Analytical Modelling of Bond-Strength Degradation of Glass Fiber-Reinforced Polymer (GFRP) Bar–Mortar Interface Under Freeze–Thaw Cycling

Grouted anchors made of glass fiber-reinforced polymer (GFRP) have gained popularity in cold-region construction projects, primarily owing to their resistance to corrosion and low density. Although prior research has addressed the bond–slip characteristics of FRP-to-concrete joints, a theoretical formulation that links cumulative freeze–thaw damage of the mortar matrix to the progressive loss of bond strength at the GFRP bar–mortar interface is still lacking. This work provides a combined experimental and theoretical examination of how the bonding capacity of ribbed GFRP bars in cement mortar declines after 0, 30, 60, and 90 FTCs. Compression and splitting tension tests were carried out on mortar cubes, while pullout specimens were used to assess the interfacial bond strength. Two mortar grades commonly used in anchorage practice (M25 and M35) and two bar diameters (12 mm and 16 mm) were selected as test variables. After 90 FTCs, the maximum bond strength fell by as much as 64.1%, whereas the post-peak residual bond strength suffered an even more pronounced drop of up to 79.3%. Meanwhile, the residual-to-peak-bond-strength ratio decreased steadily with the number of FTCs, marking a shift from a mechanically interlocked interface to a friction-governed one. The higher-grade mortar (M35) experienced clearly superior resistance to freeze–thaw attack compared to M25, while the larger-diameter bars (16 mm) degraded faster. An analytical model for estimating the bond-strength degradation is proposed, where an exponential environmental factor was introduced and the decay constants were calibrated via nonlinear regression against the bond-strength retention ratios at 0, 30, 60, and 90 FTCs. The proposed models are calibrated empirical relationships that reproduce the measured degradation well within the tested parameter ranges and indicate reasonable internal stability under leave-one-group-out cross-validation. This study provides two theoretical provisions: the freeze–thaw degradation of the GFRP–mortar bond can be effectively described by a single exponential damage law whose decay constant quantifies the rate at which the interfacial capacity is exhausted, and the residual-to-peak-bond-strength ratio serves as a mechanistic indicator of the transition from mechanical interlock to friction-controlled failure. These provisions quantitatively link mortar degradation to interfacial capacity loss, thereby providing a theoretical basis for durability design of GFRP grouted anchors in cold regions.