The yield of concrete has led to many disagreements between producers and contractors. The starting point is recalling that the total concrete batch volume shown on the ticket is computed and not measured. The computed volume is calculated from the batch weight divided by the density of each ingredient, plus the best estimate of actual total air volume at the time of delivery. But even though batch weights are pretty reliable with modern equipment, aggregate densities vary within sources, as do moisture contents.
Further, air content is sufficiently variable that most specifications include a ± 1.5% tolerance, which corresponds to a ± 1.5% variation in concrete volume for the same batch weight caused by air alone. If you were running consistently 1.5% low on air content for a 100 CY placement, you would be 1.5 yards short at the end of the pour! But all is not lost; you can make simple, standard measurements on site to verify the correctness of the concrete producer’s weight-to-volume predictions.
Recall that our ticket reports a total volume of 2.0 cubic yards. Note also that near the bottom of the ticket is a total batch weight of 7533 lb. This means that the producer is predicting a “fresh density” of the concrete of 7533 lb for the 2 cubic yard load, or 3767 lb for 1 cubic yard or dividing that by 27 cubic feet in a cubic yard results in a “unit weight” of 139.5 lb/cubic foot.
This can be verified in the field by means of the ASTM C138 Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete. The actual volume of concrete batched and transported is computed by dividing the total batch weight shown on the ticket by the fresh density measured in the field. The yield of the concrete is simply this actual, measured volume divided by the reported volume. So, let’s say our example mix arrived 1.5% low on air content. If you performed the C138 density test onsite it would come in at 141.6 lb/cubic foot. Dividing 7533 pounds (the batch weight) by 141.6 (the measured unit weight) gives us a volume in the mixer of 53.2 cubic feet instead of the expected 54 cubic feet (27 cf/cy x 2). The yield then is 53.2/54 = 0.985, or 1.5% less than promised.
While C138 checks the density of the fresh concrete by filling, consolidating, and finishing concrete in a ½ cubic foot bucket, a quick and approximate check can be made by weighing the full bottom chamber of a typical ASTM C231 pressure air meter during the standard air test (which contains about ¼ cubic foot). Either of these methods is preferable to attempting to verify batched volume from the volume of concrete placed. The problems of variable subgrade elevation, deformation of forms, effect of placing and consolidating concrete on air, and accounting for concrete wasted and used in testing make it inadvisable to go to battle over yield based on as-placed estimates of volume.