Effect of Bond-Slip on the Crack Bridging Capacity of Steel Fibers in Cemente-Based Composites

To increase the crack bridging capacity of concretes and mortars, fibers are often added to cement-based matrices. As a result, the fracture energy of a fiber-reinforced concrete (FRC) is the sum of the cohesive forces of the matrix and of the pullout resistance given by the fibers crossing the crack. In the case of fibers randomly inclined with respect to crack surfaces, the so-called frictional snubbing effect must be added to the previous contributions, if the energy released during crack growth has to be computed. Of course, snubbing forces are not present when fibers are perfectly orthogonal to crack surfaces. As the classical formulas are not always effective in the evaluation of snubbing phenomenon, a cohesive interface model is here proposed for a more precise definition of the frictional snubbing forces. The model is able to predict adequately the experimental results obtained by pulling out steel fibers at different inclinations angles from cement-based matrices. By analyzing fibers with different bond properties, the proposed model can be also used to define the crack bridging capacity of both bonded and unbonded steel fibers. Moreover, it seems to suggest a more simplified approach for the definition of the postcracking response of FRC.