CHAPTER XXV.
METHODS AND COST OF WATERPROOFING CONCRETE STRUCTURES.
Resistance to penetration by water is desirable in all concrete structures, and is essential in such structures as tanks, reservoirs, vaults, subways, basements and roofs. Concrete, as it is ordinarily made, is pervious to water, hence to secure concrete structures through which water will not penetrate some method of waterproofing the concrete must be employed. Many methods have been proposed and are being used; none of these methods is without faults, the best one of them has not yet been determined, and the evidence available as to their comparative merits is biased and conflicting. For these reasons any discussion of waterproofing for concrete is at the present time bound to be unsatisfactory.
Methods of waterproofing may be roughly classified as follows: (1) Use of mixtures so proportioned as to be impervious; (2) admixture of substances designed to produce impermeability; (3) use of waterproof coatings, washes or diaphragms. In succeeding sections enough examples of each method are given to indicate current practice; no attempt has been made to catalog all the waterproofing substances and systems being promoted—there are too many of them.
The art of waterproofing concrete is in a transition stage. Outside of the manufacturers of waterproofing material the art has received serious study by comparatively few persons. No comparative tests by independent investigators are available. Practical experience with most of the materials used has not extended over a long enough period of time to permit true conclusions to be drawn. Students of the subject are not even agreed upon the broad questions whether it is better to work toward developing an impervious concrete or toward perfecting a waterproof covering for concrete. On the minor subdivisions there is no agreement at all.
In the present state of the art one can lay fast hold to only three things. The first is that waterproofing is one component of a system of drainage; the second is that structures must, to get the best results, be designed with the fact in mind that waterproofing is a component structural element, and the third is that skilled and conscientious workmanship are essential elements in the success of all waterproofing materials and methods.
IMPERVIOUS CONCRETE MIXTURES.—The compounding of the regular concrete materials so as to produce an impervious concrete has been made the subject of numerous experiments. The most elaborate of these experiments were those conducted over a period of five years by Mr. Feret, of the Boulogne (France) Laboratory of the Ponts et Chaussees. Feret's experiments led him to the following conclusions:
"That in all mortars of granulometric composition the most permeable are those which contain the least quantity of cement.
"Of all mortars of the same richness, but of varying granulometric composition, those which contain very few fine grains are much more permeable. They are the more so where, with equal proportions of the fine grains, the coarse grains predominate more in relation to the grains of medium size.