Building Design and Construction

At the Mixing Plant-Yield Adjustments

Well-equipped concrete producers have continuous measuring devices to record changes in moisture carried in the aggregates or changes in total free water in the contents of the mixer. The same measurements, however, may be easily made manually by quality-control personnel.
To illustrate: for the example in Art. 9.5, the surface-dry basic mix is cement, 564 lb; water, 300 lb; sand, 1170 lb; and gravel, 2000 lb. Absorption is 1% for the sand and 0.5% for the gravel. If the sand carries 5.5% and the gravel 1.0% total water by weight, the added free water becomes:

Sand: 1170 (0.055 - 0.01) = 53 lb
Gravel: 2000 (0.010 - 0.005) =  10 lb
Batch weights adjusted for yield become:
Cement: 564 lb
Water: 300 - 53 - 10 = 237 lb
Sand: 1170 + 53 = 1223 lb
Gravel: 2000 + 10 = 2010 lb

Note that the corrective adjustment includes adding to aggregate weights as well as deducting water weight. Otherwise, the yield will be low, and slump (slightly) increased. The yield would be low by about

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