Hoop rupture strains of FRP jackets in FRP confined concrete

One important application of fibre-reinforced polymer (FRP) composites is as a confining material for concrete in the retrofit of existing concrete columns by the provision of FRP jackets. Such jackets are commonly formed in a wet lay-up process, with the fibres being only or predominantly in the hoop direction. It has been well established in recent studies that the rupture strains/strengths of FRP measured in tests on such FRP-confined concrete cylinders fall substantially below those from flat coupon tensile tests, but the causes are unclear. This paper presents the results of a study which is aimed at clarifying these causes. To this end, the paper reports and compares the ultimate tensile strains of two types of FRP (CFRP and GFRP) obtained from three types of tests: flat coupon tensile tests, ring splitting tests and FRP-confined concrete cylinder tests. Based on comparisons of these test results, it can be concluded that the FRP hoop rupture strains in FIR-P-confined concrete cylinders are reduced below the ultimate tensile strains from flat coupon tests by three factors: (a) the curvature of the FRP jacket; (b) the non-uniform deformation of the concrete; and (c) the effect of the overlapping zone which has an increased thickness.