Bond–Slip Performance of GFRP Rebars in Concrete Under Alkaline and Thermal Conditioning

This study investigates the bond–slip behavior of glass fiber-reinforced polymer (GFRP) bars embedded in concrete and exposed to alkaline environments at different temperatures. Although GFRP reinforcement is increasingly adopted for its corrosion resistance, the long-term bond performance of the bar–concrete interface in high-pH conditions is still not fully understood. To help close this gap, a comprehensive database of 84 pull-out tests from the literature was assembled, focusing on three key parameters: bar surface configuration, exposure duration, and conditioning temperature. The comparative analysis highlights the dominant role of surface treatment in bond degradation and reveals substantial variability across existing results. To complement the literature review, additional pull-out tests were carried out on sand-coated GFRP bars conditioned in an alkaline solution (pH = 12) for 1.5 months at ambient temperature and at 60 °C. These tests showed average reductions in bond strength of approximately 28% and 32%, respectively, compared with unconditioned specimens, together with marked changes in the post-peak portion of the bond–slip response. An analytical formulation was also applied, not as a novel bond–slip law but as a consistent mechanical framework to interpret durability-induced degradation effects, to describe the local interface shear stress–slip law, and to assess the resulting stress and slip distributions along the bonded length. Overall, the combined experimental and analytical findings emphasize the need to account for environmentally induced degradation when evaluating durability and defining design criteria for GFRP-reinforced concrete structures.