Specifications for air-entrained concrete frequently require that air content be held within a percent or so of a target value. But how easy is it to keep air content within this narrow band? This article shows that air content is influenced by a number of variables: batching sequence, fines content, haul time and temperature, to name a few. Knowing the effect of these variables can help when adjustments are needed at the batch plant or on the jobsite to keep air contents within the allowable range.
AIR CONTENT AND DURABILITY
Entrained air greatly improves the resistance of concrete to damage resulting from freezing and thawing cycles. Microscopic air bubbles produced by admixtures called air-entraining agents relieve pressures caused by water freezing in the small voids present in hardened cement paste. While the size and spacing of these air voids is important, total air content, which is more easily measured, is a good indicator of durability.
CONCRETE MATERIALS AND MIX DESIGN
Cement--Air-entraining agents become less efficient with either an increase in cement content or an increase in cement fineness.
Water--Changes in water content affect air content by changing both the water-cement ratio and the slump of the concrete. As the water-cement ratio increases, more free water is available for the generation of air bubbles, so air content increases. Above a slump of 6 to 7 inches, concrete is often too fluid to retain entrained air and air content decreases.
Aggregates--Air content is reduced by both an increase in maximum aggregate size and a decrease in sand content.
Chemical admixtures--Water reducers, retarders, accelerators and superplasticizers all increase air. Fly ash--The "Foam Index Test" may be helpful in predicting how much air-entraining agent is required by concrete that contains fly ash.
Production procedures, construction practices and environmental conditions also have an influence on air content.