Testing Fly Ash in Mortars for Air-Entrainment Characteristics

Abstract

Abstract When fly ash is used in the production of air-entrained concrete, it can affect the dosage requirement of air-entraining admixture (AEA) needed to produce the desired air content. ASTM C 618 provides a uniformity requirement for the dosage of AEA needed with a particular fly ash, and ASTM C 311 provides a test to determine compliance with the requirement. In a study of fly ash uniformity, the C 311 test was found to be ineffective in evaluating the effect of fly ash characteristics on AEA dosage requirement. The problem with the C 311 test is traced to the high levels of AEA which are required in the test and is illustrated by the use of AEA dosage versus air content curves. Modifications to the C 311 test are proposed wherein an admixture dosage is selected from the AEA dosage versus air content curve. This dosage is then used to test successive fly ash samples, and changes in air content of the test mix illustrate the variations in AEA requirement caused by the fly ash. An additional modification permits the evaluation of the fly ash's potential to lose entrained air over time.