e than the mixed macadam, and the asphaltic concrete or
sheet asphalt type of surface is employed. It is necessary to finish
the surface of the concrete base with some device that will leave the
surface rough to prevent the macadam from creeping. A knobbed tamper
which leaves numerous irregular depressions about 2 inches in diameter
and three-fourths inch deep is often employed.

=Sizes of Stone.=--For the wearing surface, stone ranging in size from
2 inches down to one-fourth inch is usually employed. If the stone is
of good quality the maximum size may be but 1-1/2 inches, but soft or
even medium stone of that size are likely to crush under traffic. The
stone for the base course should preferably be from 3 inches down, but
any available size will be satisfactory if the layer is well rolled
and bonded. The base course is constructed in the same manner as
water-bound macadam and any material satisfactory for the base course
of macadam will serve for the base course of mixed macadam. Screenings
having good bonding properties will also be required for the base
course.

=Mixing and Wearing Surface.=--Several methods are employed in mixing
the wearing surface. The simplest is to mix by hand with shovels. The
aggregates are heated in improvised heaters which may consist of
nothing more than a metal pipe two or three feet in diameter, around
which the stone is piled. The mixing platform is usually a metal plate
sometimes arranged so that it can be heated by means of a fire
underneath. The bituminous material is heated in kettles. For some
mixtures, the stone is not heated, but the bituminous material is
always heated. The batch of stone is placed on the mixing platform,
the bituminous material added and the materials mixed by hand.

Machine mixing is practiced much more extensively than hand mixing,
being both more rapid and cheaper. The mixer is similar to a concrete
mixer except that the drum is arranged so that it can be heated. The
hot stone and the bituminous binder are put into the drum and mixed
for the requisite length of time. Sometimes the stone is mixed cold,
the bituminous material only being heated.

=Placing the Wearing Surface.=--The hot mixture is carted to the road
and spread to such thickness that after rolling the wearing surface
will be not less than two inches thick. The hot mixture is dumped and
then spread by means of shovels to the approximate thickness and the
spreading completed by means of rakes