ed. The surface is
divided into sections by earthen dikes and the space inside the dikes
filled with water to a depth of two or three inches. The water
covering is maintained for two weeks or longer.

No traffic is permitted on the surface for one month, and in cold
weather traffic may be kept off the surface for a longer period.

=Expansion Joints.=--To permit the concrete slab to accommodate itself
to changes in dimension due to temperature changes, expansion joints
1/2 inch wide are placed about every thirty feet. These consist of a
sheet of some prepared bituminous material placed in position as the
concrete is poured.

Experience seems to indicate that in spite of the expansion joints,
the concrete will crack more or less and many engineers think it
advisable to omit expansion joints in constructing the pavement and
when cracks develop to pour bituminous material into them, thus
forming expansion joints.

The prevailing practice in rural highway construction is to omit the
expansion joints, but they are commonly adopted in city pavements.

=Reinforcing.=--To minimize the cracking, either bar or wire mesh
reinforcing is used in the concrete. If bars are used they are placed
in the concrete as it is poured so as to form a belt around each
section about 15 feet square. If the mesh type is employed, a part of
the layer of concrete is placed and smoothed off and a strip of the
mesh laid in place. Additional concrete is then poured on top of the
mesh to bring the slab to the required thickness.

=Bituminous Coatings on Concrete Surfaces.=--The concrete road surface
is sometimes coated with a layer of bituminous material and stone
chips or gravel pebbles. This is particularly advisable where no
really satisfactory aggregates are available and the concrete surface
would not possess sufficient durability. The bituminous material is
applied hot to the surface and is then covered with stone chips or
gravel pebbles, ranging in size from 3/4 inch down to 1/4 inch, the
resulting coating being about 3/4 inch thick. Many failures of this
type of surface have been recorded due to the difficulty of securing
adhesion to the concrete. This seems to be due in part to inability to
get the proper bituminous materials and in part to climatic effects.
Considerable progress has been made in developing this type of surface
and it may eventually become a satisfactory maintenance method.

=Characteristics.=--The concrete road is of a g