Long-term compressive performance of GFRP reinforcement in marine and alkaline environments

While the durability of Glass Fiber-Reinforced Polymer (GFRP) reinforcement has been extensively studied under tensile loading, limited information is available on its long-term compressive performance, despite its importance in GFRP-reinforced columns, piles, and other compression-critical marine infrastructure elements where bars are continuously exposed to seawater (SW) and highly alkaline solution (AS). This study addresses this knowledge gap by systematically investigating the compressive performance of GFRP bars after prolonged exposure to natural seawater (North Atlantic) and alkaline solutions, considering three bar sizes (15M, 20M, and 25M), three conditioning temperatures (20, 40, and 60 °C), and three immersion durations (1000, 2000, and 3000 h). A total of 288 bar specimens, including 270 conditioned and 18 unconditioned (control specimens), were tested using a custom-designed compression fixture. Mechanical properties, failure modes, and microstructural and chemical changes were assessed through compressive testing, SEM, EDS, and FTIR analyses. Results show that compressive strength decreases progressively with exposure time and temperature, with alkaline environments inducing the most severe degradation, particularly in larger bars, while the elastic modulus remained mostly unaffected. After 3000 h at 60 °C, strength retention ranged from 0.81 to 0.87 in seawater and 0.73–0.86 in alkaline solution across bar sizes. Seawater exposure primarily promoted moisture-induced matrix plasticization, hydrolysis of the vinyl ester resin, and gradual fiber–matrix interfacial deterioration. In contrast, alkaline exposure additionally induced resin saponification and hydroxyl-ion attack on the silica-based glass fiber network, resulting in more severe interfacial degradation and greater strength loss. Arrhenius-based service life predictions indicate 100-year retention of 0.70 (SW) and 0.60 (AS) at 10 °C (average annual temperature in North Atlantic coast), decreasing to 0.63 (SW) and 0.48 (AS) at 26 °C (average annual temperature in South Atlantic coast). The study provides novel quantitative insights into the long-term compressive durability of environmentally aged GFRP bars, supporting improved design and maintenance strategies for marine infrastructure.