Postpeak Stress–Strain Behavior of High-Strength Concrete under Different FRP Confinement Stiffness Ratios

Abstract

The ratio of fiber-reinforced polymer (FRP) confinement stiffness to concrete strength (confinement stiffness ratio) has been widely accepted to determine the postpeak stress‒strain behavior of FRP-confined concrete. It has been established that the effects of confinement efficiency are significantly different for normal-strength concrete (NSC) and high-strength concrete (HSC). This paper presents an experimental study of the stress‒strain behavior of carbon FRP (CFRP)-confined circular and square HSC columns. Various FRP confinement stiffnesses were designed for concretes with different strengths to achieve the same confinement stiffness ratios. The test results indicate that although the specimens had similar confinement stiffness ratios, the criterion values of the confinement stiffness ratio to distinguish the postpeak strain hardening/softening transition increased with increasing HSC strength. However, for FRP-confined NSC, it is suggested that a constant value of approximately 6.5 be used. In addition, this study also revealed that for specimens with weak confinement or nonuniform confinement (square cross sections), the change in confinement efficiency at the postpeak stage results in the recovery of the postpeak strain hardening behavior from the strain softening behavior. By incorporating the confinement efficiency parameter in the ultimate condition models for CFRP-confined HSC, the proposed model demonstrated satisfactory performance when compared with the test results.