Concrete yield is an important consideration on many jobs. An engineer may compute the number of cubic yards of concrete needed for a 500,000 square foot floor, and the ready mix producer may deliver the concrete in complete accordance with the specifications. Yet when the specified number of cubic yards of concrete has been delivered, the floor may be far from completed. Since this shortage can result from a number of factors, it will be helpful to take a look at the methods of calculating yield, the factors which will cause variations in yield and the techniques for policing yield.
Yield can be affected while the structure is still in the design stage. It is not unusual to encounter mix designs which simply will not yield 27 cubic feet of concrete per cubic yard of concrete ordered. When the concrete producer receives the approved mix design, he should check it for yield. This can be accomplished by applying the yield formulas. If the mix design is incompatible with correct yield, this fact should be called to the attention of the appropriate party and a revised design obtained which will produce correct yield.
After the correct mix design has been prepared and approved, problems with yield can develop at the batching plant. Contamination of the aggregates by materials of different weight may result in changes in yield, since in batching by weight the volume of aggregates will change. For example, if lightweight organic material was to contaminate regular weight aggregates, a given weight of the aggregates would produce a greater volume than anticipated. Conversely, if aggregates are contaminated with material of greater weight, yield will be reduced and strength increased.
To minimize problems with yield, contractors should observe the following rules: (1) make sure that the mix design is consistent with a yield of 27 cubic feet of concrete per cubic yard of designed mix. (2) Schedule deliveries so delays in discharging concrete from trucks will be minimized. (3) Be sure forms are not leaking. Finally, (4) do not use an excessively wet mix.