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Fiber reinforced polymers have been widely used to strengthen concrete structures owning to their high strength-weight ratio and good durability. The interfacial strength between concrete substrate and FRP plays an essential role in the structural bearing capacity. Plenty of experiments have revealed that interfacial failure typically occurs within a thin layer of concrete near the bond line. The crack path and interfacial strength are mainly influenced by the heterogeneous nature of the concrete superficial layer. Various macroscale numerical approaches have been developed to model the interfacial failure between FRP and concrete. Still, the effects of factors on the interfacial failure process cannot be investigated at the mesoscopic level.

This work focuses on the effects of the nature of concrete components on the interfacial strength and damage process between FRP sheet and concrete. A novel method named improved random walking algorithm (IRWA) is developed to achieve the mesoscale structure of concrete with user-defined aggregate shape, content, and grading. It is found that the obtained grading agrees very well with the ideal one. This method is applied to establish mesoscale models of normal tensile test and sing-lap shear test, respectively. For the former one, cohesive zone models (CZMs) are inserted into interfacial transition zones (ITZs), inner of aggregates, and mortar with different material properties. For the latter one, since the stress state of concrete is more complicated and CZMs cannot describe the damage under compression, a CZM-CDP coupled model is used to simulate the whole process of interfacial debonding. For both cases, the effect of mesoscopic factors on the interfacial bearing capacity, such as aggregate shape, content, grading, the strength of ITZs and mortar, are systematically investigated through parameter analysis. Finally, this approach is applied to model the failure of an RC beam with an externally bonded FRP sheet. The load-displacement curve agrees well with the experimental one, and the distribution of macro cracks is also similar to the failure pattern observed in the experiment.