PSI - Issue 44

Carlo Vienni et al. / Procedia Structural Integrity 44 (2023) 2262–2269 Vienni et al. / Structural Integrity Procedia 00 (2022) 000–000

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2.2. CRM system The CRM system used in this work is made by means of a preformed composite grid embedded in a mortar matrix and connected to the walls by helical steel connectors. The compressive and flexural strength of the mortar used for the CRM matrix were evaluated according to UNI EN 1015-11, obtaining an average compressive and flexural strength of f cm,CRM = 14.38 MPa (CoV 9.90%) and f f,CRM = 4.93 MPa (CoV 6.14%), respectively. The tensile strength f tm,CRM of the matrix was obtained considering the load at the first cracking observed during the tensile test on the reinforced mortar coupon. The mechanical properties of the reinforcement system were studied according to the indications of guidelines CNR-DT 215/2018 (2018) for externally bonded FRCM composites. The fiber 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 geometrical and mechanical properties of the yarns were obtained from tensile tests on dry fibers and are shown in Table 1. A schematization of the grid is shown in Fig.1.

Table 1. Geometrical and average mechanical properties of glass fibers (CoV in round brackets). Yarn direction Mesh pitch [mm] a x b [mm] or φ [mm] Resistant area of single yarn [mm 2 ] F u [kN]

σ f [MPa]

E f [GPa]

Weft (flat yarn)

5,5 x 1,8

9,9

10,70

1081 (2,1%) 641 (4,9%)

56,5 (2,1%) 35,1 (4,2%)

80x80

Warp (braided yarn)

3,9

11,9

7,62

Fig. 1. Glass fiber grid embedded in the mortar matrix

Helical steel connectors have a nominal diameter of 9 mm and a cross-sectional area of 14.9 mm 2 . They are characterized by a tensile strength equal to 16 kN, shear strength of 8 kN, and a Young modulus of 160 GPa according to the manufacturer’s technical sheet. Connectors were applied in the number of five per square meter. Clamping-grid tensile tests were carried out to investigate the tensile properties of the CRM system. Inorganic matrix composite coupons were prepared and subjected to tensile test with the ends of the rectangular specimens directly clamped by the machine wedges. The specimens had dimensions of 640x120x30 mm 3 ; each specimen included two longitudinal yarns in weft (C_X) or warp direction (C_Y). The results in terms of average load at first mortar cracking F cr , mortar tensile strength f tm,CRM (given as the ratio between F cr and mortar cross-section 120x30 mm 2 ), average ultimate load F u , coupon tensile strength σ u = F u /A f , elastic modulus of the first branch E 1 , representative of uncracked mortar stage, and elastic modulus of cracked stage E 2 are shown in Table 2. All specimens failed due to rupture of yarns joints and consequent slippage of longitudinal fibers inside the matrix.

Table 2. Average results of the clamping-grid test (CoV in round brackets). Specimen F cr [kN] f tm,CRM [MPa] F u [kN]

σ u = F u /A f [MPa]

E 1 [GPa]

E 2 [GPa] 48 (10%) 37 (9%)

C_X (CoV) C_Y (CoV)

3,06 (13%) 2,91 (14%)

0,85 (13%) 0,80 (14%)

9,96 (18%) 11,55 (7%)

503 (18%) 485 (7%)

1273 (20%) 1693 (1%)

Finally, six bond tests were carried out to study the CRM-to-masonry interface behavior. The support was realized with six brick masonry walls with dimensions of 230x120x400 mm 3 . CRM reinforcement had a thickness of 30 mm, was comprised of two longitudinal yarns, and was bonded to the support for a length of 300 mm. Three tests with longitudinal yarns in the weft direction (BT_X) and three tests with longitudinal yarns in the warp direction (BT_Y)