Water-tight concrete, or concrete made water-tight by some kind of waterproof coating, is frequently required, either for inclosing a space which must he kept dry, or for storing water or other liquids. Concrete, even when most carefully prepared from materials of the highest grade, is never of itself completely waterproof.

It is generally considered that in monolithic construction, a wet mixture, a rich concrete, and an aggregate proportioned for great density, are essential for water-tightness. With the wet mixtures of concrete now generally used in engineering work, concrete possesses far greater density, and is correspondingly less porous, than with the older, dryer mixtures. At the same time, in the large masses of actual work, it is difficult to produce concrete of such close texture as to prevent undesirable seepage at all points. Many efforts have been made to secure water-tightness of concrete in a practical manner - some with success, but others with unsatisfactory results. There are now a great many special preparations being advertised for making concrete water-tight.

It has frequently been observed that when concrete was green, there was a considerable seepage through it, and that in a short time absolutely all seepage stopped. Some experiments have been made to render porous concrete impermeable, by forcing water through a rich concrete under pressure. In. these experiments, a mixture of 1 part Portland cement to 4 parts crushed gravel was used. The concrete tested was 6 inches thick. The flow through the concrete on the first day of the experiment, under a pressure of 36 pounds per square inch, was taken as 100 per cent. On the forty-sixth day, under a pressure of 48 pounds per square inch, the flow amounted to only 0.7 per cent.

While the pressure was constant, the rate of seepage of the water decreased with the lapse of time, showing a marked tendency of the seepage passages to become closed. The experimenter is of the opinion that the water, under pressure, dissolves some of the material and then deposits it in stalactitic form near the exterior surface of the concrete, where the water escapes under much reduced pressure. Others, however, think it quite possible that fine material carried in suspension by the water aids in producing the result.

For cistern work, two coats of Portland cement grout - 1 part cement, 1 part sand - applied on the inside, have been found sufficient. About one inch of rich mortar has usually been found effective under high pressure. A coating of asphalt, or of asphalt with tarred or asbestos felt, laid in alternate layers between layers of concrete, has been used successfully. Coal-tar pitch and tarred felt, laid in alternate layers, have been used extensively and successfully in New York City for waterproofing.

Mortar may be made practically non-absorbent by the addition of alum and potash soap. One per cent by weight of powdered alum is added to the dry cement and sand, and thoroughly mixed; and about one per cent of any potash soap (ordinary soft soap) is dissolved in the water used in the mortar. A solution consisting of 1 pound of concentrated lye, 5 pounds of alum, and 2 gallons of water, applied while the concrete is green and until it lathers freely, has been successfully used. Coating the surface with boiled linseed oil until the oil ceases to be absorbed, is another method that has been used with success.

A reinforced concrete water-tank, 10 feet inside diameter and 43 feet high, designed and constructed by W. B. Fuller at Little Falls, N. J., has some remarkable features. It is 15 inches thick at the bottom and 10 inches thick at the top. The tank was built in eight hours, and is a perfect monolith, all concrete being dropped from the top, or 43 feet at the beginning of the work. The concrete was mixed very wet, the mixture being 1 part cement, 3 parts sand, and 7 parts broken stone. No plastering or waterproofing of any kind was used, but the tank was found to be absolutely water-tight although the mixture used has not generally been found or considered water-tight.

At Attleboro, Mass., a large reinforced concrete standpipe, 50 feet in diameter, 106 feet high from the inside of the bottom to the top of the cornice, and with a capacity of 1,500,000 gallons, has been constructed, and is in the service of the water works of that city. The walls of the standpipe are 18 inches thick at the bottom, and 8 inches thick at the top. A mixture of 1 part cement, 2 parts sand, and 4 parts broken stone, the stone varying from 1/4 inch to 1 1/2 inches, was used. The forms were constructed, and the concrete placed, in sections of 7 feet. When the walls of the tank had been completed, there was some leakage at the bottom with a head of water of 100 feet. The inside walls were then thoroughly cleaned and picked and four coats of plaster applied. The first coat contained 2 per cent of lime to 1 part of cement and 1 part of sand; the remaining three coats were composed of 1 part sand to 1 part cement. Each coat was floated until a hard, dense surface was produced; then it was scratched to receive the succeeding coat.

On filling the standpipe after the four coats of plaster had been applied, the standpipe was found to be not absolutely water-tight. The water was drawn out; and four coats of a solution of castile soap, and one of alum, were applied alternately; and, under a 100-foot head, only a few leaks then appeared. Practically no leakage occurred at the joints; but in several instances a mixture somewhat wetter than usual was used, with the result that the spading and ramming served to drive the stone to the bottom of the batch being placed, and, as a consequence, in these places porous spots occurred. The joints were obtained by inserting beveled tonguing pieces, and by thoroughly washing the joint and covering it with a layer of thin grout before placing additional concrete.

In the construction of the filter plant at Lancaster, Pa., in 1905, a pure-water basin and several circular tanks were constructed of reinforced concrete. The pure-water basin is 100 feet wide by 200 feet long and 14 feet deep, with buttresses spaced 12 feet 6 inches center to center. The walls at the bottom are 15 inches thick, and 12 inches thick at the top. Four circular tanks are 50 feet in diameter and 10 feet high, and eight tanks are 10 feet in diameter and 10 feet high. The walls are 10 inches thick at the bottom, and 6 inches at the top. A wet mixture of 1 part cement, 3 parts sand, and 5 parts stone, was used. No waterproofing material was used, in the construction of the tanks; and when tested, two of them were found to be water-tight, and the other two had a few leaks where wires which had been used to hold the forms together had pulled out when the forms were taken down. These holes were stopped up and no furthur trouble was experienced. In constructing the floor of the pure-water basin, a thin layer of asphalt was used, as shown in Fig. 10; but no waterproofing material was used in the walls, and both were found to be water-tight.