Slump Test Disagreement on a Pumped Placement
A ready-mix supplier delivers 8 m³ of a C30/37 mix with a specified slump class of S3 (100–150 mm) to a residential slab pour. The site engineer performs a slump test on the first truck and records 165 mm, which is outside the specified class. The driver states the load left the plant at 120 mm and that the extra workability was achieved with a mid-range water reducer added at the plant, not on site. The ambient temperature is 31 °C and the truck has been on site for 25 minutes. The engineer wants to reject the load immediately. The contractor argues the concrete is still within the supplier's own tolerance and that rejection will delay a 60 m³ pour and create a cold joint risk. No retempering water has been added on site.
1. How should the site engineer evaluate the slump result against the specification before deciding to reject?
Read the worked answer
The engineer must judge the concrete against the specified slump class, not the supplier's internal target. A result of 165 mm exceeds the S3 upper limit of 150 mm.
Key reasoning steps:
- Confirm the test was valid. Check that the sample was taken per the standard method (e.g. from the middle of the discharge, remixed, tested within the required time). An invalid test cannot justify rejection.
- Allow for measurement uncertainty. Slump is a variable test; a single result marginally outside the limit is not automatically non-conforming. If the result is close to the boundary, a repeat test on a fresh sample is reasonable before rejecting.
- Distinguish plant-added admixture from site-added water. A water reducer added at the plant is part of the approved mix design and does not itself make the concrete non-conforming, provided the mix conforms to the approved design. Site-added water would be a different issue.
- Check conformity against the specification, not the driver's claim. The driver's statement of 120 mm at the plant is not evidence of conformity at the point of delivery.
Conclusion: at 165 mm the load is outside S3 and the engineer is entitled to reject. A defensible path is to repeat the test once; if it remains above 150 mm, reject the load.
2. The contractor asks to accept the load to avoid a cold joint. What factors should govern whether the engineer accepts, conditions, or rejects it?
Read the worked answer
This is a judgement call balancing conformity against construction risk. The engineer should weigh:
- Degree of non-conformity. 165 mm is 15 mm over the limit - modest. A small exceedance in slump rarely harms a slab's strength if the water/cement ratio is unchanged (the workability came from admixture, not water).
- Whether the mix design is compromised. If the approved mix already includes a water reducer and no water was added on site, the w/c ratio may be unaffected, so the structural risk is low. If the extra workability came from water, the w/c ratio would be higher and strength at risk.
- Cold joint risk. Delaying 60 m³ creates a real cold-joint risk, which is a durability and structural concern in its own right.
- Documentation and traceability. Any acceptance of non-conforming concrete should be recorded, with the supplier's confirmation of mix constituents and, if needed, later strength verification (e.g. cores or cube results).
A defensible decision: accept only if the supplier confirms no site water was added and the mix design is unchanged, record the non-conformity, and plan strength verification. Reject if the cause of the high slump is uncertain.