Fiber Reinforced Polymer (FRP) materials have been extensively studied and applied all over the world in the last two decades. Nowadays a new generation of inorganic composite materials is also widely studied for seismic strengthening of ancient masonry construction. The new technique is studied for those cases in which a localized epoxy-bonded reinforced show its limits due to the weak nature of the substrate. In these cases Fiber Reinforced Mortar (FRM) systems are used as external strengthening coating, in forms of fibrous grids immersed in a brittle matrix made by hydraulic or cementitious mortar. The results of an extensive experimental program are presented in this study, which involved twelve full scale masonry walls subjected to diagonal shear tests. Two different configuration of masonry walls were studied: six walls were built with a single layer of limestone masonry blocks and six walls were built with a double curtain of limestone blocks separated by a layer of inconsistent mortar and connected by six transverse bond-stones. Low-strength hydraulic mortar was used as binder between the blocks. These configuration were chosen to replicate typical walls found in real heritage masonry buildings. Two different strengthening FRM systems were applied on both sides of the specimens and studied. The first consisted of an industrially pre-cured rigid Glass FRP grid immersed in a lime-based mortar. The second system was an hybrid system made by a dry glass fiber net immersed in a lime-based mortar in which a polymer primer was added as bond promoter. The results of the mechanical tests highlighted in all cases a remarkable increase of shear strength due to the weak properties of the unstrengthened walls. Brittle failure due to diagonal sliding along the joint was observed in unstrengthened masonry. The presence of the FRM has resulted in a pseudo-ductile behaviour of the specimens in both cases, even if different failure modes were observed for the two FRM systems. Fiber rupture and a full composite action was observed for the polymer added FRM system, while extensive cracks and local spalling was observed by testing the fully inorganic system.
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