The relation between slump flow and yield stress of ultra-high performance concrete (UHPC) mixtures was studied with theoretical analysis and experimentation. The relational expression between slump flow and yield stress of UHPC mixtures was built and then verified with a rheological test. The results showed that the prediction model, as a function of cone geometry of dimensionless slump flow and dimensionless yield stress of the UHPC mixtures, was constructed based on Tresca criteria, considering the geometric relation of morphological characterization parameters before and after slump of the UHPC mixtures. The rationality and applicability of the dimensionless prediction model was verified with a rheological test and a slump test of UHPC mixtures with different dosages of polycarboxylate superplasticizer. With increase in polycarboxylate superplasticizer dosage, yield stress of the two series of UHPC mixtures (large/small binding material consumption) gradually decreased, leading to a gradual increase in slump flow. Based on the prediction model of dimensionless slump flow and dimensionless yield stress, the relational expression between slump flow and yield stress of the UHPC mixtures was built. The comparison result showed that the calculated data was consistent with the experimental data, which provided a new method for predicting yield stress of UHPC mixtures with a slump test.

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.