Slump and Retempering at the Point of Placement
A pump crew is placing a 5,000 psi structural mix with a specified slump range of 4–6 in. Truck 7 arrives at 10:15 a.m. with a batch ticket time of 9:05 a.m. and a slump of 3 in. The pump operator says the mix is too stiff to pump and asks the driver to add 10 gallons of water and 1 gallon of superplasticizer to 'loosen it up.' The batch ticket shows the mix already contains a mid-range water reducer and the design water is 32 gallons per yard. The inspector is on site and must decide. The spec prohibits retempering beyond the approved water and requires slump within 4–6 in. at placement.
1. Evaluate the proposed addition of 10 gallons of water plus superplasticizer against the specification and the batch ticket.
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Adding 10 gallons of water per load (assuming a typical 9–10 yd³ load, that is roughly 1 gallon per yard) raises the w/c and is retempering beyond the approved water, which the spec prohibits. The correct approach for a stiff load is not to add water; it is to evaluate the cause and use an approved admixture adjustment only if the spec and producer's plan allow.
The superplasticizer addition is also not automatically permissible: the mix already contains a mid-range water reducer, and adding a high-range admixture without the producer's and engineer's approval risks incompatibility, set problems, and segregation. The batch ticket shows design water of 32 gal/yd³; any water added must be tracked against the approved w/c, and the spec caps it.
Decision: the inspector should reject the proposed water addition as written and require the producer/engineer to approve any admixture adjustment, documented on the ticket.
2. What should the inspector do to resolve the low slump, and what are the acceptance implications?
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Steps:
- Confirm the slump test was performed correctly on a representative sample, and re-test if needed.
- Check the batch ticket: time, water, admixtures, and whether the load is within the allowed time before discharge. If the load is old or the mix has stiffened from delay, the issue may be time, not water.
- If the slump is genuinely below 4 in. and the mix is pumpable with approved means, the producer may authorize an approved admixture (e.g., superplasticizer) per the mix design and spec, added with proper mixing and re-testing.
- If the load cannot be brought into the 4–6 in. range within the approved water, reject it and get a replacement truck.
Acceptance: the load must meet slump at placement. Retempering with water beyond the approved amount makes the load nonconforming on w/c grounds even if slump comes into range. Document the decision and notify the producer.
3. The pump operator argues that without water the mix will not pump and the pour will stop. How should the inspector weigh schedule pressure?
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Schedule pressure does not override the specification or the w/c limit. The inspector's role is to protect the owner's interest in durable, conforming concrete. Adding water to make a stiff mix pump is a common shortcut that raises w/c, lowers strength and durability, and is exactly what the spec prohibits.
Better resolution:
- The producer can adjust the mix design (higher slump with superplasticizer, or a pumpable mix) for future trucks.
- For this truck, the producer and engineer can approve an admixture adjustment, documented and re-tested.
- If the load cannot be made conforming, reject it and get a replacement; the pour may pause, but a nonconforming structural mix is worse than a short delay.
The inspector should document the request, the rejection of the water addition, and the resolution, and notify the engineer of record.