Water is certainly a necessary ingredient in concrete. However, only about 2 gallons are really needed to hydrate one 94-pound bag of cement. Additional water is required to make the concrete workable enough to be handled by the contractor. When too much water is added, however, placement costs increase drastically and the potential for concrete failure becomes much greater. If the mix is properly designed, concrete with a slump of 3 to 4 inches can almost always be handled properly by finishers. Most finishing crews prefer to place concrete at a slump in excess of 4 inches because wetter concrete requires less shoveling and permits easier screeding.
The question we must ask ourselves is what is the price of this extra "water of convenience?" Concrete at a 4-inch slump is about 800 psi stronger at 28 days than the same concrete placed at a 7-inch slump. If 1 pound of cement will produce about 10 psi, the 4-inch slump concrete has a value of approximately 80 pounds more concrete per cubic yard than the 7-inch-slump concrete. If the 7-inch slump concrete met the strength requirements, it is obvious that the same concrete at a 4-inch-slump could be produced with almost one less 94-pound bag of cement per cubic yard, amounting to a substantial saving to the contractor.
High-slump concrete can be the chief cause of concrete deterioration. Wet concrete bleeds more, shrinks more, and is weaker. Excess water can also disturb a good air void system and greatly lower the durability of concrete subject to freezing and thawing. Concrete with an optimum air void system is essential for good durability. The bubbles must be small, with an average diameter of 0.001 to 0.007 inch, and spaced closely enough so that no part of the hydrated paste is more than 0.008 inch from the surface of a bubble. This spacing is important so that water can readily move into the air spaces as it expands during freezing.