Compressive behavior of recycled aggregate concrete-filled square steel tubular columns with FRP-confined concrete core

To expand recycled aggregate concrete (RAC) application to structures, this study experimentally investigated the compressive behaviour of a composite column, called RAC-filled square steel tubular columns with FRPconfined concrete core (SCFC). The study aimed to compare the behaviour of this type of column with those using natural aggregate concrete (NAC) and explore the different confinement mechanisms between SCFC and single-steel tube confined concrete (concrete-filled steel tube (CFST)). The experimental results demonstrated the influence of RAC on the axial load-strain response. However, this effect could be eliminated by increasing the confinement. The confining effects triggered by the square steel tube and FRP tube on the concrete did not reach the maximum simultaneously. Therefore, the peak load was slightly affected. However, the ductility improved significantly, and good deformation compatibility was achieved. Accordingly, an analysis-oriented stress-strain model capable of capturing the load-strain response of RAC-filled square SCFC columns under axial compression was developed.