Dr Christoph Müller
If, as a result of a short mixing time, excess air-entraining agent is added to achieve the desired air content during the production of air-entrained concrete then the fresh concrete will contain some air-entraining agent that has not been activated. The air content of the concrete can increase during subsequent input of mixing energy. The extent of the increase (reactivation potential) is substantially affected by the active substance on which the air-entraining agent is based. In all cases a sufficiently long mixing time will prevent an increase in air content. Accurate understanding of air void formation in relation to mixing time is therefore very important for dependable production of an air-entrained concrete that meets the requirements. The investigations carried out so far have been based on laboratory trials with small laboratory mixers (volumes of about 100 l). However, the air void formation is also affected by the mixer type, size of mix, filling quantity and mixing intensity. The results of the laboratory trials cannot therefore be applied directly to practical conditions. Three air-entraining agents with different reactivation potentials (low, medium and high) will be chosen to check the relationships. The air void formation in relation to time will be determined in the laboratory and in a ready-mixed concrete plant (pan mixer of about 1.5 m3). Concretes that after a given mixing time have an air content of about 5.0 ± 0.5 vol. % will also be produced in the two test series. The resistance to freeze-thaw with de-icing salt and the air-void parameters of the concretes from the two test series will be determined. Comparison of the test results will make it possible to predict the applicability of the laboratory trials to practical conditions. The influence of the time at which the air-entraining agent is added at the mixing plant and the reactivation potential due to the transport in the mixer truck will also be examined. Recommendations will be worked out from this to ensure that the required air-void formation is achieved in practice.