Low-alkalinity seawater sea sand columns reinforced with hydrophobic SFCBs and FRP hoops: A durability and energy dissipation capacity enhancement scheme for marine engineering

The novel hybrid hydrophobic longitudinal steel-FRP composite bars (SFCBs) and FRP hoops reinforced low-alkalinity seawater sea sand concrete (SWSSC) column has been developed. It overcomes the issues of steel bar corrosion and the shortage of freshwater and river sand. However, its long-term performance in marine environment remains unclear. Hence, this study investigates the effects of marine exposure on the failure modes, long-term load capacity and ductility of hybrid-reinforced column. Results show that the hydrophobic coating on reinforcement and the low OH^–concentration of concrete effectively inhibits the degradation of FRP and the FRP-concrete interfacial debonding, reducing localized strain concentration and significantly improving the long-term performance of hybrid-reinforced column. After immersion in seawater at 55℃ for 8 months, the load capacity and pre-peak ductility coefficient of hybrid-reinforced column are 34.7 % and 25.0 % higher, respectively, than those of conventional reinforced SWSSC column. Moreover, a prediction model for long-term load capacity, incorporating the time-dependent deterioration of materials, is proposed and validated, showing superior feasibility and accuracy. It predicts that the service life of hybrid-reinforced column is approximately 3-time longer than that of conventional column. These findings provide reliable theoretical models and experimental evidence to support durable design of marine infrastructure.