Effect of macro fibers on flexural toughness and crack surface topography of concrete

In order to study the effect of different macro fibers (including macro polypropylene fiber, macro steel fiber and hybrid fiber of polypropylene fiber and steel fiber) on the flexural toughness and crack surface topography of concrete, three-point bending test was conducted on notched beams in accordance with RILEM TC 162-TDF¹⁰. The laser scanning equipment was utilized to measure the information of crack surface topography. The four roughness parameters (roughness number (R_N), fractal dimension (D), standard deviation of height distribution (σ_z) and crack tortuosity (τ)) were calculated according to the information of crack surface topography and the correlation between four crack roughness parameters and flexural toughness parameters were then analyzed and compared. The results show that with the increasing of fiber content, the flexural toughness and crack surface roughness increase. Compared to polypropylene fiber and steel fiber, the hybrid fiber illustrates the positive synergistic effect on the flexural toughness and crack surface roughness of concrete. Compared to ractal dimension D, standard deviation of height distribution σ_z and crack tortuosity τ, the correlation between R_N and flexural toughness of fiber reinforced concrete beam is the most satisfactory. Meanwhile, the relationship between roughness number R_N and flexural toughness parameters follows the exponential function, which can be applied to quickly estimate crack surface roughness by the bending test of fiber reinforced concrete.