Ultimate Condition of Fiber Reinforced Polymer-Confined Concrete

TLDR

FRP jackets are used to retrofit concrete columns, but their rupture strains in confined cylinders are markedly lower than flat coupon tests, a discrepancy whose causes are not yet understood. This study seeks to identify the causes of the reduced rupture strains in FRP‑confined concrete cylinders. The authors compared ultimate tensile strains of carbon and glass FRP from flat coupon, ring‑splitting, and confined‑cylinder tests, noting that the overlapping zone reduces average hoop rupture without altering confining pressure distribution. They found that FRP hoop rupture strains are diminished by at least three factors—curvature, localized concrete deformation, and an overlapping zone—where the first is material‑dependent and the latter two are not.

Abstract

One important application of fiber 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 layup process, with the fibers 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 that is aimed at clarifying these causes. To this end, the paper reports and compares the ultimate tensile strains of two types of FRP (carbon FRP and glass FRP) 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 FRP-confined concrete cylinders are reduced below the ultimate tensile strains from flat coupon tests by at least three factors—(1) the curvature of the FRP jacket; (2) the deformation localization of the cracked concrete; and (3) the existence of an overlapping zone. While the first factor that reduces the in situ strain capacity of FRP on confined concrete is material dependent, the last two factors that result in a nonuniform strain distribution in the jacket are independent of the FRP material properties. The third effect reduces the average hoop rupture but does not affect the distribution of the confining pressure, as the FRP jacket is thicker in the overlapping zone.