Effect of High Temperature on the Mechanical Behavior of Glass Fiber–Reinforced Polymer Composite Bars under Compressive Loading

Glass fiber–reinforced polymer (GFRP) bars have increasingly been adopted as reinforcement in concrete structures; however, their compressive behavior under elevated temperatures has not been comprehensively investigated. This study uniquely addresses this gap by conducting compressive tests under stationary conditions on GFRP bars subjected to temperatures ranging from 20°C to 300°C without the application of any cooling process, an approach that distinguishes this research. Further, the Ø10 mm helically wrapped GFRP bar specimens were meticulously designed with a slenderness ratio of 1.5 to minimize the effects of buckling and splitting, which is often a challenge in compressive testing at elevated temperatures. In addition, the degradation mechanism of GFRP bars was further analyzed by scanning electron microscopy in the microscopic level. The experimental results demonstrate that temperatures exceeding 100°C led to significant degradation of the resin-glass fiber bond, resulting in a failure mode characterized by splitting and crushing. At temperatures above 250°C, resin combustion caused fiber delamination at the ends of the specimens, along with a crushing failure. Additionally, the specimens lost approximately 70% of their compressive strength once they reached the glass transition temperature. Finally, at the temperature where the resin decomposes (300°C), the residual strength was found to be only 5% of the compressive strength at room temperature.