In plane shear behavior of calcareous masonry panels strengthened by FRCM

Unreinforced masonry construction are typically prone to brittle failures due to the nature of their constituent materials, and in many cases their strength is related to the shear strength of the primary walls. In regions affected by intense seismic events, the presence of heritage construction made by poor masonry, strongly enhance this type of vulnerability. The recent earthquakes that occurred in the past ten years in the Eurasian regions drew the attention of researchers and engineers in this sense, since entire cities formed by ancient masonry buildings were affected by extensive disasters and human losses. In order to find an effective solution to these important structural problems, composite materials in forms of Fiber Reinforced Polymers (FRP) were found to be effective and attractive in many cases, showing a good applicability both in reinforced concrete (RC) buildings and masonry construction. In this last case they are well accepted in modern masonry construction, but the use of epoxy resin as matrix and adhesive, combined with high performance fibers (i.e. carbon) have shown some limitations in the field of heritage masonry construction, in which the substrate is very poor. In this perspective two main issues obstacle an effective use of FRP: the mechanical compatibility and the saturation of the surface respect to transpiration of humidity. For these reasons in Europe there are some real applications of heritage masonry buildings in which the use of FRP is not welcome by the authorities that are asked to evaluate the strengthening proposals The problem of the mechanical compatibility is due to the differences between the stiffness and strength of the fibers (typically carbon) and the properties of the masonry substrate which may be 10-3 times those of the fibers. The problem of breathability is due to the fact that polymeric resins create an impermeable jacket which interrupts the cycle of humidity transpiration through the masonry. This may lead to a degradation of the masonry, in the long terms, because of the saline formations. For these reasons the use of a new generation of fibrous materials named as Fiber/Fabric Reinforced Cementitious Matrix (FRCM) was introduced, in order to use long reinforcing fibers into an inorganic matrix based on lime or cement mixes. These new materials have lower mechanical properties respect to FRP composites, but they show higher compatibility with poor masonry. The present manuscript illustrates the results of an extensive experimental campaign, in which masonry panels, made with limestone and poor hydraulic mortar, were tested under diagonal shear forces, until failure. The panels were tested in unreinforced configuration, then different FRCM reinforcement systems were applied and tested to compare the respective results. Both single wall and double wall panels were tested in order to represent different cases found in real applications. Totally thirty specimens were tested. The results, illustrated and discussed in the paper, show the significant increase in terms of mechanical properties that was measured in all cases of FRCM-strengthened walls. Due to the use of different mortars and fibrous systems some differences in terms of failure modes and damage at failure will be shown, even if it is reasonable to believe that for the type of masonry tested herein, all the strengthening methods resulted tremendously effective in terms of load capacity and energy dissipation, without showing a sudden brittle collapse.