LAUSR

Abstract Polyethylene terephthalate (PET) fibre-reinforced polymer (FRP) composites are an emerging form of composites based on recycled PET fibres and have a large rupture strain. A novel form of hybrid columns with PET FRP tubes, i.e., square PET FRP-confined concrete encased high-strength steel (HSS) hybrid columns (referred to as PET FRP-confined concrete-steel columns or PFCCSCs), was recently proposed. The compressive behaviour of circular FCCSCs has been investigated, but there is no research on the compressive behaviour of square PFCCSCs (SPFCCSCs). In SPFCCSCs, the inner HSS tube is expected to be effective in decreasing the confinement nonuniformity in FRP-confined square columns and enhancing the ultimate axial strain and compressive strength of the core concrete. Furthermore, the axial strength of the inner HSS tube is expected to be sufficiently utilized because its buckling is delayed due to confinement from the surrounding concrete. In total, fourteen columnar specimens were tested under monotonic or cyclic axial compression, and the effects of key parameters (i.e., the steel tube yield stress and loading patterns) were investigated. The test results demonstrate that the inward and outward buckling of the inner steel tube in SPFCCSCs can be effectively suppressed or delayed by the surrounding concrete, which was well confined by the PET FRP tube. The compressive strength of a hybrid column is approximately 20% higher than the simple summation of the corresponding strengths of an FRP-confined concrete section and an HSS tube section due to the optimal combination of the three components in the hybrid columns. The SPFCCSCs with a PET FRP tube exhibit a monotonically ascending load-strain response with excellent ductility, which is of particular interest for applications in seismic engineering and underground engineering. The loading scheme has a marginal effect on the load-strain envelope response of SPFCCSCs.