s of
a special tool known as a scarifier.

The new surface is placed and rolled in precisely the same manner as
the wearing surface of the original construction, but the layer may
not be as thick as the original wearing course. A new course will not
bond to the old surface unless the old macadam has been thoroughly
broken up first.

=Characteristics.=--The water-bound macadam is a dusty, somewhat rough
surface of low durability for rubber tired vehicles. It has long been
the standard rural highway for steel tired vehicles, but cannot carry
any considerable amount of motor traffic. It is easily repaired. When
finished with a bituminous surface its durability is greatly increased
and the dust is eliminated. It does not seem to be sufficiently rigid
for truck traffic, unless placed on exceptionally good foundation.

CHAPTER VIII

CEMENT CONCRETE ROADS

The cement concrete road is one of the later developments in highway
construction, but the type has had sufficient use to show that it is
one of the satisfactory types for heavy mixed traffic, and, where the
proper materials are available, it is one of the economical types of
construction.

=Destructive Agencies.=--It is well to have clearly in mind at the
outset that the concrete in a road surface is subjected to certain
destructive agencies not usually significant in connection with the
use of concrete, and these are so often disregarded that the average
serviceability of the concrete road surface is sometimes much lower
than it would be if built with due regard for the effect of traffic on
concrete surfaces. In most structural uses of concrete, its strength
in compression only is utilized, and the factor of safety is such as
to eliminate to some extent failures due to inferior materials or
workmanship.

The concrete road surface is subjected to compression under wheel
loads, to bending, causing tension in the concrete, to abrasion from
wheels, and to tension and compression due to effect of temperature.
The weight of the wheel loads may cause sufficient distortion of the
road slab to produce rupture. The aggregates may be crushed under
wheel loads if the material is too soft. Abrasion from steel tired
vehicles wears away the concrete unless it is hard and durable.
Changes in dimension due to the effect of change in temperature
introduce tension or compression into the road slab and may result in
cracks. Freezing and thawing in the subgrade subjects the s