FRP strengthening of masonry arches: analysis of local mechanisms and global behaviour

In the last two decades, FRP (Fibre-Reinforced Polymers) composite materials have been adopted for strengthening and repair of both modern and historic masonry constructions (buildings, bridges, towers) and structural components (walls, arches and vaults, piers and columns). Strengthening of masonry brick arches and vaults with FRP laminates can contribute significantly in the improvement of their structural capacity at a limit state, by activating local mechanisms both at material and interface levels, but also modifies the collapse mechanisms of the original structures, as the reinforcement prevents the typical brittle failure due to the formation of hinge-mechanisms. Despite the increasing number of specific studies on FRP reinforcement of masonry structures, investigations are still limited if compared to reinforced concrete applications. Moreover, few codes and recommendations are currently available. Starting from these points, the present work deals with experimental investigations on three local mechanisms involved by the collapse of FRP-reinforced masonry arches: the interface behaviour in the case of stresses normal to the surface (FRP detachment observed in structures with intrados reinforcement) has been investigated through the execution of a large number of combined tests on solid clay bricks (flexural, compressive, splitting and pull-off tests) aimed at experimentally calibrate possible correlations among the corresponding strength parameters and to observed possible influences of the fibres type and the presence of primer on the pull-off behaviour; the bond behaviour has been investigated by performing ten Double-lap Shear Tests on solid clay bricks, aimed at calibrating fracture energy value and bond-slip law for both carbon FRP and glass FRP reinforcement (moreover, a simple exponential-based bond-slip law has been proposed); the mixed-mode behaviour (fourteen tested samples) has been investigated by adapting a test setup developed for FRP applications on reinforced concrete, known as V-shape Peel Test, to clay substrate, in order to reproduce on a local scale the conditions related to the shear sliding failure of arches with extrados reinforcement. Finally, several case studies, concerning real-scale or scaled brick masonry arches and vaults reinforced at their intrados or extrados, have been collected from literature in order to compare the experimental results to the available interpretative models of the global behaviour of the structure at failure.