Influence of microfine aggregate characteristics on concrete performance

ASTM C 33 limits the amount of aggregate smaller than 75 mu m (No. 200 sieve) to be used in concrete. There is considerable interest in altering ASTM C 33 to allow a higher percentage of these microfine aggregates, particularly in the case of manufactured aggregates. However, before this can be done, the positive and negative effects of microfine aggregates on concrete performance must be fully investigated. Previous research has indicated that there is not a strong correlation between standard aggregate characterization tests and concrete performance when microfines are present. In this study, several methods of characterizing microfine aggregates were investigated. The intent was to identify the characteristics of microfines that govern their effects on concrete performance. A second goal was to identify easy-to-use test methods that could be used to simply and inexpensively identify good and poor performing microfine aggregates. Three easy-to-use tests were identified that correlated well with concrete performance: the methylene blue test, a particle packing assessment by the weight of microfine aggregates adhering to a single drop of water, and the loose packing tests described in ASTM C 1252.