Comparative Study of the Degradation of CFRP–Concrete Interfacial Bond Performance with EP and MPC Adhesives Under Sulfate Freeze–Thaw Cycles

In the saline, cold, and arid regions of Western China, the adhesive performance at the carbon fiber-reinforced polymer (CFRP)–concrete interface critically affects the long-term reliability of CFRP-strengthened structures. Replacing the organic epoxy resin (EP) with inorganic magnesium phosphate cement (MPC) has been proposed as an alternative. However, comparative studies on the deterioration of MPC- and EP-bonded CFRP–concrete under sulfate freeze–thaw cycles are limited. This study employed double-shear tests to systematically compare the failure modes, ductility, and bond performance of the CFRP–concrete interface bonded with MPC and EP after 25, 50, and 75 sulfate freeze–thaw cycles. The results indicate that, as the number of cycles increased, MPC-bonded specimens exhibited progressive interfacial peeling, whereas EP-bonded specimens underwent abrupt brittle fracture. At 0, 25, 50, and 75 cycles, the peak strains of MPC specimens exceeded those of EP specimens by 9.28%, 10.13%, 5.99%, and 0.86%, respectively, indicating greater ductility. Bond performance declined markedly for both groups as cycles increased, with MPC specimens showing greater deterioration. After 75 cycles, compared with EP-bonded specimens, MPC-bonded specimens showed a 16.56% lower interfacial load capacity, a 21.53% reduction in peak bond stress, and a 6.03% shorter effective bond length. This systematic comparison of MPC- and EP-bonded CFRP–concrete under sulfate freeze–thaw exposure provides guidance for adhesive selection and strengthening practices in saline, cold, and arid regions.