Seal Coat.—After the rolling has been completed, the surface is covered with hot bituminous cement and dressed with pea gravel or stone chips and again rolled. Traffic may be permitted in twenty-four hours.
Characteristics.—The mixed macadam is a somewhat resilient surface of excellent riding qualities and considerable durability for medium traffic. It is likely to creep and become uneven when subjected to heavy loads. The seal coat will wear off in two or three years and will require replacing.
IV. Asphaltic Concrete
Asphaltic concrete is a name given to a road surface mixture which is composed of graded stone, graded sand and asphalt cement. This type is designated as asphaltic concrete because of the analogy of the mixture to Portland cement concrete.
Asphaltic concrete is of two general types known as bitulithic, or Warrenite, and Topeka asphaltic concrete, respectively, the differences being in the nature of the mixture.
Bitulithic or Warrenite.—The stone employed for these types is graded down from a size about equal to one-half of the thickness of the wearing course, and stone passing a 1¼ or 1½-inch screen is usually specified. From the maximum size the stone is graded down to the finest particles produced by the crusher. The range of sizes of stone will vary with the source of the supply, and in order to secure the desired density in the mixture, varying amounts of graded sand and mineral dust, such as ground limestone or Portland cement, are added to the broken stone. Usually the resulting mixture contains less than fifteen per cent of voids, and to this carefully graded mineral aggregate there is added enough asphalt cement to bind together the particles.
Topeka Asphaltic Concrete.—In this type of asphaltic concrete, the mineral aggregate consists of a mixture of carefully graded sand and of broken stone of such size that all will pass a one-half-inch screen and graded down to the fine dust produced by the crusher. To this mixture is added about nine per cent of Portland cement or limestone dust. The voids in the mixture are usually about twenty-five per cent.
It will be seen that the essential differences between the Bitulithic and Topeka types are these: the Topeka type contains a larger percentage of voids and stone of a smaller maximum size than the Bitulithic. Both types have been extensively employed for city paving, but the Bitulithic and Warrenite types have also been used to some extent for rural highways. The Topeka type has been used but little for rural highways.
Foundation.—The foundation for the asphaltic concrete may be an old macadam road, a base course constructed of broken stone or Portland cement concrete, the latter being used much more extensively than either of the other types.
Sometimes asphaltic concrete is used for resurfacing water-bound macadam or gravel roads when the traffic has increased to the point where the cost of maintenance of the water-bound macadam has become excessive. The existing surface is repaired and the cross section is restored, or possibly flattened somewhat.