Slump Test Disagreement Between Two Technicians
On a bridge deck pour, technician A performs a slump test and reports 95 mm. Technician B, watching, says the result is invalid because A did not moisten the cone, and repeats the test, reporting 130 mm. The specification requires 75–100 mm. The concrete is a 28 MPa air-entrained mix with a retarding admixture, delivered 45 minutes after batching. The truck has been turning slowly. Technician A's sample was taken from the middle of the discharge; technician B's from the last third. The batch ticket shows 1.5% air target. The contractor wants to add water to 'loosen it up' because B's number is high and the crew says the mix 'looks stiff.'
1. Which result is more likely to reflect the concrete, and what procedural factors explain the 35 mm difference?
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The 35 mm spread is too large to be test noise and points to procedural and sampling differences, not a real change in the concrete over a few minutes.
Factors:
- Sampling location: A sampled the middle of discharge; B sampled the last third. With a retarding admixture and slow turning, the last portion can be wetter or drier depending on mixing efficiency and any water added to the drum; the last third is generally less representative of the batch as a whole.
- Cone moisture: an unmoistened cone absorbs water from the paste at the surface, which can slightly reduce slump - a minor effect, not 35 mm, but it is a real procedural defect in A's test.
- Technique: rodding pattern, number of layers, lifting the cone vertically, and timing between lift and measurement all shift results; a hurried test typically reads higher.
Neither result should be accepted uncritically. The correct action is a single referee test on a properly obtained sample (from the middle of discharge, per the sampling plan), with a moistened cone, correct rodding, and measurement within the specified time. The 130 mm result is not evidence that the concrete is out of specification.
2. Evaluate the contractor's request to add water and state the correct disposition.
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Adding water on site is not justified by two conflicting slump results. The mix is retarded and 45 minutes old; apparent stiffness at the discharge point is expected and does not mean the mix is out of specification. Adding water would raise the water-cementitious ratio, reduce strength and durability, and is generally prohibited unless permitted by the mix design and approved by the engineer, with water addition limits and remixing time observed.
The correct disposition: perform the referee slump test; if it falls within 75–100 mm, place the concrete. If it is genuinely above 100 mm, the load is evaluated per the specification's tolerances (which often permit rejection or require engineer review), not corrected by adding water. If it is below 75 mm, the remedy is dictated by the specification - often remixing or, if permitted, a limited water addition with engineer approval - not a field decision by the crew. Document both original tests and the referee test.