Evaluation of Tensile Strength, Elastic Modulus, Density, and Fiber Content of Glass Fiber-Reinforced Polymer Rebars During and After Exposure to High Temperatures

This study investigated the changes in the physical and mechanical properties of glass fiber-reinforced polymer (GFRP) rebars during and after exposure to elevated temperatures. The specimens were tested at 23 °C (ambient), 150 °C, 200 °C, 250 °C, 300 °C, and 350 °C in an electric kiln coupled to a universal testing machine, without exposure to flames. The mechanical properties evaluated were the tensile strength and modulus of elasticity. After exposure, the surface damage was examined using scanning electron microscopy (SEM), and resin loss was quantified through fiber content tests. At ambient temperature, the average tensile strength and modulus of elasticity of the rebars were 956.4 MPa and 44.7 GPa, respectively. Damage observed during heating was more severe than that observed after heating and subsequent cooling. At 350 °C, up to 37% of the tensile strength was lost during heating, whereas the maximum reduction in modulus of elasticity was 8.3%, indicating that the fibers themselves were not significantly compromised by heat. After exposure and cooling, the maximum reduction in tensile strength was less than 1% (after exposure to 150 °C), while the modulus of elasticity exhibited a 7.9% decrease (after exposure to 200 °C). The glass transition temperature was measured at 101.7 °C. SEM analysis revealed signs of resin degradation caused by heat, including superficial damage, microcracks, reduced resin cover over the fibers, and ruptured fibers. Fiber content tests after exposure demonstrated a direct correlation between reduced fiber content and the decline in mechanical properties. The behavior of GFRP rebars after heating and cooling can provide important insights for assessing structural safety in post-fire buildings, since natural cooling led to partial resin recovery and improvements in mechanical performance.