Many applications of concrete require resistance to the passage of water. Beyond the obvious ones, such as tanks, canals, tunnels, and retaining walls, there is need for watertight concrete in foundations, floor slabs, and other applications where ground water may be encountered. In this discussion, however, we will deal solely with the fundamental rules to observe in designing a concrete mix that has the potential to resist the passage of water. The guidelines for preparing a mix design for concrete to be subjected to water pressure include the following: use a low unit water content; specify a low water/cement ratio; employ sound, impervious, properly-graded aggregates; provide 3 to 6 percent entrained air; design a cohesive mix of proper workability. Permeability of concrete has been found to increase rapidly in mixes with water/cement ratios in excess of .55 by weight. Generally speaking no more then 6 gallons of water per sack of cement should be used in concrete that must be impermeable. For mass concrete this can be raised to 7 and one-half gallons on occasion, but in thin sections it should be no more than 5 gallons. Taking these water/cement rations as our defining points of reference, trial mixes can be derived by referring to Table I, found in the article. Assuming well-graded, well-shaped aggregates, the quantities given indicate weight of sand, coarse aggregate and water to be used per sack of cement. Adjustments will have to be made in all of these mixes for the characteristics of local aggregates, expected job practices, surface moisture of the fine aggregates, desired air content, and other variables and desired concrete properties.