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.

=Placing the Surface.=--The stone, sand and asphalt cement are heated
to the required temperature and combined in the proper proportions and
are then thoroughly mixed by a mechanical mixer. The mixture is hauled
directly to the road and is dumped and spread by means of rakes. It is
then rolled thoroughly while still hot, a three-wheeled roller being
most satisfactory. After rolling, a seal coat of hot asphalt cement is
spread over the surface and covered with hot stone chips about 1/4
inch in size. The surface can be opened to traffic immediately after
the surface has been completed.

=Characteristics.=--The asphaltic concrete surface is of excellent
riding properties, is easily repaired and of moderate durability. It
is a particularly desirable surface for pleasure automobile riding and
for horse drawn traffic.

CHAPTER XI

MAINTENANCE OF HIGHWAYS

Proper maintenance of highways is equally important with proper
construction. With nearly all types of road construction, the need for
maintenance arises soon after the surface is placed under traffic and
is continuous thereafter. The nature and amount of maintenance work
varies greatly among the several types of surface and the organization
suitable for a system of highways will depend to a considerable extent
upon the kinds of surfaces that are to be maintained.

The upkeep of a road may be conveniently considered as of two kinds,
viz., (1) that which has to do with the wearing surface and earth
shoulders or berms upon which there is some traffic and (2) that which
has to do with the side ditches and drainage structures and keeping
the roadside in presentable condition. Both kinds of work are usually
carried