Hydrothermal aging of polyester-based GFRP bars in alkaline environment

Fiber-reinforced polymer (FRP) bars are typically made of epoxy, polyester, or vinyl ester matrices. Despite their advantages such as good mechanical properties, low weight, electromagnetic transparency, and non-corrosive behavior, the matrix may degrade when exposed to moisture and alkalinity. It is well documented that epoxy and vinyl ester are more chemically resistant than polyester matrices. However, literature research demonstrated that aged polyester GFRP bars can exhibit comparable mechanical performance. Nevertheless, few studies were conducted to investigate the durability of polyester GFRP bars subjected to different conditioning times and temperatures to obtain appropriate acceleration factors. This study examines the hydrothermal effect of an alkaline environment and distilled water on the mechanical performance of polyester glass fiber-reinforced polymer (GFRP) bars. The samples were immersed in distilled water at 60 °C and in an alkaline solution at 30 °C, 40 °C and 60 °C for up to 6 months. The study analyzed the tensile strength, Young’s modulus, and apparent horizontal shear strength. Additionally, a predictive degradation model based on the Arrhenius Law was developed. The results indicate a high moisture uptake ability. The apparent horizontal shear strength retention was 69% after six months in alkaline solution, whereas the tensile strength retention was as low as 44%. In contrast, immersion in water resulted in a tensile strength retention of 53%. The scanning electron microscope (SEM) analysis revealed the occurrence of fiber-matrix interface debonding, yet no evidence of fiber degradation was observed. The Fourier-transform infrared spectroscopy (FTIR) analysis indicates the occurrence of matrix hydrolysis. The long-term behavior of the polyester GFRP bars under study was established.