Degradation of Prestressed GFRP Bars Embedded in Seawater–Sea Sand Geopolymer Mortars under Hydrothermal Seawater Aging

Abstract

The use of glass fiber‒reinforced polymer (GFRP) bars in seawater–sea sand geopolymer mortars (SSGMs) for coastal engineering has gained significant traction because of the potential to enhance the utilization of noncorrosive GFRP reinforcements and natural resources. However, the durability of GFRP bars under the combined impact of prestressing and SSGM cover in seawater environments must be further investigated. In this study, the tensile strength (TS), interlaminar shear strength, and transverse shear strength degradation of GFRP bars were evaluated through hydrothermal seawater aging tests with immersion temperatures including room temperature, 40°C, and 60°C for 83, 180, 270, and 365 days. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were performed to investigate the degradation mechanisms of the GFRP bars after conditioning. The test findings indicated that the effect of increasing the thickness of the SSGM cover on GFRP durability was insignificant. However, without the SSGM cover, GFRP bars demonstrated superior strength retention than SSGM-covered bars after 365 days of seawater immersion because of the absence of alkali ion attack. With the application of prestressing, a greater reduction in strength retention was observed due to increased microcrack formation and penetration of OH− ions and water molecules into glass fibers. In addition, the test results were compared with the bars embedded with seawater–sea sand cement-based mortars, while the SSGM-covered GFRP bars exhibited higher TS retention under identical hydrothermal aging conditions. The high addition of ground granulated blast furnace slag in SSGM contributes to a more compact microstructure, resulting in reduced water diffusion from the outer part of the SSGM. Additionally, the strong alkali-binding capacity of the gel led to a decrease in the pH of the pore solution.