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Concrete used in water-retaining structures, exposed to severe weather, or exposed to an aggressive environment must be virtually impermeable or watertight. Watertightness refers to the ability of concrete to hold back or retain water without visible leakage. Permeability refers to the amount of water migration through concrete when the water is under pressure, and also to the ability of concrete to resist penetration of any substance, be it a liquid, gas, or chloride ion. Today there is renewed interest in the permeability of concrete. It deals mainly with permeability to deleterious substances such as chloride ions from seawater and deicing salts, sulfate ions, and other aggressive chemicals. In 1986, Construction Technology Laboratories researchers studied the effects of mix design, materials, and curing on permeability of selected concretes. The concretes were subjected to tests that included: permeability to water and air, ponding with chloride solution, rapid chloride permeability, helium porosity, and volume of permeable voids. The results of the study confirmed that water/cement ratio strongly influences the permeability of concrete.