Compressive behaviour of composite columns composed of RAC-filled circular steel tube and profile steel under axial loading

Axial loading tests are conducted on composite columns composed of recycled aggregate concrete (RAC) filled circular steel tube and profile steel. This research primarily aims to investigate the compressive behaviour of composite columns on the basis of the axial compression tests of 11 specimens under uniaxial loading. The design parameters of these columns in the tests are recycled coarse aggregates (RCA) replacement percentage, diameter-thickness ratio of circular steel tube, profile steel ratio and slenderness ratio. The failure process and modes, load-displacement curves, Poisson's ratio of circular steel tube, characteristic loads and ductility of composite columns are presented and analysed. The influence of design parameters on the compressive behaviour of columns is also investigated in detail. Results show that the composite columns present good mechanical performance under axial loading. The profile steel yields ahead of the steel tube and the core RAC exhibits shear failure or crush failure. The axial bearing capacity of composite columns decreases as the magnitude of RCA replacement percentage increases, but the deformability still performs well. The decrease in diameter-thickness ratio and the increase in profile steel ratio are beneficial to the axial bearing capacity and column ductility. In addition, the axial compression behaviour of composite columns decreases significantly as slenderness ratio increases. On the basis of the test and analysis results, a modified AISC design method is proposed to calculate the nominal axial bearing capacity of composite columns composed of RAC-filled circular steel tube and profile steel.