Flexural enhancement and capacity prediction of seawater sea-sand concrete/ECC layered composite beams reinforced with steel-FRP composite bars

Seawater sea-sand engineered cementitious composite (SS-ECC) and corrosion-resistant steel-FRP composite bars (SFCB) offer promising solutions for durable marine infrastructure. However, the composite action between SS-ECC and SFCB on flexural performance and applicable design methods have not been fully explored. This study investigates the flexural behavior of SFCB-reinforced seawater sea-sand concrete/ECC layered composite beams. A total of 12 beam specimens were tested, with reinforcement type, reinforcement ratio, ECC fiber content, and ECC thickness as the variables. Results demonstrated that placing ECC at beam bottom effectively controlled pre-yield crack widths within 0.11 mm and enhanced load capacity by 5.3%–20.5% by providing tensile contribution and improving stress transfer of SFCB. Extending ECC into the compression zone exploits its superior compressive deformability, enabling improved utilization of SFCB’s post-yield stiffness and mitigating ductility loss at high reinforcement ratios. Compared with SFCB-reinforced concrete beams, SFCB-reinforced ECC beams achieved 48.6%–66.2% higher load capacity and 38.6%–96.7% higher ductility. Performance-based design recommendations for ECC configuration were proposed, and validated theoretical models of reinforcement limits and load capacity were established. This study fills the identified gap by providing experimental evidence and design guidelines for developing durable and high-performance composite beams for marine infrastructure.