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The effectiveness of load transfer in the CFRP-adhesive-concrete system highly relies on the integrity of the interfacial bond between adhesive layer and concrete. In the present paper, the effects of water immersion on the mode I fracture energy of the adhesion between CFRP adhesive and concrete were investigated experimentally and numerically. Four-point bending test was conducted to measure the mode I fracture energy of the interfacial layer between adhesive and concrete. The moisture content distribution and the hygrothermal stress were determined by using the finite element method (FEM). The mode I fracture energy was found decreasing with increasing immersion time. The difference between the mode I fracture energy at 2 weeks and 4 weeks is rare. The failure mode of the four-point bending test specimen shifts from concrete failure to interfacial debonding. The moisture content at the adhesive/concrete interface reaches equilibrium after 2 weeks of water immersion. The hygrothermal stress between adhesive and concrete is smaller than the tensile strength of concrete. Deterioration of the physical bond leads to the degradation of bonding strength. The reduction of the mode I fracture energy is more severe than that of the mode II fracture energy.

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Large tow carbon fiber reinforced polymer (CFRP) has been used to reinforce structures serving in hygrothermal conditions, which can be greatly affected by hygrothermal conditions. In this study, the hygrothermal resistance of 48K and 12K carbon fibers and their CFRPs immersed in deionized water at 25, 40, and 60°C were analyzed and compared. The sizing agent on the carbon fiber surface was deboned or hydrolyzed after immersion, resulting in a reduction in the tensile strength, an increase in dispersion, and a decrease in surface activity for carbon fibers. The retention rate of tensile strength of 48K CFRP was higher than 89.7% but was less than that of 12K CFRP due to the higher dispersion of 48K fiber filaments. The interlaminar shear strength (ILSS) and Mode I interlaminar fracture toughness of 48K CFRP was greater than 12K CFRP due to the higher fiber surface activity of 48K carbon fibers and the stronger obstructing effect of 48K fiber tows on the diffusion of water. The degrees of post‐curing and hydrolysis of CFRP were less than 10%. The degradation of the fiber‐matrix interface and a decrease in the strength and elastic modulus of the matrix led to the degradation of the tensile properties, ILSS, and Mode I interlaminar fracture toughness of CFRP.Highlights Debonding of sizing agent reduced the strength of the carbon fiber filaments. Degrees of hydrolysis and post‐curing of the matrix caused by immersion were low. Retentions of ILSS and fracture toughness of 48K CFRP were higher than 12K CFRP. 48K fiber tow possessed a stronger obstructing effect on water diffusion.