Concentric axial compressive performance of slender seawater sea-sand concrete square columns reinforced with hybrid FRP–steel bars and FRP interlocking multi-spirals

Hybrid fibre-reinforced polymer (FRP)–steel bar (HFSB) and FRP interlocking multi-spiral reinforced seawater sea-sand concrete (SSC) square columns (HFMSCs) are a hybrid upgradation of FRP interlocking multi-spiral reinforced SSC square columns and HFSB-reinforced concrete columns. Such new columns effectively combine outstanding features of both two original versions, including excellent durability, stiffness, and deformability. Existing research efforts on HFMSCs have been mainly devoted to exploring their short column behaviours, with inadequate understanding of their corresponding slender counterparts. The first insight into behaviours of slender HFMSCs under concentric axial compression was provided in this paper through detailed experimental and theoretical investigations. Effects of reinforcing bar types, FRP transverse reinforcement configurations, FRP interlocking multi-spiral spacings, FRP corner spiral dimensions, and column slenderness ratios were investigated by testing thirteen full-scale specimens. Test results confirmed the combined use effectiveness of HFSBs and FRP interlocking multi-spiral as internal reinforcement for slender reinforced SSC columns and showed that behaviours of HFMSCs decreased with increases of FRP interlocking multi-spiral spacings, FRP corner spiral dimensions, and column slenderness ratios. Finally, two versions of design models were developed for predicting column load capacities, and neglecting compression contributions of FRP outer layers in HFSBs showed more simplicity in practical design uses. •Slender HFSB and FRP interlocking multi-spiral reinforced SSC square column is tested.•Impacts of interlocking multi-spiral configuration and column slenderness are studied.•HFMSCs with column slenderness ratios up to 31.2 meet moderate deformability demands.•Interlocking multi-spiral with high elongation ratios can enhance behaviour of HFMSCs.•Two versions of design models on column load capacity are presented for such columns.