Air Content

Summary

Air Content - The air content is the amount of air contained within a concrete element, usually expressed as a percentage.

Technical Information

In concrete mixes designed for industrial flooring, 2% is considered a maximum allowable air content. This limit should be taken following the addition of all concrete constituents, including admixtures.

Care must be taken when power trowelling a concrete floor with a high air content as air voids can be pushed together, creating weak plane just below the surface. This can cause blistering, a form of delamination.

Large voids caused by entrapped air can cause a concrete to be more permeable and much weaker in compression. These air voids can be drastically reduced by compaction of the fresh concrete after it has been placed. The most efficient way to do this in a large slab is via vibration.

However, slightly higher than normal air content isn’t necessarily damaging; sometimes air can intentionally be introduced using air-entraining admixtures to combat freeze-thaw action. The air pockets caused by air-entraining admixtures are smaller than those caused by air entering concrete of its own accord. It should be noted that this is typical of external concrete slabs only and generally, air content in an internal slab should be minimised wherever possible.

The air content of fresh concrete can be tested using a Type B Pressure Meter. The allowable air content varies depending on the maximum aggregate size.