Sand-coated hybrid FRP tubes for marine applications: Bond performance with seawater-sea sand concrete under seawater

• Bonds-slip behaviour between sand coated hybrid FRP tube and SWSSC exposed to seawater. • Bond failure modes of sand coated hybrid FRP tube and SWSSC after exposure to seawater. • Long-term bond strength prediction of sand coated hybrid FRP tube and SWSSC after exposure to seawater. This study investigates the durability of the bond between sand-coated hybrid carbon-glass fibre-reinforced polymer (HFRP) tubes and seawater-sea sand concrete (SWSSC) under simulated marine conditions. Concrete-filled filament-wound HFRP tubes were immersed in seawater at temperatures of 25 °C, 40 °C, and 60 °C for durations of 30, 90, and 120 days. Push-out tests were conducted to evaluate the bond performance of the concrete-filled FRP tubes (CFFTs), focusing on failure modes, bond strength, and bond-slip characteristics. Results revealed an initial enhancement in bond strength for all conditioned samples after 30 days compared to unconditioned references, followed by a gradual decline, culminating in a maximum bond strength reduction of 9 % after 120 days. The primary failure mode for conditioned specimens occurred at the interface between the HFRP tube and the sand-coated layer, while unconditioned specimens failed at the interface of the tube and the concrete. The study found that post-peak bond behaviour varied, with bond strength either maintained or gradually reduced, mainly due to the loss of chemical adhesion and friction. Using the fib Bulletin 40 approach, a knock-down factor of 0.5 is recommended for bond strength durability over a 50-year service life in seawater environments. This research stresses the potential of sand-coated HFRP tubes and SWSSC as sustainable, marine-compatible construction materials that meet structural durability requirements.