Reinforcement mechanism of orientally distributed steel fibers on ultra-high-performance concrete.

• A novel device and method are designed to orientally distribute fine and short steel fibers in the cement mortar. • The fiber orientation coefficient of UHPC with oriented steel fiber reaches 0.87 by using image processing method. • The theoretical prediction model of flexural-tensile strength based on composite and bending moment theories is provided. • The multiscale image analysis of the fracture surface reveals the enhancement mechanism. A novel device and method were designed to orientally distribute thin and short steel fibers in ultra-high-performance concrete (UHPC). With the different volume fractions of steel fibers, the mechanical properties of UHPC with orientally distributed steel fibers (group D) and disorderly distributed steel fibers (group L) were compared and analyzed. The fiber orientation coefficient (η θ) by means of image processing and the theoretical prediction model of flexural-tensile strength based on composite and bending moment theories were studied. The multiscale image analysis of the fracture surface was applied to reveal the enhancement mechanism. The results demonstrate that the mechanical properties such as flexural strength, flexural-tensile strength, flexural toughness, and interfacial bonding strength of UHPC in group D are significantly promoted, in contrast to those in group L. The average η θ of 0.85 in group D indicates that this experimental method can substantially improve the arrangement of steel fibers, hereby the efficiency is improved. Besides, the proposed prediction model of flexural-tensile strength based on the composite and bending moment theories, showing a good agreement with the actual test values, is applicable to UHPC. Moreover, oriented steel fibers can produce excellent stress-dispersing and absorbing effects and, thus, play a significant role in strengthening and toughening. In contrast, randomly oriented steel fibers can easily fracture the surrounding matrix in a large area during the drawing process, resulting in the deterioration of mechanical properties. [ABSTRACT FROM AUTHOR]