What causes air content to vary when the amount of air entraining agent and the mix proportions have not changed?
There are many possibilities. An increase in fineness of sand can reduce the air content. If coloring agents or carbonaceous materials make their way into the mix they are also likely to decrease the air content. An inadvertent overdosage of admixture or reduction in cement content can increase the air. Organic matter in the sand often acts as a foaming agent and increases the air. Even changing from hard to soft water or from soft to hard may change the amount of air entrained by some air entraining agents. A change in dispensing sequence frequently changes the amount of air entrained . Overloading the mixer reduces mixing efficiency and decreases the amount of air. Worn mixer blades do the same thing. Changes in temperature affect the efficiency of air entrainment, and less air entraining agent is needed on a cold morning than later in the day when the temperature is higher.