PSI - Issue 64

Salvatore Verre et al. / Procedia Structural Integrity 64 (2024) 1508–1515 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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(Fig. 1). Each specimen consisted of a strip of SRG composite bonded to a concrete block. During the test, the bare textile of the SRG was pulled while the block was restrained against movement by means of steel plates. The concrete blocks had dimensions of 400  200  150 mm 3 (length  width  depth). The bonded length L f and width b f were 260 mm and 50 mm, respectively. The concrete mechanical properties are summarized in Table 1, where f c and f ct are the compressive and tensile strengths, respectively, and E c is the elastic modulus evaluated according to UNI (2006). The SRG composite was made of a unidirectional UHTS (ultra-high tensile strength) steel textile embedded in a geopolymer cement-based matrix. The textile comprised longitudinal steel fibers (cords) spaced at 6.4 mm and transversal glass fibers. The cross-sectional area of each cord was 0.538 mm 2 . The textile had the equivalent thickness t f =0.084 mm and a weight per unit area equal to 670 g/m 2 (LD textile). The mechanical properties of the textile were determined by tensile tests of five coupons, according to UNI (1995). The average tensile strength  uf , elastic modulus E f , and failure strain  uf are reported in Table 1. In this table, the compressive f c,mat and tensile f b,mat strengths of the geopolymer concrete used as matrix for the SRG composite, determined according to UNI (2007), are also reported. a b c

Fig. 1. Single lap direct shear test: (a) Photo of specimen DS-1L-1; (b) side view and (c) front view of the test setup.

Out of the 9 tested specimens, 6 and 3 had one and two layers of textile, respectively. Before the application of the SRG composite, the concrete surface was sandblasted and wetted. Small grooves were made on the concrete surface to increase its roughness. A form board was used to control the thickness of the matrix layers. For specimens with one layer of textile, the total thickness of the composite was equal to 6 mm (a first layer of thickness equal to 3 mm was applied directly to the substrate, and a further layer of the same thickness was used to cover the textile). For specimens with two layers of textile, a further layer of matrix with a thickness equal to 3 mm was applied between the textiles. Table 1. Mechanical properties of the concrete, steel textile and geopolymer cement-based matrix (Ombres and Verre 2022). Concrete Steel textile Geopolymer Concrete f c (MPa) (CoV) f ct (MPa) (CoV) E c (GPa) E f (MPa) (CoV) ε uf (%) (CoV) σ uf (GPa) (CoV) f c,mat (MPa) (CoV) f b,mat (MPa) (CoV) 14.40 (1%) 2.42 (2%) 24.54 209.3 (5%) 1.9 (9%) 3.06 (2%) 35.8 (6%) 5.6 (8%) 2.2. Test set-up Single-lap shear tests were performed in stroke control at a rate of 0.00084 mm/s. During the tests, two LVDTs were used to measure the relative displacements of the textile with respect to the concrete blocks at the loaded end. LVDTs were connected to the concrete blocks and reacted to an aluminum L-shaped plate glued to the textile near the beginning of the bonded area (Fig. 1). The average of the LVDT readings is the loaded end slip, referred to as global slip and denoted with g . Similar test setups were used by Sneed et al. (2016), Carloni et al. (2017), and Ombres and Verre, (2022). Specimens were identified by DS- nL-Z. DS indicates the type of test (single-lap direct shear tests), nL indicates the number of textile layers, and Z indicates the specimen number. 2.3. Results and failure mode The test results are summarized in Table 2 in terms of peak load P * and corresponding stress σ * , evaluated as  * = P * /( b f t f ). P avg * , and σ avg * are the average peak load and stress of specimens with the same number of layers, respectively. Fig. 2a shows the experimental results in terms of applied load P vs. global slip g , referred to as