Synergy of steel-FRP composite bars and Engineered Cementitious Composites (ECC) in ultra-lightweight slabs with high strength and ductility

The development of ultra-lightweight, high-performance slabs is crucial for marine concrete infrastructure, yet it is hindered by a critical research gap: the lack of a lightweight material system that simultaneously prevents brittle failure and fully utilizes the strength of durable fiber-reinforced polymer (FRP). This study addresses this challenge by developing a novel composite system incorporating steel-FRP composite bars (SFCB) and lightweight Engineered Cementitious Composites (ECC) fabricated with seawater, sea-sand, and industrial wastes. Twelve slabs were tested under four-point bending, examining failure mechanisms, load-deflection response, strain development, crack patterns, and ductility. Results show that while SFCB increases load capacity by 26.4%-91.1% in concrete slabs, it induces undesirable shear failure. Replacing concrete with lightweight ECC completely shifts the failure mode to ductile flexural failure, increasing load capacity and ductility by up to 87.0% and 97.5%, respectively. This synergy arises from lightweight ECC's superior tensile properties and compressive deformability, enabling full utilization of SFCB's post-yield stiffness and high strength. A simplified flexural prediction model based on strain compatibility and force equilibrium is developed, showing good agreement with experimental results (mean ratio of calculated to experimental ultimate moment capacity of 0.93). The findings provide a valuable experimental and theoretical foundation for ultra-lightweight, high-performance slabs in marine applications.