Bending and uniaxial tensile tests on concrete reinforced with hybrid steel fibers

Based on the idea of taking simultaneous advantage of the effects of different types of fibers, new materials called hybrid fiber reinforced concretes have been developed by combining fibers of different geometry and material. In the present paper, the benefits in terms of concrete toughness from a combination of micro- and macrosteel fibers are evaluated under both bending and uniaxial tensile tests on specimens of different sizes. Experimental results are very sensitive to the strain gradient in the cracked section, to the fiber geometry and to the area of the cracked surface. In fact, a larger scatter in the experimental results was observed in specimens with smaller cracked surfaces where a greater variation of the macrofiber density occurred. For this reason, beside the size effects, the fiber size and the dimension of the cracked section markedly influence the characteristic value of the fracture parameters. A numerical simulation based on nonlinear fracture mechanics of the experimental test was carried out in order to better identify the fiber contribution in the fracture propagation.