Bond Failure of Concrete Fiber Reinforced Polymer Plates at Inclined Cracks—Experiments and Fracture Mechanics Model

Bond failure is mostly brittle and often occurs a few millimeters deep in the concrete. A fracture mechanics-based engineering model of bond strength for this failure type, derived from pure bond tests exists. In a beam vertical shear crack, mouth displacements can reduce bond strength by reducing the total fracture energy required to destroy the local bond. To quantify this effect, the shear crack mouth displacements have to be calculated, dependent on the acting forces, the geometry, the reinforcement and the material parameters of the beam. Then a mixed mode fracture mechanics approach is used to quantify the of bond strength due to simultaneous action of bending and shear. In most cases, bond strength reduction due to vertical shear crack displacement will range around 5-10%. In CFRP-plates another type of debonding failure, specific to fiber reinforced plastics was observed. Interlaminar failure in the plate, preferably occurring with higher-strength concrete is considered a mixed mode fracture problem and was investigated by simultaneous measurements of the Mode I and mode II displacements of the bond crack with electronic speckle pattern interferometry (ESPI). A fracture mechanics approach to a criterion for interlaminar plate failure was derived. According to this, a surface tensile strength of the concrete of more than 3,0 MPa cannot be taken advantage of in design, since interlaminar plate failure will then govern bond failure.