PSI - Issue 44

Carlo Vienni et al. / Procedia Structural Integrity 44 (2023) 2270–2277 Vienni et al. / Structural Integrity Procedia 00 (2022) 000–000

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prescription required by the Italian guidelines for Fabric Reinforced Cementitious Matrix FRCM. Several works have been carried out using diagonal-compression tests to study the effectiveness of CRM applied on masonry walls in terms of tensile strength and shear stiffness (Gattesco & Boem, 2015, D’Antino et al., 2019, Del Zoppo et al., 2019). Besides several experimental studies based on the diagonal compression test, few works were carried out to study the effect of CRM on masonry shear compression. In Gattesco et al. (2013) an experimental and numerical study has been carried out considering the cyclic response of a stone masonry wall, showing an increase in shear strength significantly lower than the strength value of the same masonry under diagonal compression. To increase the investigation field beyond experimental tests, it is possible to perform numerical simulations, after calibrating the numerical model on experimental results. In this work, a series of experimental tests were performed on CRM components to provide a mechanical characterization of the system (Section 2). Based on experimental results, mechanical parameters of plaster, fiber grid, and mortar-to-masonry interface were obtained, and several nonlinear finite element analyses were carried out to analyze reinforcement effectiveness. In Section 3, numerical simulations of reinforced and unreinforced masonry walls under shear-compression loading were performed to study the efficiency of the reinforcement system in terms of strength, stiffness, and ductility. In Section 4 a proposal of analytical formulations was developed to consider the CRM contribution as a function of masonry mechanical parameters, masonry-to-mortar thickness ratio, and compression level ν=σ 0 /f c . 2. CRM mechanical characterization The CRM system adopted in this paper is composed of a bidirectional glass fiber grid embedded in a 30 mm thick lime-based mortar. The compressive and flexural strength of the mortar used for the CRM matrix were evaluated according to UNI EN 1015-11 on six prismatic specimens, obtaining average compressive and flexural strength f c = 14.38 MPa (CoV 9.90%) and f f = 4.93 MPa (CoV 6.14%), respectively. To define the mechanical properties of the reinforcement system, in absence of specific regulations, the prescriptions proposed by Italian guidelines CNR-DT 215/2018 for externally bonded FRCM composites have been followed. According to the Italian code, the tensile properties of CRM must be evaluated through tensile tests on dry yarns and through clamping-grid tests to analyze the influence of the mortar matrix. The yarns of the grid are composed of glass fibers impregnated with epoxy resin, the grid has a square mesh with a pitch between yarns equal to 80 mm. Longitudinal and transversal yarns in warp and weft directions are characterized by different geometry and mechanical properties. The cross-sectional area A f is the overall area of the yarns (made by two sub-yarns in the warp direction) and is equal to 9.9 mm 2 and 11.9 mm 2 in the weft and warp directions, respectively. Mechanical properties of the yarns were investigated by tensile tests of single yarns in the weft (5 tests) and warp (4 tests) directions. The average tensile strength and elastic modulus of yarns in weft direction are σ f_x = 1081 MPa and E f_x = 56.5 GPa, while in warp direction σ f_y = 641 MPa and E f_y = 35.1 GPa. Tensile properties of the CRM system were investigated using clamping-grid tensile tests. Inorganic matrix composite coupons of dimension 640x120x30 mm 3 were prepared and tested in tensile loading with the ends of the rectangular samples directly clamped by the machine wedges. Each specimen included two longitudinal yarns in the weft or warp direction: three samples with longitudinal yarns in warp direction (C_Y_n) and four samples with longitudinal yarns in the weft direction (C_X_n) were tested. A representation of the test setup is provided in Fig. 1a. The test was carried out under displacement control at a constant rate of 0.0033 mm/s. The results in terms of tensile stress σ – axial strain ε are shown in Figure 1c for longitudinal yarns in the weft direction. The results in terms of load at first mortar cracking F cr , ultimate load F u and tensile strength σ u are shown in Table 1, together with elastic modulus E 1 of the first branch and E 2 of the second branch, representative of uncracked and cracked mortar stage, respectively. The observed trilinear behavior is consistent with the results of FRCM systems, as shown in Kouris & Triantafillou (2018). All specimens failed due to rupture of transversal-to-longitudinal yarns joints and consequent slippage of longitudinal fibers inside the matrix (Fig. 1b). Tensile rupture of fibers never occurred. The contribution of CRM also depends on the typology of support on which the reinforcement is applied. To study possible detachment of the system from the support, six single lap-shear tests were carried out, considering three tests with longitudinal yarns in the weft direction (BT_X_n) and three tests with longitudinal yarns in warp direction (BT_Y_n). Six brick masonry specimens with dimensions of 230x120x400 mm 3 were used for the tests.