Q.: Could you comment on fly ash in concrete for slabs on grade? Will fly ash reduce shrinkage?

A.: Fly ash is used in concrete for flatwork to permit use of lower cement contents and to enhance workability. The proportioning of mixes containing fly ash should not be based on the simple replacement of a percentage of the cement by an equal percentage of fly ash, a practice all too commonly used. Usually more fly ash should be added than the amount of Portland cement taken out depending on the particular cement and fly ash being used. The sand content may have to be reduced somewhat. In general the use of fly ash decreases the early strength of the concrete and increases the ultimate strength depending on the proportioning and the curing. Concrete containing fly ash must be cured longer in order to derive the benefits.

According to Edward A. Abdun-Nur, ''Fly Ash in Concrete: An Evaluation," Highway Research Board Bulletin 284, fly ash tends to decrease drying shrinkage of concrete.