Axial compression behavior of seawater sea-sand concrete columns reinforced with SFCBs and closed-type winding GFRP ties

To solve the problems of steel corrosion in coastal and cross -sea concrete structures, shortage of freshwater and river sand, and slip at the overlap of pultruded glass Fiber Reinforced Polymer (GFRP) stirrups, seawater sea -sand concrete (SWSSC) structures reinforced with longitudinal Steel-FRP composite bars (SFCBs) and closed-type winding GFRP ties (CWFTs) provide a good solution. However, little research currently focuses on the axial compression performance of SWSSC columns using longitudinal SFCBs and CWFTs. Hence, twelve SWSSC columns are designed, with experimental variables including the longitudinal reinforcement type, stirrup type, CWFT spacing and width. The findings reveal that the tensile and compressive elastic moduli of SFCBs are close. Moreover, the inner steel bar can continue to support large compressive loads even after the outer GFRP layer fractures, albeit with a noticeable decrease in elastic modulus. Interestingly, replacing longitudinal steel bars with equal-stiffness SFCBs does not substantially alter the axial compression performance of the columns, including bearing capacity and ductility. Notably, the bent portion of CWFTs exhibits a strength that can reach up to 71.6% of their tensile strength of the straight part, and their elastic modulus can exceed 40 GPa. When used in SWSSC columns, CWFTs display fracture strains ranging from 0.89% to 1.42%. While increasing the volumetric ratio of CWFTs has little impact on the axial bearing capacity of the columns, it significantly enhances their ductility. When the stirrup ratio is equal, the bearing capacity and ductility of CWFT columns approximate 92% and 90% of those found in steel -tie columns, respectively. Furthermore, when the volumetric stirrup ratio of CWFT columns is double that of steel -tie columns, the former exhibits comparable or even superior ductility compared to the latter. Finally, a modified formula for predicting axial capacity is proposed, which can accurately predict the axial capacity of SWSSC columns reinforced with longitudinal SFCBs and CWFTs.