Improvement in bond behavior and thermal properties of carbon fiber-reinforced polymer strengthened steel structures

•Characterization of typical cleaning methods for steel surfaces and establishment of their relation to bonding properties.•Related variables of initial curing conditions for composite fabrication were optimized by mechanical and thermal analysis.•Effects of initial curing temperature and post-curing process on thermal properties of epoxy resin, delamination strength, and internal defect were discussed.•Moisture and hygrothermal effects on delamination strength reduction and failure mode transformation were observed. This study focused on the bond strength and environmental durability of carbon fiber-reinforced polymer (CFRP)-strengthened steel structures, by investigating the improvements in the surface treatment of the steel substrate, thermal properties of the epoxy resin, and curing process of the composite. The results showed that the steel–adhesive interfacial bonding behavior can be improved by increasing the surface roughness and using a low-viscosity primer to enhance the wettability. During the initial curing, a lower ambient temperature during the gel time can reduce the porosity of the multi-CFRP laminate. However, raising the initial curing temperature is advantageous for the mechanical and thermal properties of the composite. In addition, the thermal properties of the epoxy matrix were clarified by differential scanning calorimetry tests, based on which isothermal curing degree curves of the system were drawn. The results demonstrated that post-curing at 80 °C is vital for increasing the glass transition temperature, which is also appropriated for the primer. Finally, the moisture and thermal effects on the deterioration of the adhesive joint were examined by conducting short-period aging tests in immersion and wet–dry cyclic environments, during which delamination strength reduction and failure mode transformation were observed.