Refining the foam index test for use with air-entrained fly ash concrete

摘要

The paper describes a study aimed at improving the foam index test method for evaluating the interaction between air-entraining admixtures (AEAs) and fly ash in the production of concrete. Ten fly ashes, two commercial AEAs and a standard reagent (sodium dodecyl benzene sulfonate surfactant) were considered. Initial tests, using manual (hand) shaking, indicated that the foam index was prone to high variability between operators (coefficients of variation 19·6-27·0% for three fly ashes with loss-on-ignition (LOI) up to 7·6%) compared with that of a single operator (coefficient of variation 11·6%). In an attempt to address this, a simple automatic shaker was considered. Tests with this demonstrated that the foam index was influenced by the shaking speed and duration, suggesting that differences in the former during manual shaking contributed to the variability noted between operators. The automatic shaker also gave a more uniform foam, making the test end point (established visually) easier to determine. This approach was then examined to establish suitable working conditions and the operator variability tests were repeated. The automatic shaker reduced the coefficients of variation for the test to about half of those for manual shaking between operators, with slight reductions for the single operator. A comparison of foam index, obtained by automatic shaking, with sorptivity-related properties of fly ash indicated better correlations with specific surface area (N_2 adsorption using the BET method) than LOI. The study then progressed to investigate relationships between the foam index and AEA doses to achieve target air contents in both mortar and concrete. The results indicated very good correlations between these for both (cement-based) systems. Similar behaviour was also obtained in the various tests carried out for the commercial AEAs and standard reagent. Means of using the refined foam index test in practical situations are explored.