FRP-Strengthened Masonry: Bond Aspects

The change of social needs, the evolution of design standards, the increased safety requirements and the damage caused by materials’ deterioration or unforeseen loads are the main causes of structural deficiency in existing masonry constructions, that as a consequence need to be repaired and/or upgraded through strengthening. The demolition and the subsequent substitution of the damaged structural elements or the use of traditional techniques are often not suitable, mostly when referring to historical and monumental buildings. In recent years the development of innovative methods based on the use of fiber-reinforced polymers (FRP) are becoming an effective alternative. Composite reinforcement is generally glued to the masonry substrate through an epoxy adhesive and applied where tensile stresses become critical for the structural members. Debonding is one of the most important aspects to be taken into account when structural members are strengthened by externally bonded FRP reinforcement. It involves a brittle failure at the interface, affecting the fibers, the adhesive layer and the substrate. Therefore, the study of the interfacial behaviour is fundamental, playing a key role in the effectiveness of the strengthening system. Even if different applications regarding the strengthening of masonry structures have already been realized worldwide, especially referring to buildings of historical and monumental character, the knowledge of the bond at the interface between masonry and FRPs is still limited. In this context an experimental investigation has been planned at the University of Salento and in part already performed. The experimental programme refers to the study of the interface between Fiber Reinforced Polymer and three different natural stones (Leccese Stone, Yellow Naples Tuff, Grey Naples Tuff), being the most common stones utilized for masonry structures in Southern Italy. In this paper an analysis is presented referring to both service and ultimate conditions. An extension of models already proposed for the FRP-concrete interface has been made to predict the behaviour of the FRP-masonry interface. Comparisons between theoretical and experimental results are reported and the effectiveness of the utilized models is discussed. The fib and CNR-DT 200/2004 bond stress- slip law, available for concrete structures, fit very well experimental points when referring to the ascending branch up to the peak bond stress, while some differences have been registered for the descending branch of the bond stress-slip curves.