Ultra high performance concrete (UHPC) is characterised by excellent workability, strength and durability. Up to now, the application of this material has been limited by, among other things, the very long mixing times needed for its production. The present investigations aimed at finding the aspects of high performance concrete composition which lead to much longer mixing times compared to normal concrete. The effect of concrete constituents on the mixing time needed for optimal flow properties was investigated by systematically varying the mix composition. The shortest necessary mixing time (stabilisation time) was calculated from the variation of power applied to the mixing tool during the mixing process. It was possible to draw definitive conclusions on the effect of silica fume content, quartz flour content, w/c ratio as well as superplasticizer dosage and type. The results show that mixing time mainly depends on the ratio of the actual content of solids in concrete φ to the maximum possible concentration of solids φmax (packing density) as well as the type of cement and superplasticizer used in the mix.
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