Contribution of fiber alignment on flexural properties of UHPC and prediction using the Composite Theory

This paper investigates the effect of fiber alignment on the flexural properties of ultra-high performance concrete (UHPC) made with different steel fiber lengths (6, 13, and 20 mm) and volumes (1%, 2%, and 3%). Fiber alignment of prismatic samples was secured using a flow-induced casting method. Test results indicated that prismatic samples cast using the flow-induced method can enable 35% greater fiber orientation compared to those prepared using a conventional casting method that results in random fiber orientation. Such improvement in fiber orientation led to 60% and 80% higher flexural strength and toughness, respectively. At a fiber volume of 2%, the increase in fiber length from 6 to 20 mm can result in 45% and 195% greater flexural strength and toughness, respectively, for samples with flow-induced fiber alignment. With the increase in fiber volume from 1% to 3% for 13-mm long fibers, such enhancements were 55% and 40%, respectively. Similar flexural properties can be obtained for UHPC made with 1% of the longer fibers when the flow-induced casting method is employed compared to those prepared using the conventional casting method at 3% of the shorter fibers. The flexural-to-tensile strength ratio of UHPC increased with the increase in fiber length, volume, and orientation. For a given fiber length and volume, the flexural-to-tensile strength ratio increased linearly with the increase in fiber orientation. When the variations in flexural-to-tensile strength ratios are considered as a function of fiber length, volume, and orientation, flexural strength of UHPC can be predicted with a maximum spread of 15% using the Composite Theory.