Effects of vibrating compaction time on fiber and aggregate distribution and strength of ultra-high performance concrete

Abstract In this study, we investigated the settling velocity and distribution between steel fiber and aggregate during vibrating compaction of ultra-high performance concrete (UHPC) mixtures. In addition, we explored the effect of fiber distribution characteristics on the UHPC strength. Based on Stokes’ theorem and considering the vibration effect, a method for calculating the settling velocity of steel fiber and aggregate in UHPC mixtures during vibrating compaction was proposed. The fiber/aggregate dispersion coefficient based on the proposed method decreased gradually with time. The experimentally obtained fiber dispersion coefficient first increased and then decreased, with the maximum value appearing at 5 s. In contrast, the fiber orientation coefficient gradually increased with time. Moreover, following vibration excitations at different times, the fiber dispersion coefficient increased and the fiber orientation coefficient decreased as the yield stress and plastic viscosity of the UHPC mixtures increased. In the vibration range of 0–15 s, the compressive and flexural strength of UHPC first increased and then decreased, with the maximum values appearing at 5 and 10 s, respectively. The compressive strength was affected by the fiber dispersion coefficient, whereas the flexural strength was affected by the fiber dispersion and orientation coefficients. Furthermore, the relational expression between compressive/flexural strengths of UHPC and fiber dispersion and orientation coefficients were presented.