Q. Why is air-entrained concrete less permeable than non-air-entrained concrete? Aren't the air voids permeable to water?

A. Air-entraining agents reduce the amount of water needed to produce a given slump. At a fixed cement content, this lowers the water-cement ratio, which reduces permeability. Although entrained air bubbles increase the void content of hardened concrete, the air voids aren't interconnected. Thus, the higher void content doesn't appreciably increase permeability.

Reader Response:

The response to the question on permeability of air-entrained concrete neglected to mention a more important reason for the reduced permeability.

The slight reduction in the water-cement ratio may have some bearing, but the entrained air bubbles make the mortar, or paste, in the mixture more cohesive, or sticky, according to finishers.

Therefore, air-entrained concrete mixtures are more resistant to the settlement of aggregate particles. This results in reduced bleeding and fewer bleedwater channels through which water can enter the concrete.The response correctly states that entrained air bubbles are not interconnected. But they are so small that water will not enter them under normal atmospheric pressure, so they don't affect permeability. Water can be forced into the small bubbles, however, under the increased pressure caused by the expansion of freezing water. Thus, the bubbles provide pressure relief.

Richard O. Albright
Arsee Engineers
Noblesville, Ind.