Flexural performance of concrete slabs reinforced with GFRP rebars

Elzaroug, Omar and Forth, John and Ye, Jianqiao and Beeby, Andrew (2013) Flexural performance of concrete slabs reinforced with GFRP rebars. Civil and Environmental Research, 5. pp. 6-11. ISSN 2225-0514

Abstract

The use of non-metallic fibre reinforced polymer (FRP) reinforcement as an alternative to steel reinforcement in concrete is gaining acceptance mainly due to its high corrosion resistance. High strength-to-weight ratio, high stiffness-to-weight ratio and ease of handling and fabrication are added advantages. Other benefits are that they do not influence to magnetic fields and radio frequencies and they are thermally non-conductive. However, the stress-strain relationship for Glass FRP is linear up to rupture when the ultimate strength is reached. Unlike steel reinforcing bars, GFRP rebars do not undergo yield deformation or strain hardening before rupture. Also, GFRP reinforcement possesses a relatively low elastic modulus of elasticity compared with that of steel. As a consequence, for GFRP reinforced sections, larger deflections and crack widths are expected than the ones obtained from equivalent steel reinforced sections for the same load. This paper presents a comparison of the experimental results with those predicted by the ACI 440 code in terms of; measured cracking moment, load-deflection relationships, ultimate capacity, modes of failure, stresses and crack width. This is to investigate the suitability of using the existing ACI design equations for predicting the flexural behaviour of samples reinforced with GFRP rebars. In this investigation, it appears that the ACI code equations on the whole over predict (i.e. crack widths and midspan deflection) the experimental results. On the other hand, the maximum experimental moment satisfies the ACI condition (i.e. unfactored design moment). 1 Introduction The flexural design of concrete sections reinforced with Glass FRP (GFRP) is different from that of sections reinforced with steel because of the difference in mechanical properties of GFRP and steel. Generally, the GFRP bars used as reinforcement in concrete have tensile strengths varying between 620 and 690 MPa and a modulus of elasticity of around 40 GPa [1]. The tensile strength varies as the diameter of the bar increases due to shear lag which develops between the fibers in the larger sizes. The stress-strain relationship for GFRP is linear up to rupture when the ultimate strength is reached. Unlike steel reinforcing bars, GFRP rebars do not undergo yield deformation or strain hardening before rupture. For this reason, the flexural design of sections reinforced with GFRP has been based on: (i) ultimate strength, (ii) serviceability (the low elasticity modulus of GFRP shifts the design criteria to the serviceability limit states that check the structural behaviour aspect instead of the strength to assure functionality and safety during its life), (iii) shear and (iv) deformability (the deformability factor is defined as the product ratio of moment multiplied by curvature at ultimate failure and at serviceability [2]. For steel reinforced sections, the cross section of steel is commonly governed by the ultimate strength requirement. There are, however, some cases where the design is governed by the need to control crack width in service (e.g. water retaining structures). GFRP reinforced concrete members have a relatively low stiffness