Low-carbon repair of RC columns using sandwiched RAC and FRP-steel composite tube: Axial compression tests and design

This paper introduces a sustainable and environmentally friendly structural reinforcement method, which involves using fiber-reinforced polymer (FRP) steel composite tubes and sandwiched recycled aggregate concrete (RAC) to strengthen reinforced concrete (RC) columns. A total of fourteen specimens were designed to test under axial compression, including thirteen retrofitted columns and one original column. The effects of recycled coarse aggregate (RA) replacement ratio, external FRP layers, steel tube thickness, RAC strength grade, and confinement tube type on the structural performance of retrofitted columns were investigated. The research results demonstrated that FRP-steel composite tube composed of inner steel tube and external FRP showed efficient confinement ability and synergistic effects, which significantly improved the load-bearing capacity and stiffness of the original column. The bonding performance between sandwiched RAC and original concrete mainly depended on the confinement effect of inner steel tube. Waist bulge failure and shear failure were observed on the modified column, which can be divided into five characteristic points according to the stepwise failure behavior. Five existing strength models were reasonably modified by considering the strength contribution of each component material. Meanwhile, a prediction model was proposed based on the ultimate equilibrium theory to evaluate the load-bearing capacity of retrofitted columns. The findings of this study contribute to promoting the application of RAC in structural reinforcement, offering practical cases and references for low-carbon reinforcement strategies.