Slump Test Disagreement on a Pumped Placement
You are the special inspector on a mid-rise slab pour. The mix is a 4000 psi, 3/4 in. aggregate, air-entrained pump mix with a specified slump of 4 ± 1 in. The pump operator adds water at the hopper because the line is surging. The contractor's technician takes a slump test at the discharge end of the boom and reports 6.5 in. You take a second sample from the same truck at the pump hopper and get 5.0 in. The contractor argues the mix is fine because your number is within tolerance. The truck ticket shows 4.5 gal of water added on site beyond the batch water. The ambient temperature is 88°F and the concrete temperature is 84°F.
1. How do you resolve the two conflicting slump results, and which value governs acceptance?
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Two different sampling locations and two different times explain most of the gap. Slump changes with time, temperature, and shear history through the pump line, so the hopper sample and the boom-end sample are not the same concrete condition. The governing sample for acceptance is the one taken at the point of placement per ASTM C172 sampling practice, but it must be a representative sample of the concrete being placed, not one taken after unauthorized water addition.
Key reasoning steps:
- Establish where the sample should legally come from: the point of discharge into the form/work, sampled per C172.
- Note that the 4.5 gal on-site water addition is a change to the approved mix and must be evaluated against the approved submittal and the water-cement ratio, not just slump.
- Because the two tests disagree by more than the repeatability expectation for a single operator, treat the discrepancy as a testing/sampling problem and retest from a freshly agitated, properly sampled portion at the discharge point.
- Do not average the two values. Averaging hides the water-addition issue.
The contractor's 'within tolerance' argument fails because your 5.0 in. reading was taken at the hopper before full mixing and does not represent the placed concrete; and the 6.5 in. reading reflects added water. Neither is automatically acceptable.
2. The retest at the discharge point reads 6.0 in. What is your next action, and what documentation do you produce?
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A 6.0 in. slump exceeds the 4 ± 1 in. tolerance (upper limit 5 in.). The concrete is nonconforming on slump. Next actions:
- Reject the load or require the contractor to obtain the EOR's/ready-mix producer's written direction. Do not permit placement of nonconforming concrete without an approved resolution.
- Flag the on-site water addition on the batch ticket. Compute or request the water-cement ratio check; added water at the jobsite is generally prohibited unless specifically authorized.
- Record the concrete temperature (84°F) against the specified maximum (commonly 95°F for normal conditions, but check the project spec). High temperature accelerates slump loss and stiffening, which is likely why the pump operator wanted water.
- Document: truck number, batch time, arrival time, water added, all slump readings with location and time, tester identity, temperature, and disposition of the load.
- Issue a nonconformance report and notify the EOR and contractor in writing.
Model answer emphasizes that the inspector's authority is to test, document, and reject nonconforming material, not to 'fix' the mix by approving water.
3. The contractor proposes adding a high-range water reducer at the jobsite to restore workability instead of water. What factors determine whether you should allow it?
Read the worked answer
Jobsite admixture addition is only permissible when: (a) the project specifications and the approved mix design allow it, (b) the producer authorizes it in writing, (c) it is added within the time limits and batching window, and (d) it is dispensed and mixed per the manufacturer's instructions (usually a minimum number of drum revolutions at mixing speed).
Reasoning:
- A high-range water reducer changes the mix from the approved submittal; without written authorization it is an unapproved change.
- Adding it after significant slump loss may not restore the intended water-cement ratio or air content, and overdosage causes segregation and retardation.
- Air content must be rechecked after addition, because many water reducers affect entrained air.
- The load must still be within the allowable time/temperature window for placement.
If all conditions are met, you may allow it, retest slump and air, and document the admixture, dose, time, and retest results. If any condition is unmet, reject and require a fresh load.