Whooaaa....What do the concrete foreman, super, and truck driver have to do with this??
We're talking about AIR not WATER. I will bet my house that I never see any of those people run a test and adjust the air.
Unless on-site addition is clearly prohibited by spec, the concrete company owns the concrete until it hits the hopper in the pump and can adjust out-of-spec materials to meet spec.
When air content is under spec and the QC adds an air-pack to the truck, I've never seen it bump the air up more than 1 to 1.5% above the previously low test. For forensic work where high air contents are at fault, I would expect problems to have occurred at the batching and not from voluntary field adjustment. And if it were due to field adjusting, I would expect that it was done without testing.
I gave an example of a situation where air could be adjusted by QC agents. To clarify for all, i'm talking about the plant QC rep, not the testing company or even a testing company working for the contractor. The testing company should only be testing and reporting. Pump configuration can slaughter the air content and neither the desk engineer or the batch plant will know how this will play out during design review or during the batching.
My personal preference is to pick a mix and stick with knowing there will be some variation including air loss below tolerance, but many design professionals and GCs have different takes on it. Some interesting examples that have happened in just the last 2 years on jobs i've staffed:
1- Air content outside of 4%-7% for a portion of the tested trucks caused the design team to require the steel deck to be sprayed with fireproofing at contractor cost because of the UL Design reference concrete properties.
2- The fresh unit weight was a little high on a portion of tested trucks. The GC withheld significant money from the concrete sub who withheld on the producer. The "damage" was the steel framed slabs were "out of some degree of level" and the other subs (walls for one) were backcharging for materials. The GC connected the fresh unit weight to this.
3- A post-tensioned parking deck is constructed by pumping and by crane bucket at the same time with the same mix. Additionally, every truck has to be tested for slump and air at the point of discharge by spec. Air contents on ~5%-15% of the trucks proved out of spec and triggered freeze-thaw and petrographic testing at contractor cost
If i were a QC agent for the concrete supplier, i would feel comfortable enough in my testing and knowledge of concrete to add air on-site... but would only do it for projects with drama that would merit the time loss.