. The surface is then rolled
either with a tandem or a three-wheeled roller until thoroughly
compressed.

=Seal Coat.=--After the rolling has been completed, the surface is
covered with hot bituminous cement and dressed with pea gravel or
stone chips and again rolled. Traffic may be permitted in twenty-four
hours.

=Characteristics.=--The mixed macadam is a somewhat resilient surface
of excellent riding qualities and considerable durability for medium
traffic. It is likely to creep and become uneven when subjected to
heavy loads. The seal coat will wear off in two or three years and
will require replacing.

IV. Asphaltic Concrete

Asphaltic concrete is a name given to a road surface mixture which is
composed of graded stone, graded sand and asphalt cement. This type is
designated as asphaltic concrete because of the analogy of the mixture
to Portland cement concrete.

Asphaltic concrete is of two general types known as bitulithic, or
Warrenite, and Topeka asphaltic concrete, respectively, the
differences being in the nature of the mixture.

=Bitulithic or Warrenite.=--The stone employed for these types is
graded down from a size about equal to one-half of the thickness of
the wearing course, and stone passing a 1-1/4 or 1-1/2-inch screen is
usually specified. From the maximum size the stone is graded down to
the finest particles produced by the crusher. The range of sizes of
stone will vary with the source of the supply, and in order to secure
the desired density in the mixture, varying amounts of graded sand and
mineral dust, such as ground limestone or Portland cement, are added
to the broken stone. Usually the resulting mixture contains less than
fifteen per cent of voids, and to this carefully graded mineral
aggregate there is added enough asphalt cement to bind together the
particles.

=Topeka Asphaltic Concrete.=--In this type of asphaltic concrete, the
mineral aggregate consists of a mixture of carefully graded sand and
of broken stone of such size that all will pass a one-half-inch screen
and graded down to the fine dust produced by the crusher. To this
mixture is added about nine per cent of Portland cement or limestone
dust. The voids in the mixture are usually about twenty-five per
cent.

It will be seen that the essential differences between the Bitulithic
and Topeka types are these: the Topeka type contains a larger
percentage of voids and stone of a smaller maximum size than the
Bitulithic.