SCC and UHPC — Effect of Mixing Technology on Fresh Concrete Properties.

Although numerous regulations and guidelines cover the composition and properties of concrete and its components, the actual process of concrete production, i.e. mixing, is to all intents and purposes left to the user. According to DIN EN 206 the concrete components are to be blended so that a homogeneous mixture is produced. The duration of mixing is to be chosen to produce sufficient blending of the materials. It is decisive that water and superplasticizer are evenly distributed down to a fine scale and that agglomerates are sufficiently dispersed. If insufficient mixing energy is used, the properties expected from the composition chosen cannot be achieved. The necessary duration of mixing depends particularly on the type of mixer as well as the composition of the concrete. Due to the low water content with respect to the content of fines (<0.125 mm) and the high dosage of admixtures, the production of self-compacting concrete (SCC) and ultra high strength concrete UHPC requires more mixing energy to homogenise the concrete components. In concrete plants, mixing times above 4 min for SCC and 12 min for UHPC are not uncommon. This limits the rate of concrete production in comparison with vibrated concrete and is therefore an important financial factor. This paper considers the effect of mixing time, mixing tool speed and concrete composition on the homogenisation of ultra high performance concrete. In addition, a model approach based on mixing power is presented for the calculation of the necessary mixing time for different concrete compositions. [ABSTRACT FROM AUTHOR]