<p>Fiber-reinforced polymer (FRP) bars have been used in reinforced-concrete (RC) members, because of their good mechanical behavior and non-corrosive nature. Nevertheless, its durability reduces when exposed to humidity and alkaline ions. It is well documented that epoxy-based and vinyl ester-based FRPs exhibit greater chemical resistance than polyester-based FRPs, which might suggest they maintain superior mechanical properties after exposure to chemical environments. However, recent studies have shown that polyester-based FRP bars can retain similar tensile properties even after alkaline aging. This paper presents an experimental program intending to investigate the hydrothermal effect in RC-beams, with polyester-based GFRP bars. Pre-cracked and uncracked GFRP-RC beams were immersed in distilled water at 60&#xa0;°C for up to 6&#xa0;months and then subjected to three-point bending, employing the digital image correlation (DIC) method to assess the cracking behavior. Uncovered GFRP bars were exposed to alkaline aging for comparison. The degradation of the bars was observed in both exposure environments; however, more severe results were observed in aged uncovered GFRP bars, showing a tensile strength retention of approximately 44% after six months. In scenarios with concrete-covered FRP bars, the strength retentions were near to 60%. Furthermore, pre-cracked beams exhibited more pronounced degradation when compared to those uncracked. The change in beams behavior was more pronounced in the first three months of aging, converging to a quasi-stable strength retention level. With the aid of DIC, large crack openings were observed for unaged GFRP-RC beams. At a similar level of bending load, these values decreased significantly after three months of aging; however, after six months of aging, the crack openings increased again. Furthermore, pre-cracking did not have a major influence on the value of crack-openings at peak loads.</p>