lab to
vertical movement and discontinuous support with the result that
longitudinal and transverse cracks occur.

The foregoing indicates the importance of securing good concrete for
road surfacing, and that is accomplished by using suitable aggregates,
by proper design of the road surface and by following established
construction methods.

=Design.=--The widths usually adopted for concrete roads are: for
single track roads, 9 or 10 feet, and for double track roads, 18 or 20
feet. The thickness is 6 to 8 inches at the middle, varying with
climatic conditions and with the kind of soil upon which the concrete
is laid. The thickness at the edge is 1 inch less than at the middle
except that 6-inch surfaces are usually of uniform thickness, the
total crown being 2 inches. The thickness of the two course pavement
is the same as would be used for a single course pavement in the same
location. The surface of either width has a total crown of one or two
inches to insure water running off the surface. The earth foundation
is often flat, the crown being obtained by making the slab thicker at
the middle than at the edge. Fig. 18 shows cross section for concrete
roads.

[Illustration: Fig. 18.--Cross Section for Concrete Highway]

In the state of California, concrete roads four or five inches thick
and surfaced with a bituminous carpet mat have been successfully
constructed. Similar designs have been used in a few other places, but
for general practice it is unsafe to depend upon such a thin slab.
Climatic and soil conditions probably account for the success of the
thin roads in California.

=Concrete Materials.=--The coarse aggregate for the concrete may be
broken stone or pebbles screened from natural gravel. Durability is
necessary, but it is also important to have uniformity in the concrete
so that the road surface will wear uniformly and consequently keep
smooth. Supplies of broken stone are likely to contain a small
percentage of soft pieces and such of these as are at the surface when
the concrete is finished will crush under traffic, leaving a pit in
the surface. Pebbles screened from gravel are also likely to be
variable in durability and should be carefully inspected if they are
to be used as aggregate for concrete roads. The harder limestones,
some sandstones, pebbles from many of the gravel deposits and
practically all of the igneous rocks make satisfactory aggregates for
the concrete road.

Sometimes none of t