High-temperature tensile performance of concrete-covered GFRP bars under in-situ thermal exposure

This study investigates the high-temperature tensile performance of glass fiber-reinforced polymer (GFRP) bars embedded in concrete. To address limitations in the existing literature, where residual properties are often measured after cooling, the specimens in this study were directly tested at target temperatures of 100, 200, 300, 500, and 700°C without any cooling phase, providing a more realistic assessment of the behavior of GFRP reinforcements under in situ fire conditions. The experimental program systematically investigated the stress–strain response, tensile strength, elastic modulus, ultimate strain, and toughness of GFRP bars to identify key degradation mechanisms under thermal loading. The results indicate that elevated temperature significantly affects the tensile performance of concrete-covered GFRP bars. Compared to room temperature (20°C), the tensile strength decreased by approximately 43% at 300°C and up to 95% at 700°C, while the modulus of elasticity showed a reduction of about 34% at 300°C and 69% at 700°C. A critical transition was observed beyond 300°C, where rapid degradation in mechanical performance occurred. The ultimate strain exhibited a slight increase from 2.35% to 2.52% at 200°C, followed by a significant decrease to 0.39% at 700°C, indicating progressive deterioration of the fiber–matrix interaction. Similarly, toughness decreased drastically from 110.94 J/mm³ at 20°C to 0.85 J/mm³ at 700°C, reflecting severe loss of energy absorption capacity. These findings demonstrate that maintaining the reinforcement temperature below approximately 300–400°C is essential to preserve the load-carrying capacity of GFRP-reinforced systems under fire exposure conditions.