Hygrothermal-mechanical coupling effects on the bond behavior of CFRP-concrete adhesive joints

This study investigates the durability of the adhesive bond between carbon fiber-reinforced polymer (CFRP) plates and concrete, under sustained loading and controlled hygrothermal environments. A dedicated creep test setup based on a calibrated spring system was first developed to apply a sustained load to double-shear specimens through controlled spring compression. The device was designed in a compact configuration to allow installation inside a climate chamber and enable coupling with controlled environmental conditions. Preliminary verification tests confirmed both the symmetrical behavior of the double-shear specimens and the stability of the load applied by the springs, demonstrating the suitability of the setup for long-term durability studies under coupled hygrothermal-mechanical loading. Two CFRP strengthening systems bonded to concrete substrates were subsequently exposed to various hygrothermal conditions at 40 °C, under relative humidity (RH) levels ranging from moderate to moisture saturation, while simultaneously subjected to sustained shear loading corresponding to approximately 16–18% of the initial bond capacity. Bond behavior was assessed through time-dependent deformation monitoring, residual mechanical testing, and stress-transfer analysis using the Völkersen model derived from strain measurements. Results showed stable mechanical performance for RH levels up to 75%, with limited time-dependent deformation and no significant evolution of residual strength or stress distribution over time, although a progressive shift in failure mode from concrete cohesive failure to mixed adhesive–concrete failure was observed after aging. In contrast, moisture-saturated conditions (100% RH) led to rapid degradation, characterized by reduced residual bond capacity, joint softening, and premature creep failure of the double-shear specimens, predominantly associated with interfacial debonding. Under these severe conditions, significant differences in durability were observed between the two CFRP systems, reflecting the influence of adhesive properties in the resistance to damage induced by coupled hygrothermal-mechanical loading. These findings emphasize the critical role of moisture saturation, in combination with elevated temperature, in governing the long-term performance of CFRP-strengthened concrete structures under coupled environmental-mechanical loading.