There are two types of air in concrete: entrapped air (the normal air entrapped through mixing action) and entrained air (the air that is created by a liquid admixture to produce freeze/thaw durability). Note: Air entrainment is not needed in interior floor slabs unless they are subject to freezing/thawing cycles and in a wet environment.

Entrained air helps to create freeze thaw durability in concrete by purposely incorporating very small, closely spaced, air bubbles into the cement paste. When air entrainment is combined with the correct compressive strength and water:cement ratio, freeze thatw durability is created (air alone does not create this durability).

Entrapped air varies depending upon the actual mix constituents, quality of mixing action and admixtures used. Entrapped air should be checked at the start of each pour to ensure that the air content is less than 3%. Entrapped and entrained air in excess of 3% can lead to problems through reduced/deferred bleeding which may lead to surface delaminations, blistering or lower quality finishes. All concrete should be checked for air content at the start of each pour and with the casting of compressive strength cylinders.

A number of problems have arisen over surface delaminations on interior concrete surfaces when either there was too high an entrapped air content or air entrained concrete was shipped accidentally (generally not required for interior heated surfaces). By checking the air content at the beginning of each pour, problems can be avoided.

If you have any questions, please feel free to e-mail us for assistance.

The Concrete Floor Contractors Association of Ontario 70 Leek Crescent, Richmond Hill, Ontario, Canada, L4B 1H1 Tel: (416) 499-4000 Ext.14 Fax: (416) 499-8752