Post-heating punching shear performance of seawater sea-sand concrete GFRP-reinforced two-way flat slabs

The freshwater and natural sand depletion has promoted the utilization of seawater and sea–sand as a sustainable alternative in concrete production. Meanwhile, replacing conventional steel reinforcement with glass fiber–reinforced polymer (GFRP) bars represent an effective approach to enhancing the durability of reinforced concrete systems, particularly flat slabs. However, these systems remain susceptible to elevated temperatures and punching shear failure. This study investigates the behavior of seawater sea–sand concrete (SWSSC) flat slabs reinforced with GFRP bars under elevated temperatures, followed by punching shear loading. Key variables included slab thickness (60 and 80 mm), reinforcement ratio (0.57 and 0.85%), concrete compressive strength (35 and 60 MPa), and exposure temperature (20, 200, 400, and 550°C). Performance metrics such as punching shear capacity, cracking load, stiffness, toughness, ductility, punching angle, and load–deflection response were evaluated and compared with existing code-based and analytical models. Results show that increasing the reinforcement ratio enhances load capacity, toughness, and crack control. Greater slab thickness significantly improves load capacity, stiffness, and energy absorption. Higher concrete strength increases cracking load and reduces the adverse effects of elevated temperatures. Temperature effects reveal that mechanical properties are largely preserved up to 200°C, while notable degradation in strength, stiffness, and toughness occurs at 400 and 550°C. Additionally, increases in reinforcement ratio, slab thickness, or concrete strength led to a reduction in the punching shear angle. Moreover, any parameter increase, reinforcement ratio, slab thickness, or concrete compressive strength, led to a reduction in the punching shear angle.