itions because the weight of individual loads is the
significant factor and this does not vary so much as the volume of
traffic. A variation in volume of traffic may be compensated for by a
variation in the quality of the brick as already set forth.

The mixtures for the concrete foundation vary widely because of the
variation in the aggregates employed. If the fine and coarse aggregate
for the concrete are of good quality a mixture of one part cement, two
and one-half parts sand and five parts of coarse aggregate would
insure concrete of adequate strength. A somewhat leaner mixture is
sometimes employed and would be satisfactory if the aggregates were of
exceptional concrete making quality. Mixtures of sand and pebbles
(unscreened gravel) may also be used if care is exercised to secure a
mixture of adequate strength. The proportion will of necessity vary
with each particular material and the discussion of the various
considerations involved may be obtained from various standard works on
concrete and concrete materials.

Broken stone macadam is sometimes utilized for the foundation course
of the brick pavement and such foundations are constructed as
water-bound, which is described in a previous chapter. The thickness,
like that of the concrete foundation, varies with the soil conditions
and the weight of the loads that are expected to use the road. The
macadam is placed in a single layer and is rolled and bonded with
screenings as described in the chapter dealing with water-bound
macadam. Six inches is a common thickness for the macadam base. This
type of foundation should be employed only where the soil is quite
stable and where material costs are such as to insure that the macadam
base is materially cheaper than one of concrete. This would usually be
in locations where the cost of cement is high because of long hauls
and where suitable macadam materials may be obtained close at hand.

Old macadam roads are sometimes utilized for the foundation for the
brick surface, but the instances where this is permissible are
comparatively few in number. When an old macadam is to be used it is
reshaped to the proper cross section and re-rolled and bonded so as to
afford a stable foundation of the proper cross slope.

BEDDING COURSE FOR BRICK SURFACES

In order to equalize the variations in size and shape of the brick,
they are laid on a bedding course composed of material into which the
brick may be forced by rolling. In th