Effects of Fiber Types on Interlaminar Shear Strength Degradation of Epoxy-Based G/CFRP Bars under a Marine Concrete Pore Solution Environment

Interlaminar shear strength (ILSS) is a critical parameter for assessing the durability of fiber-reinforced polymer (FRP) bars. However, the effects of fiber types on ILSS degradation of epoxy-based glass and carbon FRP (GFRP and CFRP) bars in marine concrete environments remain insufficiently explored. This study investigates the degradation behavior and failure modes of epoxy-based G/CFRP bars through accelerated marine concrete exposure at 25°C, 40°C, and 55°C for up to 84 days. ILSS testing and microscopic observations, including super-depth-of-field and scanning electron microscopy, are conducted to examine the damage progression and degradation mechanisms. Experimental results reveal that, although carbon fibers remain chemically stable under marine exposure, epoxy-based CFRP bars exhibit more severe ILSS degradation than GFRP bars, primarily due to their larger fiber–matrix interfacial areas and weaker interfacial bonding. Moreover, the ILSS failure modes of G/CFRP bars are found to depend on the damage levels. In addition, the hydroxyl ion diffusion model (HIDM) and 2D finite element analysis are developed to predict ILSS degradation, with experimental validation of the predicted ILSS retentions. The findings systematically identify the ILSS durability of epoxy-based GFRP and CFRP bars under marine exposure and provide an efficient modeling framework for the durability assessment of FRP-marine concrete structures.