How to use the concrete slump and air content check
Ready-mixed concrete keeps losing slump between the batch plant and the placement, and hot weather makes the loss much faster. Enter the slump at batching, the concrete temperature, and the haul plus waiting time to see the predicted slump at arrival and whether it still falls inside the tolerance band in your project specification.
The slump loss model follows the field rule used in ACI 305R for hot weather concreting: about 1 in. of slump is lost per 30 minutes at 70 F, and the loss rate roughly doubles for each 20 F rise in concrete temperature. Actual loss varies with mix design, admixture type, and drum revolutions, so calibrate the result against your own truck data when you have it.
Air content is judged against the plus or minus 1.5 percentage point tolerance in ASTM C94, measured by the pressure method of ASTM C231. Haul time is checked against the ASTM C94 limit of 90 minutes from batching to complete discharge, or 300 drum revolutions, whichever comes first.
Adding uncontrolled water on site raises the water-cement ratio and permanently lowers strength and durability. If the slump arrives low, use the single tempering water addition allowed by ASTM C94 or a high-range water reducer, and let the testing agency and engineer of record confirm acceptance before placement continues.
Frequently asked questions
ASTM C94 requires discharge within 90 minutes of batching, or within 300 drum revolutions, whichever comes first. The limit can be extended only when the purchaser approves it in writing.
Only the one-time tempering water allowed by ASTM C94, and only if it keeps the mix under the specified water-cement ratio. Uncontrolled water raises w/c and lowers strength and durability.
ASTM C94 allows plus or minus 1.5 percentage points from the specified air content measured by ASTM C231. Too much air lowers strength, too little reduces freeze-thaw resistance.