izes, the smaller being 1-7/8 inch in diameter when new, and the
larger 3-3/4 inches in diameter when new. Ten of the larger spheres
are used and the balance of the charge is made up of the small size.

When tested in the standard manner the loss allowable for the several
classes of service are as follows:

------------+---------------+----------------
| | Maximum Loss
Traffic | Average Loss | for any Brick
------------+---------------+----------------
Heavy | 20 per cent | 24 per cent
Medium | 22 per cent | 26 per cent
Light | 25 per cent | 28 per cent
------------+---------------+----------------

=Other Tests.=--Sometimes the absorption test is specified for paving
brick, but it is rarely a vitrified brick that will pass the rattler
tests which fails to pass a reasonable absorption test. Absorption of
water in an amount exceeding 4 per cent indicates incomplete
vitrification and failure of such brick is almost certain during the
rattler tests.

The cross breaking test is also sometimes employed, but generally
only to check the general quality of the brick. Failure in service
more frequently occurs from excessive wear than from any other cause
and the cross breaking test has little significance, except for brick
less than 3 inches thick, which are to be laid on a sand bedding
course.

=Foundation.=--The foundation for brick roads is usually of Portland
cement concrete, the thickness varying with the nature of the traffic
and the kind of soil upon which the pavement is built. For well
drained soils and normal highway traffic, 5 inches is the ordinary
thickness of foundation. Under favorable conditions such as locations
with sandy soils or in semi-arid or arid regions where the soil is
always stable, the foundation may be four inches thick, and a
considerable mileage of brick road has been built with concrete
foundations less than four inches thick.

In other locations the soil and traffic conditions require a base six
inches or more in thickness, and the proper thickness can be
determined only after all of the factors involved are known and have
been analyzed. It is impractical to adopt a standard thickness of
foundation that will be equally economical for all locations and all
kinds of traffic. As the brick pavement is essentially a heavy traffic
type of surface, the design cannot be varied greatly with similar
foundation cond