Fracture K Value of Pre-Coated Concrete-Epoxy Mortar Interface Based on a Proposed Bilinear Softening Model

To study the fracture performance of the concrete-epoxy mortar interface (CEMI) pre-coated with epoxy solutions with different concentrations, a total of nine specimens were fabricated to be subjected to four-point bending tests. DIC technology was used to monitor the deformation of the pure bending region of specimens. A triple-fold stiffness model was developed based on the test results of applied load–displacement curves. A generalized method for determining the parameters of the bilinear softening model was proposed and validated by the test results. Additionally, the fracture performance and crack extension of CEMI specimens were deeply analyzed using the double-K fracture criterion. The fracture initiation toughness KICini was calculated by introducing the cohesive fracture toughness, and the crack extension resistance KR curves of the CEMI specimens were calculated by combining the linear-elastic fracture mechanics and the proposed bilinear softening model. It was indicated that the initiation locations and extension paths of interfacial cracks could be effectively identified by the DIC technique, with an error of less than 8% between test results and predictions. The bridging effect was strengthened by pre-coating with an epoxy solution of the CEMI specimens by filling the microscopic defects on the concrete surface, thereby improving KICini, delaying unstable crack extension, and enhancing interfacial fracture resistance.