Hot-Weather Load: Site Water, Air Content, and the Discharge Clock
Truck 14 is batched at 10:05 with 8 yd³ of a 4,000 psi air-entrained mix: 600 lb/yd³ cementitious, 270 lb/yd³ of design water (w/cm 0.45), 0.75 in. nominal maximum aggregate. The specification permits w/cm up to 0.50, slump of 4 plus or minus 1 in., air content of 6.0 plus or minus 1.5%, and a maximum concrete temperature of 95 degrees F as placed. The truck arrives at 10:50 with roughly 210 drum revolutions on the counter. The technician measures a slump of 2.5 in. and a concrete temperature of 96 degrees F. The contractor asks the driver to add 12 gal of water and mix 10 more minutes so the pump crew can place the load; the water goes in without notifying the inspector. After remixing, slump is 4 in. and the pressure meter reads 5.2% air. The technician reports 5.2% and passes the load. The laboratory's current aggregate correction factor for this aggregate is 0.8%. The pump crew expects to finish discharging at about 11:40.
1. Was the 12 gal addition permissible? Support your answer with the w/cm calculation and the procedural requirements that apply to water added at the site.
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Two separate tests apply: the w/cm ceiling and the procedure.
Water balance: 12 gal divided by 8 yd³ is 1.5 gal/yd³, or about 12.5 lb/yd³. The load moves from 270 lb to roughly 282.5 lb of water; divided by 600 lb of cementitious that is a w/cm of about 0.47. Since the specification allows 0.50, this particular addition does not by itself put the load over the w/cm limit - the load has about 300 lb of water available at 0.50.
Procedure: it still fails. Water added at the site requires the approval of the purchaser's authorized representative before it is added, must be measured, added with the drum turning, recorded on the delivery ticket, and followed by at least 30 revolutions at mixing speed before the concrete is sampled or discharged. Here the water went in unannounced, was not approved, and the technician sampled before the required remixing was complete. Approving it after the fact is not the same as permitting it - record the quantity, recalculate the w/cm, and retest slump, air, and temperature after the full remix.
Judgment point: the underlying cause is a 96 degree F load losing slump to heat, not a dry mix. The right fix is plant-side (retarder, chilled water or ice, shorter haul), not a water can at the chute. On any mix with a 0.45 or lower cap, the same 12 gal would have been an immediate rejection.
2. Evaluate the reported air content. What value should appear on the record, does the load conform, and what would you do before accepting or rejecting it?
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The pressure meter does not measure entrained air alone; it also responds to air trapped in the pores of the aggregate. The aggregate correction factor is determined for the specific aggregate combination and is subtracted from the gauge reading. The correct value here is 5.2 minus 0.8, or 4.4%.
Acceptance range for 6.0 plus or minus 1.5% is 4.5% to 7.5%. At 4.4% the load does not conform, and reporting 5.2% would have created a false acceptance record - a documentation failure as serious as the test error.
Before rejecting, rule out test error: confirm the meter was zeroed and calibrated, that the base was filled in accordance with the method and consolidated properly, that the correction factor is current for this aggregate, and that the portion was obtained within the sampling time limits after remixing. Also consider the history of this load: added water plus 10 extra minutes of mixing can knock down air content, so the 4.4% may be real. Retest a fresh portion from the same truck and check the next truck's air as well.
If the retest confirms something below 4.5%, the load does not meet the air requirement for the freeze-thaw exposure and should be rejected unless the design professional accepts it in writing. Document the correction factor, the raw gauge reading, the corrected value, and the disposition.
3. Considering discharge time, drum revolutions, and temperature together, is the load still placeable at 11:40? State the governing limits and the disposition you would recommend.
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There are three independent limits and the load trips at least two of them.
Time: ASTM C94 practice limits discharge to 90 minutes after batching, or 300 drum revolutions, whichever comes first, unless the engineer approves otherwise. Batched at 10:05, the load must be out of the drum by 11:35; finishing at 11:40 is past the limit.
Revolutions: 210 at arrival plus about 10 minutes of mixing at 8 to 12 rpm adds 80 to 120 revolutions, putting the drum at or beyond 300 before discharge is complete. That clock has effectively expired too.
Temperature: the measured 96 degrees F exceeds the 95 degree F maximum as placed. That is a nonconformance on its own, and it also explains the slump loss and accelerates strength loss and air instability.
Put together, this is a hot, over-revolved load with unapproved site water and air below tolerance. My recommendation is do not place it; divert it to nonstructural use or reject it, notify the plant, and change the remaining deliveries (retarder or hydration control, chilled water, shaded sampling, shorter hauls). Record batching time, arrival time, revolutions, temperature, water added, and the final disposition on the ticket and in the inspection log so the outcome is defensible later.