“In practice, knowing the time (t1) and the size of the time window (t2-t1) with optimum fresh concrete properties is of decided importance for concrete production. The dispersion phase should be rather short for economical concrete production.” While addditional mixing is necessary, it is also important to limit that to the period of optimal fresh concrete properties. Overmixing will reduce workability.

Mixing intensity

On the side of mixing technology, shortening the mixing time can be achieved by increasing the mixing tool velocity. With increased tool velocity, the dispersing phase shortens. The mixing time for reaching optimal flow properties can, therefore, be shortened significantly.

At the same time, the duration of the phase with optimal flow properties also shortens. Therefore, overmixing is reached more quickly, combined with a decrease in flowability. Also note that the maximum flowability decreases somewhat with increased tool speed. Temperature increase in the mix as a factor could be excluded.

How much the mixing time can be shortened by increasing the tool velocity and how far this is combined with a decrease in flowability depends, again, on the mixture proportion. The decrease in flowability is influenced by the content of coarse aggregate.

The optimal velocity is therefore not a fixed value, but has to be determined for every type of concrete, depending on mixing time and maximum flowability. If high tool velocities are to be used then, especially for high coarse aggregate contents, an increased water demand in the mixture proportion possibly has to be taken into account.

However, reducing mixing time and increasing the tool velocity can significantly contribute to increasing a ready mixed concrete plant's capacity. At mixing times of 60 seconds, including time for adding water and superplasticizer, the mixing time of SCC is similar to that of common types of vibrated concrete.”

Hybrid mixing sequences

To further reduce mixing times, tests were carried out with hybrid mixing sequences. In hybrid mixing sequences, mixing is carried out in several stages with varying mixing intensity. If the idea of a hybrid mixing process is transferred to producing SCC in the ready mixed concrete plant, it would be possible to reduce the running time of the plant mixer still further.

In this case, only the intensive water and superplasticizer dispersion takes place in the plant mixer; the final homogenizing can take place at low mixing intensity in the ready-mix truck. When using a hybrid mixing process, the mixing time in the factory mixer could be reduced to 30 seconds for SCC. Tests showed that the effective mixing time in the plant mixer can be clearly reduced by using hybrid mixing and also with compulsory mixers at low mixing tool velocities.

Conclusions

Without a doubt, numerous concrete technological and economical advantages are combined in using SCC concretes. However, long mixing times reduce concrete plant capacity and can also cause supply bottlenecks onsite.

By increasing the mixing tool velocity, it was possible to reduce the mixing time to 60 seconds, including time to add water and superplasticizer.