th 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 th