Efficient combination of steel-FRP composite bar and seawater sea-sand ECC permanent formwork for high-performance slabs: Experimental and analytical investigation

Steel-FRP composite bar (SFCB) is promising for marine infrastructure by combining high stiffness, strength, and ductility with superior durability. However, the construction of marine infrastructure is still hindered by the brittleness of concrete, the lack of freshwater and river sand, and harsh construction conditions. This study proposes a novel solution by integrating SFCB with seawater sea-sand engineered cementitious composite (ECC) permanent formwork and seawater sea-sand concrete. Experimental and analytical investigation was conducted on composite slabs, examining the effects of reinforcement type and ratio, ECC thickness, interface treatment, and fiber content. Results indicate that bottom ECC acted as permanent formwork, suppressed crack propagation, and prevented shear failure. While SFCB maintained high stiffness and strength, slabs with high reinforcement ratios were prone to shear failure, which was mitigated by ECC layer. Longitudinal grooves outperformed transverse ones at ECC-concrete interface, while low fiber content in ECC caused ineffective shear crack restraint and interfacial failure. Further adding ECC at the top of slabs improved deformability, increasing bearing capacity and ductility by up to 87.7 % and 107.9 %, respectively. Cross-sectional analysis identified three failure modes, and calculation methods for reinforcement limits and flexural capacity were proposed. These results support the safe design of high-performance composite slabs.