Postfire Axial Compression Behavior of Unloaded RC Columns Strengthened with CFRP Jackets

Fiber-reinforced polymer (FRP) composites have been widely recognized as an effective solution for strengthening and repairing RC structures, owing to their mechanical properties and ease of application. Among various FRP-based strengthening methods, external confinement using FRP jackets has proven particularly effective in enhancing both the load-carrying capacity and the deformation performance of RC columns. This paper presents an experimental investigation into the postfire axial compression behavior of RC columns wrapped with carbon fiber–reinforced polymer (CFRP) jackets. A comprehensive experimental program was conducted to examine the effects of key parameters, including fire exposure time (2 and 3 h) to induce different levels of thermal damage, the number of CFRP jacket layers (2 and 4), and concrete cover thickness (20 and 40 mm) on the axial compression behavior of the specimens. The experimental evaluations focused on failure modes, load-carrying capacity, load–displacement response, secant stiffness, ductility indices, CFRP hoop strains, and lateral dilatation behavior. The results demonstrated that the use of CFRP jackets significantly improved both the load-carrying capacity and the ductility of fire-damaged RC columns. Increasing the number of CFRP layers significantly enhanced the load-carrying capacity, while a thicker concrete cover delayed CFRP rupture, resulting in higher hoop rupture strains and more effective confinement. A cross-sectional analytical method was developed to predict the axial load–carrying capacity of CFRP-wrapped fire-damaged RC columns. The proposed method accounted for the nonuniform thermal degradation of concrete and steel reinforcement, as well as the confinement effect provided by the CFRP jackets on the thermally damaged concrete core. The validity of the analytical method was confirmed through a close agreement between experimental results and theoretical predictions. The novelty of this study lies in the integration of full-scale experimental testing with a validated analytical framework that comprehensively captures the combined effects of the thermal degradation of materials and CFRP confinement on the postfire axial compression behavior of RC columns strengthened with CFRP jackets.