One case of application of composite materials in civil engineering is the strengthening of reinforced concrete beam in situ by an externally bonded fiber reinforced plastic (FRP). In this paper, an iterative analytical model, capable of simulating the bond-slip and the material non-linearity, is proposed to determine the ultimate capacity of these beams. To define the bond-slip effect, the knowledge of interaction concrete/adhesive/plate is important. So a new single lap specimen is suggested to study the mechanical behaviour and to determine the mechanical properties of the interface. The theoretical model based on the compatibility of deformations and the equilibrium of forces is also able to predict the failure modes due to the tensile fracture of FRP, the crushing of concrete in compression or the plate separation. In this study the premature failure mechanism and the plate debonding detail will be discussed. A parametric study using the non-linear model is made to obtain the analytical equation capable of calculating the maximal shear stresses at the interface plate/concrete aimed to anticipate the debonding load.
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