Interfacial deterioration mechanisms in polymer repair mortar and concrete under salt freeze thaw cycles

The durability of the interface between polymer-modified mortar and old concrete remains a critical concern, particularly under salt freeze-thaw cycles. This study investigates the interfacial deterioration behavior of ethylene vinyl acetate (EVA) repair mortar and old concrete subjected to freeze-thaw cycles in 3.5% and 7.0% NaCl solutions. The evolution of interfacial bonding strength, ultrasonic pulse velocity (UPV), failure mode, microhardness, pore structure and chloride transport was systematically evaluated. The results indicate that salt freeze-thaw cycling leads to progressive interfacial degradation, characterized by reduced bond strength and UPV, along with a transition in failure mode from cohesive substrate failure to adhesive interfacial debonding. The 4% EVA modified repair mortar exhibits the best overall performance, effectively enhancing interfacial bonding and retarding freeze-thaw deterioration, whereas excessive EVA addition reduces the interfacial bonding strength. Microstructural analyses indicate that freeze-thaw damage is governed by pore structure coarsening in the interface, while appropriate EVA incorporation effectively suppresses crack development and stabilizes the pore structure. A strong correlation between UPV and interfacial bonding strength further indicates that UPV can reflect the deterioration trend of the interface. Moreover, specimens exposed to 7.0% NaCl exhibit less severe deterioration than those in 3.5% NaCl, which may be associated with reduced ice-induced damage resulting from freezing point depression and changes in pore-water freezing behavior. These findings enhance the understanding of interfacial deterioration mechanisms and support the durability design of polymer repair mortar under salt freeze-thaw conditions.