Masonry columns confined with fabric reinforced cementitious matrix (FRCM) systems: A round robin test

The conservation and the preservation of existing masonry buildings, most of them recognized as culturalheritage, require retrofitting techniques that should reduce the invasiveness and assure reversibility andcompatibility with the substrate. In this perspective, the strengthening system should be able to improvethe bearing capacity of the structural member and, at the same time, to assure mechanical and materialcompatibility. The use ofFabric Reinforced Cementitious Matrix(FRCM) composites is now recognized to besuitable for these purposes. Indeed, the inorganic matrix has comparable properties with respect to theexisting historical mortars while the fabric has relevant tensile strength. At the same time, these systemsassure satisfactory level of reversibility (or at least removability). In this scenario, the present researchaims to investigate the FRCM-confinement of masonry columns focusing on the influence of specificparameters, still poorly investigated, in order to deeply understand their effect on the mechanicalresponse. In particular, the experimental variables are: the type of masonry substrate (Tuffand clay brickwith lime-based mortar), the type of FRCM system (glass dry mesh + lime-based mortar and steelmesh + lime-based mortar) and the number of plies (1, 2 and 3). In addition, a detailed experimentalcharacterization of the utilized materials has been carried out, including bond tests between the rein-forcement and the substrate. The results evidenced that the FRCM is an effective solution for masonrycolumns confinement once a proper design is performed, taking into account all involved parameters.The different strengthening systems exhibited different failure modes. Generally, a single ply of externalreinforcement produced a negligible increase of bearing capacity. Both strengthening systems appliedwith multi-ply strengthening schemes produced a significant increase in terms of strength and ultimateaxial deformation. This benefit was observed for bothTuffand clay masonry columns. Two availabledesign-oriented formulas, reported in the Italian CNR (National Research Council) and ACI (AmericanConcrete Institute) guidelines have been utilized, in order to further investigate their accuracy, mostlyin the case of multi-layered reinforcement. The performed comparisons highlight that the two design relationships provide similar and accurate results when referred to the GFRCM (Glass-FRCM) system in 1-and 2-layer configurations, while the predictions appear conservative when 3 layers of GFRCM are uti-lized, for both masonry types. Considering the SRG (Steel Reinforced Grout) system, the results predictedby the two models are more scattered, mostly when the number of layers increases. In addition, the for-mulation proposed by CNR appears more accurate in the case ofTuffmasonry while the ACI predictionsare closer to the experimental results in the case of clay brick masonry