f the total
fine aggregate (material under one-fourth inch in size) to be
retained on a two hundred-mesh sieve."

"_Screened Gravel and Sand Mixtures_.--Where it is impossible to
obtain run of bank gravel containing the necessary binder in its
natural state, screened gravel shall be used and the necessary
sand and clay binder added as directed by the engineer. Gravel
and sand shall be delivered on the work separately. Clay binder
shall be obtained from approved pits and added as directed by the
engineer."

"_Run of Bank Gravel_.--When run of bank gravel is permitted
either for one course or two course work, the size shall not
exceed that specified for bottom or top course. If necessary, the
contractor shall pass all the material through a two-inch screen
for the bottom course, and through a one-inch screen for the top
course. When the work consists of only one course, the material
shall be of the sizes as specified for the top course. The
necessary binder shall be contained in the material in its
natural state, excepting that a small percentage of clay binder
may be added as directed by the Engineer."

=Wearing Properties.=--A certain amount of grinding action takes place
on the road surface under the direct action of wheels, especially
those with steel tires. Where rubber tired traffic predominates, this
action is much less severe than where steel tired vehicles
predominate, but the tendency exists on all roads. In addition, there
is distortion of the layer of gravel under heavy loads which causes
the pieces of stone in the surface to rub against each other and to
wear away slowly.

The gravel road in the very best condition is slightly uneven but
there is comparatively little jar imparted to vehicles, and,
consequently, little impact on the surface. When somewhat worn, the
impact becomes a factor of some importance and the pounding of
vehicles has a very destructive action on the surface. Soft pebbles
will be reduced to dust in a comparatively short time.

The degree to which any gravel resists the destructive action of
traffic depends upon the varieties of rock represented by the pebbles
in the gravel. If the pebbles are mostly from rocks of good wearing
properties, that quality will be imparted to the road surface. If
mostly from rocks of little durability, the same characteristic will
be imparted to the road surfac