Questions often arise between concrete suppliers and buyers regarding yield, or volume of concrete supplied. A major reason for this is that often the actual yield may be less than the yield calculated from the volumes of ingredients. For example, if the mix temperature varies, less air may be entrained; or if the sand becomes drier and no corrections in batch weights are made, the yield will be under that calculated.
If the specific gravity (sp. gr.) and absorption (abs.) of the aggregates have been determined in advance, accurate, yield calculations can be performed as often as necessary to adjust the yield for control of the concrete.
Example
Yield of Non-Air-Entrained Concrete. The following material properties were recorded for materials used in trial batches: fine aggregate (sand) sp. gr. = 2.65, abs. = 1%; coarse aggregate (gravel) sp. gr. = 2.70, abs. = 0.5%; and cement, sp. gr. = 3.15 (typical). These properties are not expected to change significantly as long as the aggregates used are from the same source. The basic mix proportions for 1 yd3 of concrete, selected from the trial batches are
Cement: 564 lb (6 bags)
Surface-dry sand: 1170 lb
Surface-dry gravel: 2000 lb
Free water: 300 lb /yd3 (36 gal /yd3)
Check the yield:
These results indicate that some rapid field checks should be made. Total weight, lb, divided by the total volume, yd3, reported on the trip tickets for truck mixers should be about 4000 on this project, unless a different slump was ordered and the proportions adjusted accordingly. If the specified slump for the basic mix was to be reduced, weight, lb /yd3, should be increased, because less water and cement would be used and the cement paste (water plus cement) weighs 864/7.68 = 113 lb/ft^3 < 149.4 lb / ft^3. If the same batch weights are used for all deliveries, and the slump varies erratically, the yield also will vary. For the same batch weights, a lower slump is associated with underyield, a higher slump with overyield. With a higher slump, overyield batches are likely to be understrength, because some of the aggregate has been replaced by water.
The basic mix proportions in terms of weights may be based on surface-dry aggregates or on oven-dry aggregates. The surface-dry proportions are somewhat more convenient, since absorption then need not be considered in calculation of free water. Damp sand and gravel carry about 5 and 1% free water, respectively.
The total weight of this free water should be deducted from the basic mix weight of water (300 lb /yd3 in the example) to obtain the weight of water to be added to the cement and aggregates. The weight of water in the damp aggregates also should be added to the weights of the sand and gravel to obtain actual batch weights, as reported on truck-mixer delivery tickets.
