Slump Test Disagreement on a Pumped Placement
A ready-mix truck arrives with a batch ticket showing a design slump of 4 in. The specification permits a tolerance of ±1 in at the point of placement. The driver added 8 gal of water at the plant before departure, and the ticket records that addition. A technician performs a slump test on a sample taken after the first 10% of the load has been discharged, obtaining 6.25 in. The foreman insists the load is fine because the truck was charged 40 minutes ago and the day is hot (95 F). A second technician takes a sample from the middle of the discharge and reads 5.5 in. The pour is a 12 in. thick elevated slab, and rejection would delay the finishing crew by two hours.
1. Is the concrete within slump tolerance, and what does the discrepancy between the two readings tell you?
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The first reading of 6.25 in. exceeds the upper limit of 5 in. (4 in. design + 1 in. tolerance). The second reading of 5.5 in. is still above tolerance. The two readings differ by 0.75 in., which is within normal single-operator repeatability but shows the load is not uniform - a real signal on a hot day with added water. Per ASTM C143, a single test is the average result, but when two samples from the same load straddle or exceed the limit, the controlling value is the one that shows nonconformance. Judgment: the load fails the acceptance criterion for slump. Do not average your way into acceptance. The correct action is to reject or require a documented adjustment approved by the engineer, not to proceed because the schedule is tight.
2. The producer proposes adding a high-range water reducer to bring slump down. Evaluate this proposal.
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A water reducer cannot remove water already added; it disperses cement and improves workability at a given water content, which typically increases slump, not decreases it. Adding HRWR to an already over-slump load does not restore the original w/c ratio recorded on the ticket. The only legitimate paths are (1) reject the load, or (2) if the engineer authorizes it, accept the load under a documented variance with the understanding that the effective w/c has increased. Because the ticket already records 8 gal of added water, the batch water plus that addition should be reconciled against the maximum permitted w/c for the specified strength and exposure. Judgment: the proposal is technically unsound and should be refused; the decision belongs to the engineer of record, documented in writing.