In water-bound macadam the stones are held in place by a weak cement composed of stone dust and water, and this cement is not sufficiently strong to hold the stones in place when they are subjected to the shear of automobile tires. In finishing the water-bound macadam surface, the spaces between the stones are filled with screening and in addition a layer about one-fourth inch thick is left on the surface. The automobile traffic first brushes aside all of the screenings and smaller particles of rock, exposing the larger stones. These gradually loosen as the road is used and are brushed aside. When this effect begins, the road is said to be raveling. Various lengths of time may elapse from the time the road is first finished until raveling begins, depending upon the character of the stone, the weather and the amount of motor traffic. During the period before raveling starts, it is comparatively easy to restore the road surface at any time by the addition of screenings or clay and sand. Usually there will be a few small areas of the surface that, on account of faulty construction, will ravel or become rutted much earlier than the remainder of the surface. These can be repaired by the methods described in the chapter on "Water-bound Macadam Construction." When the surface begins to ravel seriously, maintenance becomes much more difficult and in order to prevent raveling and the difficulties of maintenance thereafter, the macadam surface is often coated with a bituminous material. [Illustration: Fig. 20.--Oiling a Gravel Road] If there is any dust or screenings on the road surface, the bituminous material will not adhere to the stones and will soon flake off under traffic. The surface of the macadam must therefore be thoroughly cleaned before the bituminous material is applied. The usual practice is to finish the road as water-bound macadam, and permit traffic on it for a sufficient length of time to show any weak places in the surface and at the same time thoroughly to season the surface. If any defective places appear, they are repaired and when the surface exhibits satisfactory stability, but before it begins to ravel, the bituminous surface is applied. There will ordinarily be some stone dust and some screenings remaining on the surface at the time bituminous treatment is undertaken, and there may also be some caked mud or other foreign material. All of this must be removed so as to expose the stones throughout. =Applying the Bituminous Binder.=--The bituminous binder may be delivered in tank cars, which is desirable if the work is near a railroad siding, or ample tank wagon service is available for long hauls so that the tank will not be held up too long. Often it is desirable to purchase the binder in barrels and haul these to the site of the work in advance of beginning the construction of the surface.