Watertightness of Concrete

The ideal concrete mix is one with just enough water required for complete hydration of the cement, but this results in a mix too stiff to pour in forms. A mix fluid enough to be poured in forms always contains a certain amount of water over and above that which will combine with the cement. This water eventually evaporates, leaving voids, or pores, in the concrete. Penetration of the concrete by water is still impossible if these voids are not interconnected. They may be interconnected as a result of slight sinking of solid particles in the mix during the hardening period. As these particles sink, they leave water filled channels that become voids when the water evaporates. The larger and more numerous these voids are, the more the watertightness of the concrete is impaired. The size and number of the voids varies directly with the amount of water used in excess of the amount required to hydrate the cement.

To keep the concrete as watertight as possible, you must not use more water than the minimum amount required to attain the necessary degree of workability. You may want to make the concrete mix as wet as possible to reduce the labor of placing the concrete. This is not a good idea because adding water can result in more shrinkage and lower strength concrete, which may not meet the specifications for the project. You might also end up delaying when finishing work can begin.

The quality of troweling can impact the watertightness of concrete. Troweling is one of the final stages of finishing concrete, completed after the surface of the concrete has been floated and the bleed water has evaporated. Troweling makes the surface of concrete hard and dense, which contributes to its watertightness.