Moisture content of aggregates affects the properties of both fresh and hardened concrete. Wet aggregates add water to the mix, increasing slump and decreasing compressive strength. Dry aggregates absorb mix water, decreasing slump and making it more difficult to place and finish. To correct for moisture, producers must adjust batch weights.
Aggregates are subject to four moisture conditions, depending on the amount of water in the pores or on the surface of the aggregate particles. Aggregate that contains no water in the pores or on the surface is oven-dry. Aggregate that has a dry surface but contains some water in the pores is air-dry. Aggregate that has surface pores filled with water but no free water on the surface is saturated surface-dry. Aggregate that has surface pores filled with water and has free water on the surface is wet.
Aggregate quantities actually weighed out for concrete must account for moisture in the aggregates. Aggregates generally are wet, and their batch weights must be increased by the percent of absorbed and surface water they contain. The mixing water added to the batch must be reduced. For air-dry aggregates, water is absorbed into the aggregates, reducing the mix water. Therefore the amount of water added should be increased.