In the work of the Massachusetts Highway Commis sion: "All broken stone used is ,separated into three sizes by passing it through a screen with meshes - inch, z inches, and 2i inches in diameter. The largest size is placed at the bottom and is covered with the succes sive smaller sizes, The different sizes of stone are spread in courses. The sub-grade and each course of stone are rolled thoroughly, and the top course is watered before rolling. " It was the practice of McAdam to require that all the stone used upon his roads should be as nearly as possible of a uniform size, and that no foreign sub stance be mixed with it. In more recent practice it has been found advantageous to use a certain amount of finer material to fill the interstices between the stones, and thus aid in the compacting of the road as well as render it less pervious. Some engineers place a thin layer of binding material upon the surface of the road and work it into the surface voids, while . others distribute the binding material through the entire mass of stone composing the road.
It is agreed that an impervious surface cannot be formed of blocks of hard broken stone without the ad dition of some small material to fill the voids. It has also been found that when the rock is hard, such as is needed for good wear in a road surface, it will compact with difficulty, and that a certain amount of binding material is necessary in order that the road may be brought to a surface.
Binding material may consist of the screenings from the broken stone used in the road, of sand or small gravel, or of loam. Loam has been used in some instances with satisfactory results, but the wisdom of its use is questioned by most road builders. When
sand is used, it should contain a considerable admix ture of fine particles, and will usually be improved by the addition of some fine screenings. The presence of considerable dust in a finely divided condition is important in securing the binding together of the road surface.
In cases where local stone is being used for the lower courses of a road to be surfaced with trap or other more durable rock, or where a flint gravel surface is used upon a soft limestone base, the screenings from the stone used in the lower layer may often advan tageously be used in binding the surface, the whole run of the crusher except the screenings through about a one-half inch screen being used in the lower course of the road.
In some instances the binding material is mixed with the surface stone before placing upon the road. The following extract from the specifications of Mr. James Owen for roads in Essex County, New Jersey, shows this practice: "When the two courses are rolled to the satisfaction of the Engineer and Supervisor, a coat of fifty (5o) per cent of three-quarters (I) inch stone and fifty (so) per cent of screenings properly mixed is to be spread of sufficient thickness to make a smooth and uniform surface to the road; then again rolled until the road becomes thoroughly consolidated, hard and smooth. " This specification is remarkable for the large quantity of screenings used, and needs great care in securing a proper mixture of the two materials.