Issue 35

S. Tarasovs et alii, Frattura ed Integrità Strutturale, 35 (2016) 271-277; DOI: 10.3221/IGF-ESIS.35.31 274 applied load. As was shown in previous studies, the embedded depth of the “hook-end” fiber (H) has relatively small effect on the pull-out force [11, 12]. Experimental scheme and resulting pull-out force are shown in Fig. 5. Experimental pull-out curves show that maximal pull-out load is achieved at 40 degrees, other authors have shown, that the maximum peak load is attained at 45 degree inclination [13]. Lower stiffness and increased displacement at maximal force for fibers embedded in 60 degree could be explained by matrix spalling and fiber bending. Figure 5 : Experimental scheme and single fiber pull-out force at different angles. In present work 3-point and 4-point bending tests were used to characterize concrete elements reinforced by steel fibers. 3-point bending tests were used to determine mechanical properties of plain concrete. In order to evaluate the influence of steel fibers on concrete strength, four-point-bending tests with larger specimens were conducted. 3-point and 4-point bending test are schematically shown in Fig 6. Commercially available concrete matrix “Sakret C25” (compressive strength 25 MPa) was used to mix the specimens, with maximal grain diameter equal 8mm and water content 1l/10kg. Specimens were tested 28 days after material mixing. Figure 6 : Geometry and loading scheme of a) 3-point bending test, b) 4-point bending test. Dimensions of specimens are shown in Tab. 2. Specimens had an initial crack with length 2a 0 . The results of 3-point bending tests are shown in Fig. 7. These curves were used to estimate cohesive properties of plain concrete and the numerical curve also shown in Fig. 7. For the current simulations bi-linear softening curve was used to model the traction- separation law of cohesive elements with fracture energy G Ic =150 J/m 2 .