Q.: We've had trouble with low entrained air contents in concrete containing fly ash. What causes this problem and what can we do about it?
A.: Unburned carbon in fly ash will absorb air entraining agents and may necessitate an increase in the air entraining agent (AEA) dosage. Loss on ignition (LOI) is a measurement of unburned carbon remaining in the ash. Specifications of the American Society for Testing and Materials (ASTM C 618) for fly ash used in concrete limit the LOI to 6 percent. Values greater than 2 to 3 percent, however, will probably require you to significantly increase the air entraining agent dosage. You might also try changing the type of AEA used. Several admixture manufacturers have developed AEA's specifically for use in fly ash concrete.