Adding water to concrete mix in field

I know its a big no-no but what are the exceptions to this rule.

I see it done all the time. The concrete mixing truck will back up to the job site and spin out a tiny bit of concrete down its chute so the foreman and inspector can take a look at it. If the mix seems a bit dry it's acceptable to add water on the spot as long as it's thoroughly mixed before placement. Any QC/QA tests run on the concrete should be run on the middle 80% of the truck's load so if the slump gets out of tolerance because of too much added water it will be caught and dealt with.

Look at the Design and Control of conc. Mixtures. 13th edition page 96: " Under careful supervision a small amount of water may be added to remix the concrete providing the following conditions met: (1) maximum allowable cement ratio is not exceeded; (2) maximum allowable slump is not exceeded; (3) maximum allowable mixing and agitating time (or drum revolutions) are not exceeded; (4) concrete is remixed for at least half the minimum required mixing time or number of revolutions."

I've also seen it done on site, but I always look at the batch ticket to see if the plant added all the water as required in the mix design, or not.

I've had a couple of contractors not like the way it looked (consistancy wise), but had to deal with it as the mix already had all of the allowable water, other times, they can add a few gallons and remix per the requirements stated above.

Generally this practice is not ideal as there can be issues if the re-mix is too short / long, etc... Testing should be done between 50-75% of the truck when possible as it will be the most uniform.

HTH

Thanks everyone

For short hauls with cool temps we usually under batch the water and add on site if needed. miloc has the best answer.