Seawater-mixed glass fibre-reinforced polymer-reinforced concrete deep beams: experimental and analytical study

This study investigates the structural behaviour of deep concrete beams made with seawater and reinforced with glass fibre-reinforced polymer (GFRP) bars. Eight beams, each 150 mm wide, 500 mm deep and 1600 mm long, were cast and tested to failure under four-point loading. The experimental variables included mixing water type (seawater or freshwater), reinforcement type (GFRP or steel) and cement type (ordinary Portland cement or sulfate-resistant Portland cement). The results showed that seawater-mixed GFRP-reinforced beams reached ultimate loads between 358 kN and 529 kN. Although these values were slightly lower than those of comparable steel-reinforced beams, the GFRP beams offer superior resistance to corrosion and environmental deterioration. Their failure behaviour was marked by wider crack development, while beams cast with sulfate-resistant cement showed better crack control. Importantly, seawater-mixed beams demonstrated mechanical performance comparable to that of freshwater-mixed beams, supporting the sustainable use of seawater in concrete construction. Overall, the findings confirm the feasibility of using GFRP-reinforced seawater concrete in marine structures where durability is critical. A theoretical assessment based on the modified compression field theory also predicted the shear capacity of the deep beams with good agreement to the experimental results and validated the analytical model predictions effectively.