Question: For a hard-troweled concrete floor, what is the maximum percentage of air that should be allowed?
Answer: Air is intentionally entrained in concrete to provide protection against damage by freeze/thaw action. Some designers will specify entrained air for an interior slab that could be exposed to cold and wet weather during construction, before the building is closed, in an attempt to prevent freeze/thaw damage. In the 1990s, though, as trowels got heavier, it became clear that hard-trowel finishing of air-entrained concrete led to problems with blistering and delamination. The troweling tends to concentrate air and water in a layer just below the surface (for more information, visit here). So there’s a trade-off to consider. American Society of Concrete Contractors Position Statement #1 indicates that, “ASCC concrete contractors will hard-trowel air-entrained concrete if required by specification, but only with the acknowledgment that the risk associated with delamination or blistering and the changes in hardened air void parameters are entirely the responsibility of the specifier.” ACI 302.1, “Guide for Concrete Floor and Slab Construction,” states “In most cases, concrete for trowel-finished interior concrete floors made with normal-weight aggregates should not include an air-entraining admixture; the maximum total air content for this concrete should normally not exceed 3% at the point of placement. Air contents in excess of 3% make the surface difficult to finish and can lead to surface blistering and peeling during finishing. Troweled concrete with intentionally added air will typically not retain the proper bubble size required to provide scale resistance and freezing/thawing durability for most applications.”