WARREN'S METHOD. Upon the subsoil is placed a 4-inch layer of broken stone which is thoroughly rolled. On this stone foundation is spread a coat of thin asphaltic cement, which enters the interstices of the stone holding its fragments together and forming a surface with which the wearing coat will readily and firmly unite. The asphalt macadam consists of a mixture of as phaltic cement and broken stone, the fragments of the latter vary ing from 1 to 2 inches in the largest dimensions to fine dust. The ingredients of the asphaltic macadam are mixed about as described for the wearing coat of the ordinary asphalt pavement (§ 627). The mixture of asphaltic cement and stone is spread, while still hot, of such a thickness as to be 2 inches after being thoroughly rolled with a road roller (§ 336) weighing 15 to 20 tons. On top of the asphalt macadam is spread a layer of asphaltic cement, partly to seal the surface against the entrance of air and water, and partly to bind together the fragments forming the wearing surface. While the surface of the asphaltic cement is still sticky there is spread over it a thick coat of fine stone chips, which are then rolled and the road is ready for traffic.
The finished roadway presents a rough gritty surface, which has more of the characteristics of an ordinary broken-stone road than of the usual asphalt pavement. Less asphaltic cement is required for a given thickness of asphalt concrete than for the asphalt mortar of the wearing coat of the ordinary asphalt pavement, since the larger the fragments of the aggregate the less the per cent of voids, and consequently the less cement required. It is claimed that no single stone has been dislodged in any of the seven cities in which experimental sections have been built. It is also claimed that asphalt macadam is superior to ordinary asphalt pavement, since the angular fragments of the broken stone used in the former are less mobile than the rounded sand grains used in the latter, and hence the cement in the former may be made softer and may also be worked at a lower tempera ture than in the latter. The softer the asphaltic cement, the more durable it is; and the lower the temperature at which it is worked, the less the danger of damage by overheating it.