About twenty minutes into the placement the crew asks the driver to add 15 gallons of water so the mix flows better around the steel, and the ticket describes a 3,000-pound-per-square-inch mix. How should the inspector respond?
- AAccept it; water added at the site is a normal adjustment and the strength of the mix is set at the plant, not on the job
The strength is specified at the plant and destroyed at the site. That is exactly why adding water is controlled rather than left to the crew's preference for a mix that flows.
- BAccept it; the mix will still reach the specified strength provided the crew vibrates the concrete thoroughly after placement
Vibration consolidates concrete and removes voids, which is a different problem. It does nothing about a water-cement ratio that has already been raised past the design value.
- Reject the addition; the strength depends on the water-cement ratio, so water beyond the design allowance must not be added
- DReject the entire load; once any water is added at the site the concrete is no longer the mix the plans specify
Some water addition is contemplated and permitted within the design allowance, so a blanket rejection of the load overstates the rule. The question is how much water and whether it was within what the mix design held back.
Why C is correct
Concrete strength is controlled by the water-cement ratio. Water within the design allowance held back at batching may be added and documented, but water beyond it lowers strength and cannot be recovered by placement technique. The addition is stopped, and any water already added beyond the allowance calls for evidence that the concrete meets the specified strength.
What this question is testing
Whether the candidate knows what actually controls concrete strength, understands that a limited site water addition is contemplated and documented, and can pick a proportionate response rather than accepting or rejecting wholesale.
On the job
Watery mixes are requested constantly, especially on walls with congested steel or on hot days when the pour is going slowly. Inspectors who watch the truck and the ticket catch it; those who arrive after the pour never will. Where an addition happened, the department can require cylinder tests or a core, which is expensive and slow, and that possibility is usually enough to stop the practice on a job.
Memory technique
Every gallon added is strength poured out.
Exam tip
Water is the enemy of strength. Ask whether the addition was within the design allowance on the ticket.
Where to look it up
IRC Chapter 4, foundations, the concrete materials paragraph and the specified compressive strength table.