Reactivation potential of air-entraining concrete admixtures; part 2

From some job sites of concrete pavement, it was reported that the air void content of the hardened concrete was higher than that of the fresh concrete. Investigations with commercial air-entrainers based on synthetic and natural active substances showed that a substantial increase in air content can occur only if the fresh concrete is supplied with air-entrainer that is inadequately activated. The air void content may increase if mixing energy is subsequently applied during placement or transport. However, it was not possible to make an accurate determination of the influence of the type of active substance on the extent of this increase, the reactivation potential. The air void formation was studied as a function of the type and quantity of the admixture added and of the mixing time. Furthermore, the precipitation of admixtures in calcium hydroxide was investigated. The test results were combined in a model that explains the effects of mixing time, type and addition quantity of active substance on air void formation. A subsequent increase in air content is to be expected only with admixtures based on synthetic tensides due to their better solubility in the pore solution. The knowledge of the working mechanisms allows giving recommendations for practice to avoid an increase in the air content.