PSI - Issue 44

Matteo Canestri et al. / Procedia Structural Integrity 44 (2023) 2198–2205 Canestri et al./ Structural Integrity Procedia 00 (2022) 000–000

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After the curing process (i.e., 28 days in laboratory-controlled environment) and before the application of the reinforcement, the four vertical edges of the columns were grinded with a radius of curvature equal to 20 mm to minimize the risk of stress concentration. Subsequently, columns were retrofitted with a variable number of layers of unidirectional steel fibers and lime-based mortar matrix, following these steps: (i) immersion of columns in water filled tanks for about an hour; (ii) application of a first mortar layer, 5 mm thick, on the entire lateral surface of the specimen; (iii) positioning of the first reinforcement grid along the full perimeter of the column, (iv) application of a second mortar layer either to complete the wrapping or to provide a base to add the second and third layer. Specimens were named following the notation reported in Fig. 1, where “C” stands for compression test, “URM” represents the unreinforced specimen, “nL” is the number of reinforcing layers, while X is the sample number. The overlapping length was equal to 375 mm for all layers (Fig. 1). Only one specimen (C_1L_MOD) was characterized by a reduced overlapping length of 250 mm. To cover the full height of the column, two 260 mm-wide horizontal strips were overlapped at mid-height. Furthermore, a small gap (2 cm) was left bare both on top and bottom of the samples, to avoid direct contact of the loading plates with the SRG composite cover and to prevent its instability. Before testing, the strengthened columns were cured in a controlled environment for at least 28 days. 2.2. Mechanical characterization of the materials The brick compressive strength f b,c was derived from uniaxial compression tests (EN 772-1) on 10 cylindrical samples, with a diameter of 50 mm, drilled from the clay bricks. It resulted equal to 19.3 MPa (C.o.V. 5.1%). An elastic modulus E b , equal to 6.8 GPa (C.o.V. 4.3%), was found through cyclic compression tests (EN 12390-13) performed on 5 additional specimens. Mechanical characterization tests were also conducted on the mortar used for the masonry joints ( low strength mortar type) and on the mortar matrix of the SRG strengthening system ( high strength mortar type). Following the indication of the European Standard EN 1015-11, mortar compressive strength f c and flexural strength f fl were obtained by testing 12 prismatic samples (160×40×40 mm 3 ). Results, for both mortars, are reported in Table 1. These tests have been executed simultaneously with the compression tests on the columns, to achieve an equivalent curing time between specimens and their constituents. The SRG strengthening system was characterized by the use of high-strength unidirectional galvanized steel fibers, characterized by a weight density of 650 g/m 2 , a tensile strength of 3000 MPa and an elastic modulus of 190 GPa, as declared by the manufacturer. To obtain the tensile performance of the SRG system, 6 SRG coupons, with dimensions equal to 40×10×400 mm 3 , were subject to tensile tests under displacement control conditions (rate equal to 0.2 mm/min in the uncracked phase and 0.5 mm/min at the end of the cracking phase). A servo-hydraulic machine, with a capacity of 100 kN, was used to carry out all the tests. Stress values were obtained by dividing the applied load by the cross section of the textile ( A f = 2.7 mm 2 ), while deformations were acquired by an extensometer, with a gauge length equal to 150 mm. After an initial undamaged stage, a series of cracks started developing along the full length of the samples. After the stabilization of the cracking process, the tests were conducted up until failure, which occurred due to fiber rupture between one of the formed cracks. The average tensile strength σ u , the ultimate strain ε u and the fiber elastic modulus E f resulted equal to 2990 MPa (C.o.V. 1.4%), 1.99% (C.o.V. 7.7%) and 180 GPa (C.o.V. 8.1%), respectively. Finally, to assess the bond strength between the SRG system and the masonry substrate, 5 single-lap shear tests were carried out following the procedure proven effective in previous experimental campaigns (Bellini et al., 2021; Ferretti et al., 2022). The results of these tests, expressed in terms of stress vs slip curves, are reported in Fig. 2, along with an example of the obtained failure mode. Slip values were obtained by means of two LVDTs (Linear Variable Displacement Transducer) as the relative displacement between the reinforcement textile and the masonry Table 1. Mechanical tests on mortar (C.o.V. in parenthesis) Mortar Type f c (MPa) f fl (MPa) Low strength High strength 5.7 (15.0%) 17.4 (6.3%) 1.9 (14.2%) 5.5 (6.3%)