Mitigating Degradation of GFRP Bars in Seawater–Sea Sand Concrete by Reducing Alkalinity under Accelerated Hydrothermal Seawater Aging

Glass fiber‒reinforced polymer (GFRP) bars are increasingly recognized as promising reinforcement options for seawater–sea sand concrete because of their chloride resistance. However, the internal alkalinity of concrete remains a crucial factor impacting the durability of concrete-wrapped GFRP bars in marine environments, particularly when used in prestressed applications. To increase the durability of prestressed GFRP bars in seawater–sea sand concrete, the effectiveness of reducing internal alkalinity by incorporating phosphogypsum was investigated. The accelerated aging test of GFRP bars was conducted under various exposure periods (30, 60, 120, and 240 days), temperatures (25°C, 40°C, and 60°C), and prestress levels (0% and 20%). The long-term performance of the GFRP bars wrapped with different-alkalinity concrete was evaluated. Scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis were employed to investigate the failure mode of the conditioned GFRP bars and the degradation mechanism. The results showed that 10% phosphogypsum incorporation significantly mitigated the deterioration of the GFRP bars. Specifically, the tensile strength retention after 240 days improved by 13.0%–22.3% for non-prestressed FRP bars and 5.1%–17.9% for prestressed FRP bars when wrapped with low-alkalinity seawater–sea sand concrete. Unlike non-prestressed bars, the occurrence of delaminated resin and fiber fracture is more pronounced for prestressed bars. The addition of phosphogypsum neutralizes hydroxide ions in the cement matrix and improves the mechanical strength of concrete, thereby mitigating alkali attack and inhibiting water molecule infiltration within GFRP bars. The findings provide experimental data and validation for existing theories on the durability of prestressed GFRP bars in aggressive environments, supporting future advancements in marine infrastructure design.