Air-entrained Concrete

The Portland Concrete Association (PCA) defines air-entrained concrete as containing billions of microscopic air cells per cubic foot.  These tiny chambers or microscopic bubbles act as reservoirs to allow water to expand when subjected to freezing temperatures and helps to relieve some of the internal pressure on the concrete. 

Air-entrained concrete is not only designed to make concrete more resistant to freeze/thaw cycles but also to make it more deicer-scaling resistant.  In addition the air-entraining concrete acts as a lubricant in the mix to improve the workability while the concrete is still plastic and increases its slump (a measure of the consistency of fresh concrete). 

Air-entrained concrete is produced by using air-entraining portland cement or by the introduction of air-entraining agents (chemical admixtures).  The amount of air bubbles are generally in the range of 10 to 500 micrometres in diameter (0.0004 to 0.02 in) and are spaced very closely together.  These air pockets or bubbles relieve much of the pressure from the freezing water by giving it a place to safely flow into.  The PCA explains that, “The amount of entrained air is usually between four and seven percent of the volume of the concrete, but may be varied as required by special conditions.”

The procedure was first started in the 1930’s, and today, most concrete is air-entrained particularly the concrete that is used for exterior projects in colder climates.  Another advantage it offers is that with the increased slump, it allows less water to be used in the mix to produce a higher strength concrete.  The benefit of less water in the mix also means there is less drying shrinkage.  Air-entrained concrete is also used to protect against sulfates and alkali-silica reactivity.

Sources:

cement.org/cement-concrete-basics/working-with-concrete/air-entrained-concrete
en.wikipedia.org/wiki/Air_entrainment
nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs141p2_023438.pdf

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