Experimental research on the flexural behavior of seawater sea-sand ECC composite beams with U-shaped and channel-shaped GFRP profiles as permanent formwork

To advance coastal and island infrastructure construction and research on seawater sea-sand engineered cementitious composite (SS-ECC) members with fiber-reinforced polymer (FRP) profiles, this study proposes an innovative composite beam integrating high-ductility SS-ECC with U-shaped and channel-shaped glass fiber-reinforced polymer (GFRP) profiles serving as permanent formwork, forming seawater sea-sand ECC (GFRP-SS-ECC) composite beams with U-shaped and channel-shaped GFRP profiles as permanent formwork. Five specimens were subjected to four-point bending tests to investigate flexural behavior. The composite beams exhibited progressive flexural failure with favorable ductility. U-shaped GFRP wall thickness significantly influenced flexural behavior, while SS-ECC strength primarily delayed interfacial slip and enhanced member ductility. At low loads, mid-span strain distribution conformed to the plane section assumption, which gradually became invalid at higher loads. Based on experimental observations, a flexural capacity calculation method was developed by combining the strip method with the infinitesimal element method. Validation against test results confirmed that the method accurately predicts ultimate flexural capacity, providing guidance for designing durable, high-ductility GFRP-SS-ECC composite beams in marine environments.