METHODS OF WATERPROOFING The various methods devised for the water proofing of concrete may be classified under two broad divisions as follows: 1. Integral methods, in which the mass of the con crete itself is waterproofed prior to setting.
2. Methods of external or surface treatment, in which a protective, impervious covering is applied to the sur face of concrete or placed between successive strata dur ing the erection of the structure.
Both of these methods may be—and in many instances are—effectively combined in the same construction.
Each of the above classes into which water proofing methods are divided may be further subdivided into a number of special methods dif fering from one another in marked characteris tics.
Integral methods, for example, comprise three great subdivisions as follows: (a) The use of carefully graded and propor tioned and thoroughly compacted mixtures to give maximum density to the concrete.
(b) The introduction of special waterproof ing materials into the mixture; these may be in the form of a powder, a paste, or a liquid.
(c) The special treatment of Portland ce ment itself during the process of manufacture, so that as it comes from the mills it already con tains all the waterproofing elements required, with appropriate aggregates, to give an imper vious mixture.
methods of waterproofing may be subdivided according to the materials used: (a) One method that has been very widely adopted and has been found of great practical efficiency, even under conditions of great sever ity, consists in the use of asphalt or of asphalt or tar-product mixtures, either used alone or in combination with felt or similar waterproofing fabrics. This method will be more fully described later.
(b) A method that has been proved ef fective where the requirements are not very severe consists in plastering the wall—immedi ately after the removal of the forms, if possible a coat of very rich cement and sand mor tar. This coating is usually from one-half to three-quarters of an inch thick, and lime paste is sometimes added for smoothness in working. It is essential that the coating be well troweled and smoothed off, preferably with a wooden float, as the hard skin developed by such treat ment is practically waterproof. A neat cement
wash is sometimes used, applied with a brush. On horizontal or inclined surfaces, the surface of the concrete should be worked and troweled, giving the effect of a cement coating.
(c) Another method that has given satis factory results in preventing water penetration and absorption under conditions of moderate pressure, consists in the application of washes that depend, for their effect, upon chemical action. Perhaps the most widely used example of this method is what is known as the Sylvester process of waterproofing, which consists in the alternate application of hot solutions of castile soap and alum. The application is preferably made while the concrete is still "green;" but the process may be used for the water proofing of old walls either of cement or lime plaster, stone, brick, or other more or less porous material. The walls in every case should be clean and as dry as possible, and the temperature of the air not lower than 50° F. The soap is first dissolved in water—three-quarters of a pound of soap to a gallon of water—and, while boiling hot, is spread over the surface with a flat brush. Twenty-four hours later, the alum solution—one pound of alum to eight gallons of water—should be applied in the same manner, at a temperature of about 65° F. The process is repeated every twenty-four hours, four coats being usually suf ficient to effect the desired result. From the chemical combination of the soap and alum, an insoluble compound is formed which effectively fills the pores of the structure to an appreciable depth and checks the penetration of moisture.
A process similar to the above in its action has been used for making mortar non-absorbent. Powdered alum (one per cent by weight) is thor oughly mixed with the dry cement and sand; and about one per cent of any potash soap (such as ordinary soft soap) is dissolved in the water used in mixing the mortar. A wash consisting of five pounds of alum dissolved in two gallons of water, to which one pound of concentrated lye has been added, has been successfully used on green concrete surfaces. Boiled linseed oil, applied in successive coats until it ceases to be absorbed, has also been used effectively.