Q.: I once saw some rules of thumb about the effects of adding water to a concrete mix, but I can't find them.
A.: The basic rule of thumb is that adding 1 gallon of water per cubic yard increases the concrete slump by about an inch. Because of the more fluid mix, air content also is likely to increase by 0.5% to 1.0%. Concrete compressive strength decreases because the increased water-cement ratio produces a weaker paste and the increased air volume means that there is less solid material present. The compressive strength decrease could be 200 to 400 psi, considering both factors.