Stability and Retention of Entrained Air in Conventional and Self-Consolidating Concrete

Concrete is entrained with air in order to improve its long-term resilience against cyclic freeze-thaw damage. However, a significant amount of entrained air bubbles can exit the fresh material during mixing, transportation, handling, placement, and vibration. This loss of entrained air can then result in the loss of resiliency to cyclic freeze-thaw damage. The movement of an air bubble within concrete can be simplified as a fluid mechanics problem as an undeformed sphere within a yield-stress fluid (i.e. a Bingham plastic). However, concrete as a highly granular material complicates this neat-fluid assumption. In this study, the stability and retention of air bubbles in concrete and model materials is studied as the rheology, aggregate volume fraction, and extent of external vibration are changed. It is found that the granularity of concrete contributes significantly to the loss of entrained air for comparable rheology. Moreover, the yield stress of the granular fluid appears to dominate the onset of movement of air. These results suggest that it is possible to fine-tune the extent of vibration (peak acceleration and frequency) to retain entrained air if the appropriate rheology and aggregate volume fraction of a conventional or self-consolidated concrete are known.