Novel FRP interlocking multi-spiral reinforced-seawater sea-sand concrete square columns with longitudinal hybrid FRP–steel bars: Monotonic and cyclic axial compressive behaviours

•Combined uses of FRP interlocking multi-spirals, SSC and hybrid FRP–steel bars are explored.•Hybrid FRP–steel bars as longitudinal reinforcement endure structural controllability.•Reducing spacings of middle or/and corner spirals improves post-peak deformability.•Unloading/reloading cycles foster plastic deformation and load degradation in FMSSCs.•Design-oriented load–strain model based on full-section confining stress is proposed. Fibre-reinforced polymer (FRP) interlocking multi-spiral reinforced-seawater sea-sand concrete (SSC) square columns (FMSSCs) can optimize the merits of FRP and SSC while providing excellent confinement to the concrete in square columns. However, existing works on FMSSCs are limited to columns with longitudinal FRP bars, whose complex compressive properties and natural brittleness challenge the structural ductility. To this end, this paper develops a variant of FMSSCs by employing hybrid FRP–steel bars (HFSBs) as longitudinal reinforcements to break out of existing limitations. The HFSBs with the inner steel bar and outer FRP layer possess distinctive post-yield stiffness and remarkable corrosion-resistant performance. A series of monotonic and cyclic axial compression tests are conducted to investigate effects of concrete types, reinforcement configurations, and loading schemes on behaviours of FMSSCs. Test results demonstrated that the use of SSC had a negligible effect on the short-term structural performance, and the presence of longitudinal HFSBs endured FMSSCs to introduce damage controllability, especially under cyclic loading. Additionally, repeated loading/reloading cycles exerted cumulative impacts on the plastic deformation and load degradation of FMSSCs but affected cyclic envelope behaviours little. Finally, a design-oriented load–strain model was proposed to predict monotonic and cyclic axial responses of FMSSCs with longitudinal HFSBs.