On a recent job we had excessive fluctuations in air content. The mix contained 490 pounds of cement per cubic yard and was made with 1-inch maximum size aggregate. The air was required to be between 3.5 and 6.5 percent, but sometimes it was considerably higher and sometimes a little lower. What causes such changes?
Many things affect it. Air content is decreased if the concrete temperature rises, slump is lower, or the cement or pozzolan is finer. It is increased if calcium chloride is used in rich mixes but usually not in lean mixes. Long mixing can increase air content but still longer mixing may decrease it. Changes in brand or type of cement or kind of pozzolan may make the air content go up or down.