Durability of recycled GFRP concrete under accelerated aging in alkaline solution: Mechanical and microstructural properties

This study examines the accelerated aging of concrete incorporating recycled glass fiber-reinforced polymer (GFRP) fibers obtained from decommissioned boat hulls. To simulate the alkaline environment of concrete pore solution and expedite deterioration, concrete specimens with and without GFRP were immersed in a heated alkaline solution. Mechanical performance was evaluated after 30, 60, and 90 days of immersion, focusing on compressive strength, modulus of elasticity in compression, and splitting tensile strength. Microstructural investigations were carried out using mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results showed that recycled GFRP concrete experienced a progressive decline in mechanical properties under alkaline exposure. The MIP results revealed an increase in total pore volume and gradual pore coarsening, while XRD analysis indicated a partial reduction in portlandite content. These observations offer new interpretive insights, demonstrating that the evolution of pore structure and portlandite content reflects the intrinsic degradation mechanisms of recycled GFRP concrete. SEM observations further confirmed degradation and microcracking within the fiber–matrix interfacial zone, indicating the weakening of interfacial bonding. The resin–cement interface, however, remained relatively stable. In contrast, the control concrete without recycled GFRP exhibited marginal strength gains, attributed to continued hydration and pore refinement. The study contributes to understanding the microstructural mechanisms of degradation under alkaline conditions and underscores the importance of durability strategies for achieving long-term performance in recycled GFRP concrete.