Improvement effect of steel fiber orientation control on mechanical performance of UHPC

In order to improve mechanical properties of ultra-high performance concrete (UHPC), one L-shape device with a narrow horizontal channel was developed to control the flow of fresh mixture and then the orientation of steel fibers was significantly improved. Mixtures with water-to-binder ratios of 0.20, 0.22 and 0.24 were prepared by introducing steel fibers with 1%, 2%, and 2.5% of the total volume respectively. Three fiber orientation parameters (fiber orientation angle, fiber orientation number and fiber orientation coefficient) were quantitatively evaluated by the image analysis method. Based on experimental results, mechanical properties (compressive strength, flexural strength, toughness, deflection at modulus of rupture and maximum deflection) were notably increased with the higher dosage of steel fibers. Compared with specimens prepared by a direct cast, this flow control method improved fiber orientation number and fiber orientation coefficient from 0.6 to 0.7 and 0.4–0.6 to 0.7–0.9 and 0.6–0.8 respectively. The best improvement on flexural strength, toughness, deflection at modulus of rupture and maximum deflection reached 64.3%, 65.1%, 77.1% and 14.9% respectively. Based on experimental results, linear relationships were regressed for fiber orientation parameters and mechanical properties of UHPC. Therefore, a potential method was developed to improve the reinforcement effect of steel fibers in UHPC.