"[A]ir-entrainment adds to the fluidity or slump of concrete and it is possible to restore the initial degree of stiffness by adding solids or subtracting water. Air cannot be added to a mixture without introducing unbalances in consistency and yield of the plastic concrete. ...[T]he strength of the hardened concrete depends on the consistency of the plastic concrete. Experiments have shown that for the same workability, less water is required than for ordinary concrete."