Natural embedment versus accelerated ageing: Degradation of BFRP and GFRP bars in alkali-activated concrete

Fibre-reinforced polymers (FRPs) offer a corrosion-resistant alternative to steel, but their long-term durability in alkali-activated concrete (AAC) remains a critical study area with elevated pH levels compared to Portland concrete. To evaluate degradation mechanisms, this study utilizes a three-tier exposure framework: natural FRP embedment in AAC (20°C) for up to 730 days, immersion in chemically replicated AAC pore solution (20°C), and thermal acceleration in the same solution (60°C). Glass (GFRP) and basalt (BFRP) bars were evaluated, isolating fibre and resin vulnerability by comparing single-manufacturer composites with identical vinyl-ester resins alongside commercial epoxy variants with testing the identical neat vinyl-ester under identical environmental conditions. While GFRP demonstrated strong stability across all environments, BFRP variants exhibited substantial deterioration. BFRP-VE specimens showed non-Fickian moisture uptake exceeding 20%, while BFRP-epoxy experienced severe accelerated structural breakdown. Epoxy matrices demonstrated high susceptibility to leaching and mass loss (≈12% in BFRP-EP1), offering limited protection at elevated pH. This rapid fibre-matrix and matrix degradation exposed underlying basalt fibres to accelerated alkaline attack, causing visible fibre etching as supported by SEM-EDX. Crucially, natural concrete embedment caused a 70% interlaminar shear strength reduction for BFRP-EP1 within 540 days (representing 1.5% of a 100-year service life), confirming degradation pattern in simulated environment and demonstrating BFRP vulnerability to AAC pore solutions. Comprehensive molecular, microstructural (SEM-EDX, FTIR), thermomechanical (DMTA), and elemental leaching (ICP-OES) analyses identified the degradation mechanism as the inherent chemical instability of basalt fibres in alkaline media, not just matrix hydrolysis, while the selected vinyl-ester outperformed the specific tested epoxy system.