Postfire Strengthening of RC Cylinder and Prisms Using FRP-Based Discontinuous Systems

Abstract

Postfire retrofitting of reinforced concrete (RC) columns using fiber-reinforced polymer (FRP) confinement can restore axial capacity, yet the performance of discontinuous, hybrid, and prestressed systems under severe thermal damage remains insufficiently understood. This study investigates the axial compressive behavior of circular, square, and rectangular RC elements exposed to 700°C for 90 min and retrofitted using four strategies: (1) full wrapping with carbon fiber-reinforced polymer (CFRP), (2) CFRP partial strips, (3) a hybrid system consisting of a full glass fiber-reinforced polymer (GFRP) jacket with outer CFRP partial strips, and (4) prestressed CFRP partial strips applied via the strip constriction technique. Axial compression tests were conducted to evaluate strength recovery, stress–strain response, stiffness restoration, and confinement efficiency. Full CFRP wrapping achieved the highest strength recovery, in some cases exceeding original unheated capacity, while partial strip wraps provided greater deformability but limited stiffness restoration. Hybrid systems enhanced both strength and stiffness by mitigating stress concentrations in unwrapped zones. Prestressed confinement significantly increased initial stiffness and peak load by activating early confinement, albeit at the expense of reduced ultimate ductility. To generalize these findings, a unified strength model is proposed for FRP-confined heat-damaged concrete, incorporating (1) a thermal modification factor to account for fire-induced degradation, (2) a stiffness-dependent interaction formulation for hybrid confinement, and (3) a prestress enhancement factor for actively confined systems. Validation against an extensive experimental database demonstrates accurate and unbiased strength prediction across various cross sections and confinement configurations. The proposed framework provides a comprehensive basis for performance-based rehabilitation design of fire-damaged elements.