Adding water on-site to concrete mixtures has been a controversial topic for as long as concrete has been used as a construction material. The addition of water is clearly covered in ASTM C94, Standard Specification for Ready Mixed Concrete where it is allowed to adjust fresh concrete properties so the material is suitable for the specific application.
Concrete that is delivered with a low slump may lack the workability for proper placement consolidation and finishing operations. Additionally, low slump concrete mixtures in some cases may be deficient in air content for durability in freezing and thawing environments or where the material may be exposed to deicing chemicals. While on-site additions of water may potentially reduce the final concrete strength properties, in many cases the consequences of adding the water may be less detrimental than attempting to place, consolidate, and finish a concrete mixture that that lacks proper workability and/or air content.
How Much Can I Add?
The general rule is that water may be added to adjust the slump of the material to comply with specifications upon arrival as long as the maximum specified water-cement ratio is not exceeded. Typical additions of water on-site are in the order of one to two gallons per cubic yard of concrete (this would amount to 10 to 20 gallons of water in a 10 cubic yard load). A general rule of thumb for the effect of the addition of water to a concrete mixture is an increase in slump of approximately 1 inch per gallon of water added to a cubic yard of concrete.
In short, a small addition of water (one to two gallons per cubic yard of concrete) can be beneficial to the quality of the hardened concrete when appropriately added (not exceeding the maximum water-cement ratio or maximum revolutions) on-site during the normal course of concrete construction.
Further Reading: Portland Cement Association