Experimental calibration of the bond strength of FRP, externally glued on masonry elements, based on design provisions

The application of Externally Bonded Fibre-Reinforced Polymers (EB FRP) has became a widespread technique for strengthening and repair of existing masonry structures, particularly in the case of Architectural Heritage buildings, where strict requirements need to be met. The bond behaviour of FRP is a crucial issue that influences the effectiveness of the intervention and it is still far from being exhaustively investigated for masonry substrates, which present ample variability. Guidelines released by the Italian Research Council, CNR DT-200 (2004), face this topic by adapting formulations based on a fracture energy approach originally developed for concrete substrates and adopted by other guidelines such as the fib Bulletin 14 (2001). This approach leads to an estimation of the maximum axial force, pertinent to the reinforcement, that depends on the fracture energy of FRP-to-masonry interface, besides FRP mechanical properties and geometry. The present work deals with experimental calibrations of the coefficient that correlates interface fracture energy and mechanical properties of the substrate and, therefore, allows to estimate the design axial force of the reinforcement. Such a coefficient, due to a lack of experimentations at the time those Guidelines were released, was proposed as half the corresponding one in the case of concrete substrate. Herein presented calibrations are based on the collection and analysis of shear tests, available in literature, performed on masonry elements, such as clay bricks and natural stones, and on masonry prisms.