Structural to interfacial fracture transition in epoxy coated hydrating cement

In this work, the fracture behavior of an epoxy coated hydrating cement paste system is investigated with the aid of a slant shear test. A finite element model based on the phase-field method (PFM) is used to simulate the crack propagation and understand the transitions from structural to interfacial fracture and vice versa. At construction sites, it is a usual practice to coat epoxy only after the concrete is completely cured, i.e. after 28 days. However, during hydration, the surface energy of the cement paste varies with time. Therefore, it is necessary to investigate the temporal dependency of the interfacial strength on the instant (time) of coating. With this in mind, we investigated fracture processes in the cement paste, coated with epoxy at various time instants, from an early age (2 days) to the later age (28 and 50 days) of cement paste. From the experimental and numerical results, the cohesive failure of cement paste (bulk or structural failure) is primarily observed for 2-days hydrated cement paste. In contrast, adhesive failure at the interface between cement paste and epoxy (interface fracture) is observed for 28 and 50-days hydrated cement paste. In our fracture analysis, we have incorporated the surface roughness and the bulk porosity of cement paste which evolves over time during the hydration process. From the results of numerical analysis, it is found that the fracture behavior of the cement-epoxy system can be controlled by tuning the material properties of the cement paste and the interface i.e., elastic modulus, E and fracture energy Gc. Also, it is observed that the surface roughness and the bulk porosity of cement paste significantly influence the crack trajectory as well as the load–displacement response of the cement-epoxy system.