Mode II fracture energy of FRP-concrete interface: Its evaluations and roles in interface modeling and anchorage design

Since interface delamination is a most possible factor leading to the overall failure of concrete structures externally strengthened with FRP laminates (plates/sheets), studying the interface behaviors between the FRP laminates and concrete has become a keen interest in the past decade. The interface between FRP laminate and concrete is easy to exceed the peak interface cohesive stress even though there is only a low level of tensile stress in FRP sheets. Therefore, rather than interface bond strength, fracture mechanics-based interface parameters show their more powerful functions on describing the interface softening and the cohesive crack propagation, and also their advantages on evaluating the interface damage due to various negative environmental factors. For interfaces between concrete and externally bonded FRP laminates under shear, Mode II fracture energy, which is related to the work done by the bond stress and affected by all interfacial components, becomes an most important interface characteristic parameter. This paper firstly compares test results of Mode II fracture energy from single and double-lap shear bond tests and parametrically discusses the effects of all test variables. In particular, the large scatter of test results is discussed based on an extensive experimental database (test results of 231 specimens from 11 researchers). The Mode II interface fracture energy is directly used in this paper to model the bond behaviors of FRP sheet-concrete interfaces under shear. A two-parameter fracture energy based bond stress-slip model is introduced. Besides its simplicity, a most significant characteristic of the proposed two-parameter bond model is that other important interface parameters like interface peak cohesive stress and the corresponding slip, which are difficult to be calibrated in shear bond tests, can be related to the Mode II fracture energy mathematically. Finally, the fracture energy-based design models for the bond strength and anchorage length of FRP/concrete interface are proposed. In a summary, an overall picture for the evaluation as well as the uses of Mode II fracture energy of FRP/concrete interface for numerical modeling and engineering design can be seen through the presentations in this paper.