Experimental study and constitutive modelling of bond degradation for ribbed GFRP bars in cement mortar under freeze-thaw cycles

Cement-grouted anchors are widely used for ground reinforcement and structural stabilization in civil and mining engineering. Glass fiber-reinforced polymer (GFRP) bars offer a corrosion-resistant alternative to steel in harsh environments, yet their long-term anchorage performance is threatened by bond degradation at the bar-mortar interface under freeze-thaw cycles (FTCs). Predicting the durability of GFRP anchors in cold regions remains a critical challenge due to the lack of appropriate theoretical models that account for FTC-induced interfacial bond deterioration. This study presents an integrated experimental and analytical investigation into the bond behavior between ribbed GFRP bars and cement mortar under FTCs, with the aim of developing a predictive bond-slip model for durability assessment of grouted anchors. A total of 48 pullout specimens were tested after 0, 30, 60, and 90 FTCs, considering variables such as bar diameter, mortar strength, and cover thickness. The results show that FTCs cause a clear transition in bond failure mechanism from mechanical interlock dominance to friction dominance. After 90 FTCs, the bond strength, bond stiffness, residual bond strength, and fracture energy decreased by up to 63.95%, 58.04%, 79.26%, and 71.07%, respectively. An FTC-dependent reduction factor f ( n ) was proposed and verified to quantify the degradation of the bond strength, bond stiffness, residual bond strength, and fracture energy. Finally, a novel FTC-dependent bond-slip model was developed and integrated into an analytical framework to predict the load-displacement response, maximum load, and initial stiffness of GFRP grouted anchors under FTCs. The proposed model shows good agreement with experimental data and provides a practical tool for evaluating the residual capacity and service life of GFRP grouted anchors in cold-region infrastructure.