ranular texture and is
not slippery. It is of course rigid and noisy for steel tired
vehicles. It is an excellent automobile road and its low tractive
resistance makes it a desirable surface for horse drawn vehicles. It
possesses a high degree of durability if properly constructed. It is
likely to crack indiscriminately but as a general rule the cracks are
not a serious defect.

=Maintenance.=--The cracks that appear in the concrete surface are
filled once or twice a year, tar or asphalt being employed. The dust
and detritus is cleaned out of the cracks and the hot filler poured
in, with enough excess overflowing to protect the edges.

CHAPTER IX

VITRIFIED BRICK ROADS

Vitrified brick roads consist of a foundation course of Portland
cement concrete, broken stone or slag macadam, or of brick laid flat,
the first named being by far the most generally used, and a wearing
course of vitrified brick.

=Vitrified Brick.=--Vitrified brick are made from clay of such a
character that when heated to the required temperature they will fuse
into a glassy texture. Brick roads are constructed on roads carrying
the severest of traffic and the brick must therefore be tough and of
high resistance to wear.

Not all of the clays from which brick may be manufactured will produce
a product suitable for road construction, and paving brick, even
though truly vitrified, are of different degrees of durability,
depending upon the nature of the clay and the care exercised in the
manufacture.

Paving brick are manufactured by the stiff mud process, which means
that the clay is molded into form in a relatively dry condition. To
accomplish this, considerable pressure is exerted in forcing the
column of clay through the dies, which form the prism from which the
brick are cut. If the clay is unsuitable in character or is not
properly ground and mixed, the brick will possess planes of weakness
between the various layers of clay which have been pressed together,
and these planes, called laminations, are a source of weakness if too
marked. It is usual to specify that the brick used for road surfaces
shall be free from marked laminations.

If the brick is not properly burned it will be only partly vitrified
and therefore not of maximum durability. It is customary to specify
that the brick shall show a glassy fracture indicating complete
vitrification.

Various defects of a minor nature occasionally develop in the brick
during the suc