The use of pumps for transporting and handling structural lightweight aggregate concrete is increasing. Lightweight aggregates are inherently porous, due to their method of manufacture, and will absorb greater amounts of water then conventional concrete aggregates. At atmospheric pressure this absorption may vary from five to 25 percent by weight for different lightweight aggregates; under the pressure exerted by pumping it can be considerably more. If the absorption is significantly increased during pumping, water loss from the mortar can impart the pumpability of the mortar. By using a series of sprinklings some producers have been able to satisfy absorption requirements of lightweight aggregates. The aggregates are sprinkled at the producers' stockpiles during loading for shipment and again after unloading at the concrete batch plant. From two to three days of repeated sprinklings may be required to adequately presoak coarse lightweight aggregates but the actual time should be based on tests or experience with a particular aggregate. To permit more uniform slump control, free water should be allowed to drain from aggregates for several hours before they are used in concrete. Lightweight fine aggregates, if used, should be presoaked but presoaking of lightweight fines in stockpiles or bunkers is difficult because water will not penetrate the fine material easily. However, fine particles absorb water faster than coarse particles. Accordingly, presoaking is occasionally done right in the mixer by adding two-thirds of the mixing water before adding other ingredients. With this method presoaking lightweight fines takes only about five minutes. When designing lightweight concrete mixes, the total water requirement is different from that used for normal weight mixes. For simplicity, the total water in a lightweight mix may be considered divided into two segments: active water and absorbed water. It is the active water that establishes the slump and water-cement ratio. The absorbed water is contained within the lightweight particles and, as such, does not directly affect the quality of the paste. A slump loss that takes place in a lightweight mix between the pump hooper and the discharge end of the line is usually caused by further aggregate absorption under pumping pressure. This is often due to insufficient presoaking of the aggregates, an amount of coarse aggregate that is excessive for a pumpable mix or abnormally high pump pressure.