Bond Behavior Between External Composite Reinforcements and Masonry Substrates

The need to guarantee higher safety levels of masonry structures under both short and long term conditions, have led to the use of new materials and technologies, in conjunction or in place of traditional ones. In this context fiber-reinforced composite materials have gained an increasing success, mostly for strengthening, retrofitting and repair existing structures. As well known, the bond performance between the reinforcement and the masonry substrate is a critical aspect as it influences the effectiveness of the technique. The bond depends on many parameters as mechanical proprieties of the substrate, interface and reinforcement, bond length, type of test, environmental conditions etc. The research work of the authors was devoted to this topic from many years and several of the above parameters were analysed. In the present paper the most recent experimental results are reported and discussed; they refer to the analysis of bond between masonry made by natural stones and reinforcement. At this scope a single face shear test has been carried out, varying the substrate configuration and the stiffeness of the FRP reinforcement. In particular a kind of calcerous stone, typically used in the Mediterranea area, was considered and syntetic and natural fibers were used as reinforcement. The effect of these analysed parameters was investigated in terms of bond strength, mode of failure and strains path along the bonded length. In addition a theoretical analysis was done based on the suggestions available in the Italian guide-line. The obtained results show that both the stiffness of the reinforcement and the presence of the mortar joints influence the interface behavior; in particular the tests on masonry substrates furnish higher bond strength with respect to those made using stone unit subtrates, despite the poorer mechanical properties of masonry. In addition the analytical bond strengths evaluated according to the Italian Guideline furnish a good estimation of the excperimental data in the case of specimens realized with stone units.