Hysteresis behaviour of seawater sea-sand coral aggregate concrete columns reinforced with steel-FRP composite bars

To address severe corrosion issues in marine infrastructure and promote sustainable island-reef construction, this study elucidates the seismic performance of Seawater Sea-sand Coral Aggregate Concrete (SSCAC) columns reinforced with novel Steel-FRP Composite Bars (SFCBs). A high-fidelity three-dimensional finite element model (FEM) was developed, incorporating SSCAC damage evolution, SFCB fracture behaviour, and calibrated bond-slip effects. Validated against experimental data, the numerical model accurately predicts cyclic responses with a remarkably low average peak load error of 2.63 percent. Extensive parametric analyses reveal that the superior secondary stiffness of the composite bars extends the plastic hinge length of the columns by approximately 25 percent, increasing it from 0.50 h to 0.625 h compared to conventional steel-reinforced counterparts. This structural extension significantly enhances rotational capacity and post-earthquake reparability. Quantitative evaluations indicate that while raising the axial load ratio from 0.3 to 0.5 improves initial stiffness, increasing the ratio further up to 0.77 causes severe ductility loss and accelerated degradation. Thus, maintaining an axial load ratio strictly below 0.5 is recommended. Furthermore, employing higher-strength coral concrete or increasing the steel ratio within the composite bars accelerates concrete crushing, thereby reducing overall ductility. Optimizing the yield strength of the inner steel core proves highly effective for maintaining deformation capacity, whereas an outer FRP elastic modulus exceeding 35 GPa triggers premature brittle rupture. Finally, a degraded three-segment restoring force model is established through theoretical derivation and regression analysis, offering a precise and practical analytical tool for the seismic design of resilient composite structures in marine engineering.