Mixing of high performance concrete: effect of concrete composition and mixing intensity on mixing time

Abstract

At present the use of ultra high performance concrete (UHPC) is limited to a small number of special applications. This is essentially due to the time needed to blend the UHPC mix which is considerably longer than required for ordinary concrete. The present investigations aimed at the determination of the composition features which result in the longer mixing time of UHPC. The individual components of the concrete mix (silica fume, quartz flour, water, superplasticizer and coarse aggregate) were systematically varied in order to investigate their effect on the mixing time needed to achieve optimal flow properties. Besides concrete composition, the effect of mixing tool speed was considered. The shortest necessary mixing time (stabilisation time) was calculated from the evolution of the power applied to the tool during mixing. It was confirmed that high w/c values resulted in short stabilisation times. In addition, the contents of silica fume and quartz flour as well as the type of cement and superplasticizer affected the stabilisation time significantly. It was possible to describe the effect of the individual variables in terms of the relative solid concentration ϕ/ϕmax which depends on concrete composition. This is the ratio of the actual volumetric concentration of solids in the concrete mix ϕ to the maximum possible concentration of solids ϕmax calculated from the volumes of particles (particle size distribution). The relative solid concentration is the basis of a model for the calculation of the mixing time of UHPC of a given composition and for given tool speed. High tool speeds and low actual concentrations of solids shorten the stabilisation time.