Seawater sea-sand concrete beams reinforced with steel–FRP composite bars under coupled seawater immersion and sustained load: Flexural behavior and degradation modeling

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Steel–fiber reinforced polymer (FRP) composite bars (SFCBs) exhibit high post-yield stiffness and good corrosion resistance, making them a reliable form of reinforcement for seawater sea-sand concrete (SWSSC). Nevertheless, the durability of SFCB-reinforced SWSSC beams has not been fully elucidated, which limits their practical application in marine infrastructure. This study investigated the flexural performance of SFCB-reinforced SWSSC beams under coupled seawater immersion and sustained load (CSISL). Ten SFCB-reinforced SWSSC beams were prepared, and the effects of seawater immersion time and sustained load level were examined. The test results show that the failure mode transitioned from SWSSC crushing to GFRP rupture with increasing duration and severity of CSISL. The deformation and energy ductility indices initially increased and then decreased with conditioning duration. The degradation mechanism under CSISL is associated with reduced SFCB tensile resistance and deterioration of the SFCB–SWSSC bond. Sustained load can aggravate these effects by keeping flexural cracks open and promoting seawater ingress. Finally, a degradation influence coefficient k and an empirical–mechanistic model were proposed to describe the evolution of flexural capacity under CSISL. The model showed good agreement with the calibration data within the investigated range. These findings provide valuable insights for evaluating and improving the durability of SFCB-reinforced SWSSC structures under CSISL.