(d) In addition to the above methods of surface treatment, there are on the market a large number of special patented waterproofing compounds and processes of more or less merit, manufactured according to formulee, and sold under various proprietary trade names. These are listed and described in greater or less detail below.
It must be understood that there is no hard and fast line of demarcation between the integral and surface methods of treatment. A method that is in its nature essentially integral, involving the incorporation of special materials throughout the mass of the concrete directly affected—as, for example, in the case of "hydro lithic" cement—may, in the manner of its appli cation, have all the appearance of a purely sur face method of treatment, the waterproofed material being spread upon the surface or depos ited between successive strata in much the same way as asphalt and other bituminous products are applied.
As already explained, the relative merits of different methods of waterproofing will depend upon the severity of the requirements to be met, and upon local conditions peculiar to the indi vidual work in hand. It would be beyond the province of this general work of reference to advocate any one method as universally superior to any other, our purpose being merely to place in our readers' hands a clear and intelligent classification and description of the various at tempts that have been made to solve this great problem under the conditions of up-to-date practice—a comprehensive summary of the re sults achieved and the claims set forth.
There are some points of vital bearing on the problem of waterproofing, regarding which there is no dispute. It is universally conceded, for example, that proper proportioning of ingredi ents, proper grading of aggregates, thoroughness of mixing, and careful methods of placing con crete are factors of the utmost importance. The densest concrete that can be made with the given materials will be the most impervious to water. Also, the richer the mix, where no special com pounds are used, the more nearly waterproof will the concrete be. It is evident, therefore, for practical reasons, that, for waterproofing work where the concrete alone is depended upon, a tendency to excess of fine materials is better than the reverse. Especially in monolithic con struction, it is now generally conceded that a wet mixture, a rich concrete, and a proportioning of aggregates to give great density, are essential for securing a waterproof structure.
Concrete which was not water-tight when placed has sometimes been observed to become so after the lapse of time. Where constantly subjected to only moderate water pressure, there is a marked tendency of the seepage pas sages to become closed as the result of the opera tion of natural causes. Impermeability, like hardness itself, increases with age. Various explanations of this phenomenon have been given. Some think it due to the crystallization, near the surface, of certain materials dissolved by the water during its passage through the concrete; others attribute it to the gradual accu mulation of silt carried in suspension by the water.
Where a heavy pressure is constantly main tained against the concrete, on the other hand— and especially if the work is near salt water— these results cannot be looked for. Where the work is of such magnitude as to extend over several days for construction, seepages are likely to occur, especially along the line where one day's work joins that of another, the flow, in stead of diminishing, becoming freer with the lapse of time. In such cases a special water proofing process becomes an imperative neces sity. In any case it is advisable to make pro vision for thorough water-tightness from the start.
The amount of pressure exerted by water against walls and beneath floors is frequently underestimated, especially where the hydrostatic head is low or is prevalent for but a short time after a rainfall. Table XVITT, showing the pressure exerted on floors and walls by water under various hydrostatic heads, will assist the reader to a clear understanding of the force which waterproofing methods must be designed to withstand.
Integral Methods of Waterproofing Under this head, as before explained, come all those details that relate to the securing of maximum density in the concrete itself by the use of carefully graded and proportioned and well-compacted mixtures. In addition, how ever, this heading embraces a great variety of special products intended for direct incorpora tion in the concrete mass. The majority of these on the market are in powder form, but some are in the form of a solution.