What is air-entrained concrete ?

Air-entrained concrete contains billions of microscopic air cells per cubic foot . These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes . Air -entrained concrete is produced through the use of air - entraining portland cement , or by the introduction of air - entraining agenets , under careful engineering supervision as the concrete is mixed on the job . The amount of entrained air is usually between 4 percent and 7 percent of the volume of the concrete , but may be varied as required by special conditions .