PSI - Issue 44

Sara S. Lucchini et al. / Procedia Structural Integrity 44 (2023) 2206–2213 Lucchini et al. / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction Un-Reinforced Masonry (URM) walls in existing structures often show a high-vulnerability to Out-of-Plane (OP) actions occurring during seismic events. Considering the bearing walls of masonry buildings, the seismic vulnerability to OP loads is typically increased by the simultaneous presence of in-plane actions, which further reduce the ability of the wall to withstand lateral loading. During earthquakes, bearing walls may experience OP collapses especially when there is not sufficient continuity in the masonry of perpendicular sets of walls or floors are not adequately connected to them. Moreover, existing URM structures are usually more vulnerable than those recently constructed because seismic design has been included in structural code recommendations only in the last decades. Considering all these aspects, it is clear that retrofitting interventions are crucial to mitigate the seismic vulnerability and prevent future collapses. The literature reports different innovative methods to retrofit existing buildings. Many experimental studies (Valluzzi et al., 2014) proved the ability of Fibre-Reinforced Polymer (FRP) strips externally bonded to masonry to improve the OP flexural resistance of both bearing walls and infills. As an alternative to FRP based techniques, reinforced coating can be applied on the masonry surface to improve the in-plane and the OP behavior. Typical applications well documented in the literature are those adopting Composite Reinforced Mortar (CRM), Textile Reinforced Mortar (TRM) or Fabric Reinforced Cementitious Matrix (FRCM) systems (Donnini et al., 2021; Gattesco and Boem, 2020; Meriggi et al., 2020). The relatively recent introduction of structural concrete reinforced with randomly diffused fibers, also known as fiber-reinforced concrete, promoted the use of this type of composite materials even for retrofitting interventions. High performance fiber reinforced materials such as the Engineered Cementitious Composites (ECC) showed to be really effective in improving the seismic resistance when used for plastering the masonry surface. As shown in Facconi et al. (2020), significant seismic performances can be also attained by using single- or double-sided strengthening carried out by using coatings made with normal strength cement-based mortar containing short steel fibers, hereafter defined as Steel Fiber Reinforced Mortar (SFRM). The present experimental research aims at investigating the OP performance of hollow block masonry walls strengthened on one side by a 30 mm thick layer of SFRM. The two one-way masonry walls investigated herein were part of a full-scale building tested in a previous research (Lucchini et al., 2021). Therefore, the properties of the wall restrains are representative of the real boundary conditions generally occurring in real buildings. Both a monotonic test and a reverse cyclic quasi-static test were performed to simulate the OP seismic response. The results of these tests will be summarized and briefly discussed in the manuscript. Moreover, the results of non-linear finite element analyses will be reported and compared with the experimental ones to provide a better comprehension of the specimens’ behavior. 1. Experimental program 1.1. Material properties As mentioned above, the walls tested in this research were portions of a full-scale building constructed in the laboratory of structural engineering of the University of Brescia to investigate the seismic performance of a real structure after retrofitting with SFRM coating. Therefore, the mechanical properties of materials can be found in the manuscript (Lucchini et al., 2021) reporting the results of that experimental investigation. Here, for the sake of brevity, the main material properties are summarized. The URM forming the bearing walls of the building was made with 250 (length) x 190 (height) x 200 (width) mm fire clay blocks with vertical holes (void area: 62%). A ready-mix cement based mortar (cement: hydraulic lime:sand – 1:2:9 by volume) with a mean compressive strength of 3.4 MPa (CoV=7.2%) filled the10 mm thick head and bed joints. From the uniaxial tests on URM wallets performed according EN 1052-1 a mean compressive strength of 2.93 MPa (CoV=28.8%) and an elastic secant modulus of 8,980 MPa (CoV=26.3%) were obtained. The specific weight of masonry was approximately equal to 7.45 kN/m 3 . The SFRM (specific weight = 21.4 kN/m 3 ) consisted of a commercial ready-mix cement-based mortar containing 60 kg/m 3 (0.76% by volume) of double hooked-end steel fibers uniformly dispersed within the mortar matrix. Steel fibers had a length of 32 mm, a diameter of 0.4 mm and a tensile strength higher than 2100 MPa. The cylindrical compressive strength of the material was equal to 35.1 MPa (CV=15.6%) whereas the elastic modulus was about 20,400 MPa