The effect of the number of strengthening layers on the FRP-concrete bond behaviour

Bonding is a very important issue when Fiber Reinforced Composites (FRP) are used to strengthen reinforced concrete beams. Bonding depends on the mechanical and physical properties of concrete, composite and adhesive. A number of studies have been published concerning the investigation of the relationship between these aspects and the ultimate debonding strain, demonstrating its crucial role. At the same time, when the strengthening system is composed of multiple layers of FRP, it is common practice to sum up the thickness of each layer and introduce the result in the predictive formulas, like it was only one layer. It is not clear if this procedure can be done without any modification to the bond predictive laws. The present experimental study is mainly devoted to the investigation of the FRP-concrete debonding process when a variable number of FRP layers are considered. The results have been compared with the predictive formulas by the Italian Guidelines, showing their reliability. Finally, the dependency of the fracture energy of multiple-layers reinforcement from its axial stiffness has been observed.