DEBONDING OF THIN CEMENT-BASED OVERLAYS

Thin bonded cement-based overlays include surface repairs, toppings, and linings, especially on slabs, slabs on soil, concrete pavements, and tunnel walls. The aim of this paper is to show that, contrary to a widely held belief, debonding of cement-based overlays is governed not by the shear stress along the overlaybase interface but by the built-in tensile stress perpendicular to this interface. After a brief comparison with laminated composite materials and with beams reinforced by bonded strips (made of metal or of composite), the specific case of thin cement-based overlays is presented. Different conditions of debonding of an overlay are investigated. In all cases, the tensile stress perpendicular to the base-overlay interface is confirmed as the designing parameter. In terms of fracture mechanics, debonding of cement-based overlays is governed by a mixed mode in which mode I is widely dominant. There is a lack of appropriate tests to accurately characterize the mechanical behavior of the interface. Nevertheless, finite-element calculation relying on the mean values of the shear and tensile strength of the interfacial bond (calculation was performed with CESAR code) provides a good approximation of debonding initiation and propagation.