What is concrete yield and how do I avoid yield discrepancies?
Concrete yield is defined as the volume of freshly mixed concrete from a known quantity of ingredients. Ready mixed concrete is sold on the basis of the volume of fresh, unhardened concrete‐in cubic yards or cubic meters as discharged from a truck mixer. The volume of freshly mixed and unhardened concrete in a given batch is determined by dividing the total weight of the materials by the average unit weight or density of the concrete determined in accordance with ASTM C‐138. Three unit weight tests must be made, each from a different truck. Further, the volume of hardened concrete may be about 2% less than its volume in a freshly mixed state due to reduction in air volume, settlement and bleeding, decrease in volume of cement and water, and drying shrinkage.
To prevent or minimize concrete yield problems; check concrete yield by measuring concrete unit weight in accordance with ASTM C 138. Repeat these steps if a problem arises. Be sure that the scale is accurate, that the unit weight bucket is properly calibrated, that a flat plate is used for strike off and the bucket is cleaned prior to weighing. Measure formwork accurately, and take your measurements off of the forms at the placement, not from the drawings. The American Concrete Institute says, allowances should be made from 4‐10% for waste and other causes such as a concrete pump. Construct and brace forms to minimize deflection or distortion. For slabs on grade, accurately finish to appropriate grade height, and compact the sub‐grade to proper elevation. 1/8‐inch error in a 4‐inch slab would mean a shortage of 3 percent or 1 yard in a 32 yard order.