As defined in ASTM C138, yield is the volume of concrete obtained per sack of cement. Yield is sometimes expressed as the number of cubic feet in a theoretical cubic yard of concrete. The latter definition of yield, also called the "return," can be derived from the following equation: total batch weight for nominal one cubic yard divided by fresh unit weight of concrete equals actual cubic feet of concrete. Even under well controlled conditions, a loss of approximately 2 percent in actual volume of air-entrained concrete can be expected for concrete in place. This is the result of a partial loss of entrained air, evaporation of water, bleeding, settlement, and other factors on the jobsite. The air content of lightweight concrete is at least as high, and sometimes higher, than that of conventional concrete to promote workability and accommodate the generally smaller maximum size aggregate. This has a considerable bearing of yield if the air content varies significantly from the design values. Air content can be checked by the volumetric method. If frequent checks of the unit weight indicate variations of more than 2 percent from the established fresh unit weight value, the air concrete or aggregate batch weight value, the air content or aggregate batch weighs should be adjusted to obtain the proper unit weight. Another factor which sometimes results in unacceptable variation in yield is contamination of the aggregate stockpiles. It is important that lightweight aggregates be effectively separated from standard weight sand, gravel, and stone. Assume we are given a mix design calling for 564 pounds of cement, 1300 pounds of sand, 900 pounds of lightweight aggregate and 39 gallons of water with 6 percent air content. What would be the effect of a 10 percent by weight contamination of the lightweight aggregate by gravel? The inadvertent change in batching weights would alter the 564, 1300, 900, and 39 gallons to 564, 1390, 810, and 39 gallons. The loss in yield would be more than 1 percent.