Influence of fiber alignment and length on flexural properties of UHPC

This paper investigates the influence of fiber alignment induced during casting on flexural properties of ultra-high performance concrete (UHPC). Straight steel fibers with different lengths of 6, 13, and 20 mm were used at 2% volume ratio. Pullout load and energy of fibers with different lengths were evaluated. Flexural strength and toughness were determined for prismatic samples cast using a flow-induced device that can improve fiber orientation during placement. The horizontal outlet height of the device was adjusted to 10, 20, and 30 mm in order to secure different degrees of fiber orientation. Test results indicate that the longest fiber had approximately 5- and 27-time greater pullout load and pullout energy than those of the shortest fiber. Compared to UHPC made with the shortest fiber, UHPC cast with the longest fiber had approximately 75% and 245% higher flexural strength and toughness, respectively. The fiber dispersion and orientation improved when the horizontal outlet height decreased, resulting in greater flexural strength and toughness. Such enhancement was up to 75% and 100%, respectively, compared to similar UHPC cast using the conventional method that can result in random fiber orientation. Synergistic improvement exceeding 130% in flexural strength was obtained for samples cast with fiber length of 13 mm and horizontal outlet height of 10 mm, compared to samples cast with shorter fiber of 6 mm using the conventional casting method. The smoothed particle hydrodynamics simulation indicated that the ratio between fiber length and horizontal outlet height of 1.1 was more efficient to improve flexural properties of UHPC, compared to the ratios of 0.6 and 1.6 where the highest value led to blockage.