Air Void Formation in the Laboratory and in Practice

As a result of a short mixing time excess air-entraining agent (AEA) is added during the production of air-entrained concrete to achieve the required air content. In this case, the fresh concrete contains AEA that has not been fully activated. If there is a subsequent input of mixing energy, the air content of the concrete can rise. The extent of the rise (the re-activation potential) is substantially affected by the active substance of the AEA. Any increase can be avoided if the AEA is fully activated due to a sufficient mixing time. The knowledge of the air void formation in relation to the mixing time is precondition for the production of air-entrained concrete conforming to the requirements. The functioning of an AEA can be characterized in the laboratory during extended initial testing. However, the air void formation is also influenced by the mixer type, quantity of mixture and mixing intensity. The extent to which laboratory results can be applied to practical construction conditions has not yet been clarified. In a research project the air void formation in fresh and hardened concrete in relation to the mixing time was therefore determined in the laboratory and in a ready-mixed concrete plant. Comparison of the test series shows that the results of laboratory trials can be applied to practical conditions.