ESPI measurement of bond-slip relationships of FRP-concrete interface

Fiber-reinforced polymer (FRP) composites are widely used for strengthening reinforced concrete structures because of their superior properties. Reliable performance of the bond between the external FRP and the concrete in maintaining the composite action between them is crucial for this strengthening technique to be effective. To fully understand and model this bond behavior, a rigorous bond-slip law is essential. This paper presents an experimental study in which the displacement fields in a carbon fiber-reinforced polymer (CFRP)-to-concrete double shear test were measured using the nondestructive and noncontact electronic speckle pattern interferometry (ESPI) technique. Full-field in-plane displacements were measured in 33 CFRP specimens with concrete strengths varying from 23 to 69 MPa. The measurement results were used to infer the bond-slip behavior between the FRP and the concrete. The inferred bond-slip curves include a nonlinear ascending branch and a descending branch. The strength of concrete is found to have significant effect on the peak bond stress but little effect on the slip corresponding to the peak bond stress. A logarithmic model and a simpler parabolic model are proposed to represent the experimental bond-slip constitutive curves.