Experimental study on axial compression performance of CFRP-strengthened concrete-filled steel tube composite columns after high-temperature exposure

This study examined a specific composite column system, herein referred to as a CFST composite column, comprising an inner concrete-filled steel tubular (CFST) core and an outer reinforced concrete (RC) layer. Exposure to high temperatures severely damaged the external RC layer, leading to significant degradation of the column's overall load-bearing capacity and ductility. While carbon fiber-reinforced polymer (CFRP) strengthening is widely used, its effectiveness for rehabilitating such composite columns after fire remained insufficiently understood. We systematically investigated the axial strengthening effect of CFRP on square-section CFST composite stub columns after high-temperature exposure through experimental testing and numerical simulations. Thirteen specimens subjected to temperatures ranging from 200 to 800 °C were repaired with CFRP and tested under axial compression; their failure modes and load-bearing capacity variations were analyzed. The results showed that the axial bearing capacity markedly decreased with increasing temperature, with a maximum reduction of 41.5% at 800 °C compared with the ambient-temperature specimen, whereas increasing the number of CFRP wrapping layers effectively restored and even enhanced the axial capacity—a finding validated by finite element models developed in ABAQUS. Based on the experimental and numerical results, a calculation method for predicting the axial compressive capacity of CFRP-strengthened CFST composite columns after high-temperature exposure was proposed, thereby advancing the mechanistic understanding and providing a predictive tool for the post-fire rehabilitation of this composite column type.