Often when a discussion turns to watertight concrete, we think strictly in terms of hydraulic structures. In reality, there are many other applications of concrete in which permeability plays an important part in the performance picture. For example, virtually every home basement is a job in need of watertight concrete. In fact, concrete must be impermeable for nearly every below ground application. Above ground, too- as in parking garage floor slabs- concrete must often withstand water or vapor pressure. First, the concrete ingredients should be of uniformly high quality. This means sound, well-graded aggregates held together by an impermeable, hardened cement paste. The bearing that aggregate quality has on the impermeability of concrete is multi-faceted. With a well-graded aggregate, the amount of cement paste, often the culprit in leaky concrete, is minimized. Also, the resulting concrete is easier to handle and consolidate. The aggregates must be sound and inert to obviate popouts or alkali-aggregate reactivity which would open passageways for water through the concrete. The aggregates themselves should have a low absorption factor and they should be reasonably free of organic matter. Concreting practices must also be of high quality. Handling and placing practices must be such tat segregation is avoided. The concrete should not be allowed to free drop more than 6 feet and vibration should not be used to move the concrete horizontally. Curing is also highly important in achieving watertightness. Watertight concrete must be cured at least 7 days and should not be subjected to hydrostatic pressures before this period has ended. A number of treatments are also available such as cementing a fabric membrane and applying a mortar incorporating metallic aggregates and an oxidizing catalyst.