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The Foundation

concrete, asphalt and surface

THE FOUNDATION - Since the sheet-asphalt wearing surface has no power in itself to act as a bridge, it is essential that it be placed upon a firm unyielding foundation; and consequently it is nearly always placed upon a bed of hydraulic cement concrete. prepared as described in Art. 2. Chapter IX, page 367. For heavy city traffic, the concrete is usually six inches thick; and for light traffic, it is sometimes only 4 inches thick. The proper thickness will depend upon the weight of the traffic, the strength of the con crete, and the bearing power of the soil.

Occasionally the asphalt is laid upon a bituminous-cement con crete (563). The chief advantage claimed for the bituminous con crete is that the asphalt wearing-surface adheres more firmly to it than to the hydraulic concrete, and thus prevents weather cracks ( 654) and waves ( 655). An additional advantage of bituminous over hydraulic concrete is that less time is required for the former to set. On the other hand, the bituminous concrete is weaker and less reliable, and usually is more expensive. Further, in repairing the wearing surface, it is difficult to separate the top coat from the bituminous base so as to secure a uniform smooth surface. Bitu

minous concrete never was so common as hydraulic-cement con crete for a foundation, and has now practically been abandoned.

It is necessary that the concrete he thoroughly dry before the asphalt mixture is laid upon it, as the generation of steam caused by placing the hot material upon a damp foundation will produce blistering and possibly disintegration of the wearing coat. This is a matter that needs close attention in laying an asphalt pavement. To dry the foundation after a rain or during damp weather, fine hot sand is sometimes spread over the concrete and then swept off; but this method is expensive and not very effective, and besides there is liability that enough sand will be left upon the foundation to interfere with the adhesion of the asphalt. Often an old pavement of broken stone, cobble stones, granite blocks, or brick is used for a foundation for an asphalt pave ment. All of these give fairly good results, but it is important that the surface of each shall be perfectly clean and dry when the asphalt wearing coat is laid.