Page:Popular Science Monthly Volume 61.djvu/218

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MODERN street is laid on a concrete foundation, the surface of which may consist of brick, asphalt block or sheet asphalt.

It is of the greatest importance that the foundation should be properly constructed. It is true that in cities where a large area of Belgian block pavement has been already laid, sheet asphalt is often laid upon these blocks; but while streets can be made in this way, they are, when so constructed, more or less liable to criticism in several respects; but chiefly from the fact that the Belgian block, as compared with concrete, is an unstable foundation, liable to yield under unusual or excessive strain and always sure to carry the surface with it. While this cause of the criticism is usually absent, especially upon residence streets, there have been conspicuous examples of its presence in almost every large city, where the custom of laying asphalt surfaces on Belgian block prevails. We therefore consider the normal modern street as laid on a concrete foundation.

The French engineers determined the elements of this problem, nearly half a century ago, by very costly experiments for which the city of Paris was mainly responsible. As all the roadways of France are more or less completely under governmental supervision, the results of these experiments have been described in several masterly memoirs by some very able French engineers, in the Annales des Ponts et Chaussées, which is the official organ of the French governmental supervision of bridges and roads. Space forbids mention here of more than a summary of these results. It was found that of all the materials that were available, only a concrete made of Portland cement of good quality mixed with clean sharp sand and broken stones was always to be depended upon. If a concrete foundation could always be laid on well drained earth properly rolled or rammed, that was never invaded by frost and never disturbed by openings for all the multitudinous purposes that disturb modern streets, concrete that will not stand such rigid tests might be used instead of that made of Portland cement. But, streets laid under such conditions and subjected to such use are an exception, indeed they are rare, and for that reason should not be made the rule. The French engineers determined that under any circumstances, nine inches of concrete would hold any traffic; that six inches