Q.: Is it acceptable to batch concrete with aggregates below the saturated surface-dry moisture state? If not, what standard prohibits it?

A.: For normal-weight concrete, batching dry aggregates shouldn't cause any problems. Most air-dry aggregates don't absorb more water than a percent or so by weight of the aggregate. The batch plant operator can correct for the amount of water absorbed by adding more water at the plant.

Batching very dry lightweight aggregates can cause problems if the concrete is to be pumped. Absorption of mix water by the aggregate may cause rapid slump loss and rock jams in the pumpline. But it's difficult to predict exactly how much water will be absorbed and correct the amount of water added at the plant to eliminate the problem. To avoid this problem, presoak lightweight coarse aggregate by sprinkling if the concrete will be pumped.

An even better solution is saturating the aggregate at the plant where it's produced, using either a vacuum or thermal method. Then keep it from drying before it's batched to prevent unnecessary pumping problems. For more information on pumping lightweight concrete, see our June 1985 article, "Pumping Structural Lightweight Concrete," on page 505.