Working Mechanisms of Air-Entraining Admixtures and their Subsequent Activation Potential

From some job sites it was reported that the air void content of the hardened road concrete was considerably higher than that of the fresh concrete, especially, when air-entraining agents based on synthetic active constituents have been used. Investigations in the Research Institute with commercial air-entrainers based on synthetic and natural active agents showed that a substantial increase in air content occurs only if, as a consequence of a shortened mixing time during production, excess air-entraining agent has been added to the fresh concrete to achieve the desired air content. The fresh concrete then contains inadequately activated air-entrainer. The air void content may increase if mixing energy is subsequently applied during placement. However, it was not possible to make an accurate determination of the interactions between the type and addition level of active agent and mixing time. Investigations were therefore carried out with commercial air-entraining agents and pure chemical substances. A subsequent increase in air content is to be expected only with admixtures based on synthetic active agents due to their better solubility in the pore solution. The knowledge of the working mechanisms allows to give recom-mendations for practice to avoid a subsequent increase in the air content during the construction work.