Mechanical behavior of GFRP-recycled concrete-steel multitube concrete columns under axial compression

This paper proposed a novel GFRP-recycled concrete-steel multitube concrete column (RA-MTCC), which consisted of an external circular GFRP tube, a group of small internal steel tubes in a circular array, and recycled aggregate concrete filling for all the space inside of these tubes. The mechanical behavior of eleven specimens were investigated under uniaxial compressive test considering parameters of the GFRP tube thickness, the configurations of internal steel tubes, the strength and the replacement ratio of recycled coarse aggregates (RCAs). Test results indicated that: (1) the replacement ratio of RCAs had no significant impact on the failure mode and ultimate load bearing capacity but noticeable adverse effect on the stiffness of RA-MTCCs; (2) the small internal steel tube could significantly increase the ultimate load bearing capacity and stiffness of RA-MTCCs, with a maximum enhancement of 82% and 114%, respectively; (3) specimens with GFRP outside exhibited excellent ductility behavior due to the effective confinement provided by GFRP jacket and steel tubes. An analytical model considering the replacement ratio of RCAs was proposed and verified with the present test results. The predicted results showed that the proposed model could accurately predict the full range stress–strain response of RA-MTCCs. Finally, based on the proposed analytical model, a simplified calculation formula for the ultimate bearing capacity of RA-MTCC was proposed.