Density, Yield, and Gravimetric Air Check
A 1.00 ft³ measure weighs 9.2 lb empty and 153.0 lb filled with consolidated concrete. The batch ticket shows cement 650 lb, water 280 lb, fine aggregate 1,180 lb, coarse aggregate 1,920 lb, and a design volume of 27.0 ft³. No water was added at the jobsite. The concrete is normal-weight with 1 in. nominal maximum aggregate. A pressure air meter on the same sample reads 6.0%. The specification requires 5 to 7% air. The technician computes a gravimetric air content that does not match the pressure meter and is unsure which value to report.
1. Calculate the fresh density, yield, and gravimetric air content. Show equations and units.
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Mass of concrete in the measure = 153.0 lb − 9.2 lb = 143.8 lb.
Fresh density = mass / volume = 143.8 lb / 1.00 ft³ = 143.8 lb/ft³.
Total batch mass = 650 + 280 + 1,180 + 1,920 = 4,030 lb.
Theoretical density (assuming no air and the stated design volume) = 4,030 lb / 27.0 ft³ = 149.26 lb/ft³.
Gravimetric air content = (theoretical density − measured density) / theoretical density × 100 = (149.26 − 143.8) / 149.26 × 100 = 3.66%, about 3.7%.
Yield = total batch mass / fresh density = 4,030 lb / 143.8 lb/ft³ = 28.03 ft³ = 1.038 yd³.
Cement factor = 650 lb / 1.038 yd³ = 626 lb/yd³.
These calculations assume the batch weights are accurate and no water was added after batching.
2. The pressure meter reads 6.0%. Which result should be reported, and what should be checked?
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Do not average the two values. A difference of more than about 2 percentage points is a warning that one method or the sampling is wrong. The gravimetric calculation depends on accurate batch weights, an accurate design volume, and no unrecorded water addition. The pressure method depends on a properly calibrated meter, correct consolidation, a stable gauge reading, and the aggregate correction factor for the actual aggregate combination.
Check the batch ticket against actual weights, verify that no water was added at the jobsite, confirm the aggregate correction factor for the pressure meter, check the meter calibration, and confirm that the pressure sample was properly consolidated and the gauge tapped. If the batch weights are accurate and no water was added, the gravimetric result is a strong check and suggests the pressure result may be uncorrected. If any batch weight or water addition is uncertain, the gravimetric air content is not reliable. Resolve the discrepancy with a second method before accepting or rejecting the load.
3. Assume the investigation confirms the batch weights and no water addition. What decision and communication are appropriate if the correct air content is 3.7%?
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If the verified air content is 3.7% and the specification requires 5 to 7%, the concrete does not meet the air requirement. The technician should notify the supplier and the contractor promptly and document the results and the method used. The load should not be accepted for air-entrained concrete unless the specification allows a lower value or an authorized representative approves a variance.
The technician should not add an air-entraining admixture at the jobsite without approval, because jobsite addition is difficult to control and may not be permitted. Low air content can reduce freeze-thaw durability and may also indicate a batching or testing problem. Before final rejection, confirm that the sample was representative and that the test method was correct. If acceptance is critical, obtain a new representative sample and retest air content by an appropriate method.