Steel–FRP Composite Bars and Closed-Type FRP Ties of Concrete Columns for Synergistic Enhancement in Axial–Flexural Stiffness, Strength, and Ductility

Durable fiber-reinforced polymer (FRP) bars are well-suited to marine infrastructure, particularly in applications involving seawater sea-sand concrete. However, their adoption is constrained by the low elastic modulus and brittle failure behavior of longitudinal FRP bars, as well as the inadequate confinement provided by conventional pultruded FRP ties. These limitations adversely affect serviceability (e.g., deflection and crack width), strength, and ductility of concrete columns subjected to axial–flexural loading. To address these challenges, this study proposes a novel hybrid reinforcement scheme for concrete columns, combining longitudinal steel–FRP composite bars (SFCBs) with closed-type FRP ties to enhance axial–flexural performance. An experimental program was conducted on hybrid longitudinal SFCB and FRP tie reinforced concrete (hybrid-RC) columns under various loading conditions to elucidate axial–flexural failure mechanisms, quantify the mechanical contribution of SFCBs, and evaluate the confinement behavior of closed-type FRP ties. Similar to conventional steel bars reinforced concrete columns, hybrid-RC columns exhibit compression-controlled and tension-controlled failure modes while achieving comparable serviceability, strength, and ductility. Notably, the post-yielding stiffness of SFCBs increases the load-carrying capacity by 11.3% under tension-controlled failure. In addition, closed-type FRP ties provide significantly improved confinement compared with pultruded FRP ties, resulting in substantial enhancement of axial–flexural capacity. An analytical model is further developed to predict axial–flexural failure modes and strength, incorporating the distinctive constitutive behavior of the hybrid reinforcement system and the experimentally observed failure mechanisms of hybrid-RC columns.