Diluting concrete with water to make it workable

[Mtviewdude]

General rule of thumb, you can add 1-2 gallons per cubic yard to increase the slump without it being too much of an issue. If I had to choose between 1-2 gallons -vs- poor slump, I would add the water, any more I would reject the batch.

However if you are curious, from the studies I have read.
Adding ~2.5 gallons per cubic yard reduces the strength ~25%
Adding ~5.0 gallons per cubic yard reduces the strength ~50%
Adding ~7.5 gallons per cubic yard reduces the strength ~75%

Hawaii concrete mixes (due to their aggregates) tend to be stronger then specified. On the 4k psi mix we ordered the cylinders were breaking at ~5.2k.

Usually on the batch form it will specify "max add water".