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Page:Popular Science Monthly Volume 61.djvu/219

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would hold up all ordinary traffic and that four inches would hold in many special cases. They adopted a thickness of six inches of Portland cement concrete as a general rule, and that rule has been followed in England and the United States with very general satisfaction.

It is imperatively necessary that this foundation should present to the material laid upon it an absolutely unyielding surface, for the reason that the bricks or blocks being jointed and the thin sheet of asphalt, while continuous, possessing very little strength in itself, either of them is bound to follow a yielding foundation with disastrous results. As before stated, an unyielding foundation is therefore, within reasonable limits, an absolute necessity; for, while the average street in our northern cities is liable to the vicissitudes of frosts, and still more liable to the vicissitudes of frequent openings, the evil effects of these vicissitudes can be for the most part avoided by constructing a concrete foundation sufficiently strong to form a bridge over any such weak spots of limited extent, and thus hold up the surface. This very obvious requirement will never be found in a concrete that is neither thick enough nor strong enough.

The action of the Commissioners of Accounts of the City of New York in insisting that only Portland cement of good quality should be used in the street foundations of Greater New York only confirmed and was confirmed by the conclusions reached by the French engineers more than a generation ago. Why such a question should have been raised at this time by engineers presumably familiar with the literature of their profession is not apparent.

In constructing this foundation the contractor is required to proceed as follows:

The concrete shall be composed of one part cement, three parts of sand and six parts of broken stone. … Unless machinery be used, concrete shall be mixed in batches, one barrel of cement with the requisite proportion of other material, on suitable tight platforms, not less than twelve feet by twelve feet in size. The cement and sand shall be thoroughly mixed dry and then made into a mortar with as little water as possible, after which the broken stone, having first been watered, shall be added. The whole mass shall then be turned and worked until a moist resultant is obtained, with the stone uniformly distributed.

It is very necessary that the concrete should be thoroughly set and dried out before the surface should be placed upon it. It is therefore advisable that a modern street should be constructed during dry and warm weather.

If the surface is to be of brick, a brick made especially for the purpose is used that is uniformly burned until vitrified. The size should be as uniform as possible, they should be hard enough to resist abrasion and of a texture that renders them impervious to water. When they