Influence of rheological properties of cement mortar on steel fiber distribution in UHPC

This paper aims to study the distribution of steel fibers in UHPC specimens prepared by different rheological cement mortar. Four levels of superplasticizer dosage were used for every water to binder ratio (0.18, 0.20, 0.22, 0.24) to adjust rheological properties of cement mortar mixtures (without steel fibers). Steel fibers were added by volume of 1%, 2% and 3% for every cement mortar mixture respectively. Cube specimens (70.7 × 70.7 × 70.7 mm3) were cast and cured under the same condition for every mixture and the distribution of steel fibers along casting depth was determined by digital image processing. The experimental results reveal that the flowability of fresh cement mortar increases and the yield stress decreases with the higher water to binder ratio and superplasticizer dosage. The plastic viscosity of fresh mixture is decreased by the higher water to binder ratio and little influenced by the variation of superplasticizer dosage. The fiber distribution along depth mainly depends on rheological parameters of fresh mixture, especially the yield stress. Moderate rheological parameters are necessary for obtaining an uniform distribution of steel fibers in UHPC. On the basis of this study, the optimal yield stress of fresh matrix ranges among 900-1000 Pa, 700-900 Pa and 400-800 Pa respectively for UHPC mixtures with 1%, 2% and 3% fiber volume fraction.