After resin curing, the bar surface was sand-coated to improve bond and force transfer between reinforcing bars and concrete. The mechanical characteristics of these reinforcing bars were obtained by carrying out tensile tests on three specimens of each

Test setup and instrumentations

Fig. 1(a) and (b) show the experimental setup of the simply and continuously supported slabs tested, respectively. Each span of the continuous slabs was loaded at its mid-point and supported on two end rollers and a middle hinge support. Each slab was instrumented with two load cells to measure the reactions at one end support and the main applied load from the hydraulic ram. Moreover, deflections at the two mid-spans of continuously supported slabs and the mid-span of simple slabs were

Crack propagation and failure modes

The first visible cracking load of each slab tested is presented in Table 3. The steel reinforced concrete slab exhibited a higher first cracking load than slabs reinforced with BFRP owing to the higher axial stiffness of steel bars than that of BFRP bars. The amount of BFRP reinforcement at different locations for each slab tested has also affected the first cracking load; for example slabs C–B–OU and C–B–UO experienced the first crack at the lower reinforcement location.

Fig. 2 sketches the

Conclusions

Tests results and code modelling of two simply and four continuously supported concrete slabs reinforced with BFRP bars have been presented in this paper. The following conclusions are drawn:

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The continuously supported BFRP reinforced concrete slabs developed earlier and wider cracks, and larger deflections than the control concrete slab reinforced with steel owing to the lower elastic modulus of BFRP reinforcing bars.

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At initial stages of loading, the experimental reactions of all slabs tested

Acknowledgments

The experimental work presented in this paper was conducted at the Heavy Structures Laboratory in the University of Bradford; the assistance of the laboratory staff is acknowledged. The authors are also gratefully to MagmaTech Ltd. for providing the BFRP reinforcement.

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