Compressive performance of double steel tube confined geopolymer concrete columns incorporated with waste rubber

To achieve the harmless disposal of waste rubber, this paper proposes a novel type of composite column, i.e., double steel tube confined rubberized geopolymer concrete columns. In this composite column, the external tube is discontinuous at the beam-column joints to fully utilize its confinement effect, and the internal tube is directly connected to the joint, which is filled with rubberized geopolymer concrete. The dual confinement of internal and external tubes in this composite column can effectively improve the mechanical properties of traditional rubberized geopolymer concrete members. Axial compressive tests of 21 circular columns with a diameter of 206 mm were conducted to investigate the effects of key parameters, such as configuration of external tubes, diameter ratios of the internal and external tubes, steel tube thicknesses, and rubber ratios of core concrete. The results indicate that as the rubber ratio increases from 0 % to 50 %, the reduction in load capacity of double steel tube confined concrete columns is within 25 %, while the load capacity of unconfined rubberized concrete columns decreases by more than 40 %. Moreover, under the same total volume of rubber, the load capacity and ductility of the composite column enhance with the increase of the internal tube diameter, which can provide guidance on the type selection of the composite column for the recycling of waste rubber. Finally, considering the coupling effects of the rubber ratio and the diameter ratio of the internal and external tubes on the peak and residual behavior of core rubberized geopolymer concrete, a new stress-strain model is proposed for core concrete under the dual confinement of internal and external tubes. The reliability of the proposed model has been verified by comparing with the experimental results.