The Abbeville press and banner. [volume] (Abbeville, S.C.) 1869-1924, 1899-01-25
CLAY AS ROAD METAL
MOT DESIRABLE EXCEPT IN SMALL
QUANTITIES.

Acts rs . Cement to Knit the Stone To-
gether When Properly Used-Action Of
Water on the Roadbed - Prevents Dust
and wear-Trees and Hedges.
The part played by water on crushed
stone while undergoing rolling is sew
eral fold. In practice broken stone COM
pletely wet down is thoroughly rolled
with a roller weighing ten or more
tons. The wet surface of the angular
rock fragments permits more thorough
compacting, as the water acts as a lu-
bricant, allowing the stones to slip by
one another with greater freedom than
would be the case were the fragments
dry. At the same time the water re-
tains the powdered rock resulting from
abrasion Of the particles and holds it
between the fragments.
This process is reactionary and cumu-
lative, for the presence Of the powder
of attrition acts through capillarity to
take up and retain still greater quan-
titles Of water until the spaces between
the pieces Of broken stone composing
the upper part Of the road become COM
pletely filled with powdered rock. Not
q little of the cementing or bonding OF
a road during rolling is in reality the
effect of capillarity existing between the
grains of powdered rock and the ad-
joining walls.
This principle may be observed on
our seashores and sand roads. While
wet, the beach or road may be firm and
unyielding, allowing heavily loaded
teams to pass over them, but when dry
such places are impossible for heavy
teams and difficult of passage for all
kinds of vehicles. In this case cement
ing, as ordinarily understood, plays no
part in producing adhesion between
the grains. Upon drying, the grains are
entirely free to move over one another,
having lost the water which served to
bind them togetheA
When macadam road is thoroughly
compacted, a careful inspection will
show that the fragments Of broken stone
are closely packed together and the
spaces between are filed with a fine
powdered rock, which, if derived from
q suitable road material, carries small
percentage of clay. Any Of our com.
monty used road stones contains an ap-
preciable quantity Of clay disseminated
in little PARTICLES in the feldspar whence
1t has been derived by the weathering
Of the rock prior to it's removal from
the quarry. '
The microscope shows that the field
spar of our. traps and GRANTEES and other
road stones is never entirely free from
considerable amount Of kaolinization
or alteration to clay. This change has
taken place in the crust Of the earth to
a great depth, and no road material can
contain feldspars free from more or less
of this mineral depending upon the
character of the rock and the amount
of weathering to which it has been sub
jeoted. By the gradual wearing OF the
feldspar, by the abrasive action of the
roller, the wheels Of carriages, and the
feet of animals, small but important
quantity Of clay is liberated, and this
in combination with any clayey ma
terial that may have been added to the
road serves to furnish the necessary
quantity of cement to knit the broken
stone firmly together. 1t is not to be
understood that an appreoiable quantity
of CLAY acts otherwise than objection
ably when used in roadbuilding. As
rule nothing is more undesirable than
the clay element in highway construo-
tion, yet a little mixed with some OF
our southern limestones serves to in.
crease their oementing and enduring
qualities. Upon drying, the powdered
rock sets much after the manner Of
sandy clay and serves not only to bind
the pieces Of rock together, thus giving
rigidity to the way, but to retard the
wear due to any differential motion OF
the fragments over one another while
the load is passing along the road. A
wet5:ng Of the superficial portion Of
roadway during rains tends by the ex-
pansion Of the cement to knit the sur
face together and make 1t impermeable
to the passage Of water.
From theoretical standpoint it
would seem probable that certain con-
dition of moisture instead Of being OBJ
jectionable to roadway undergoing
constant use is rather desirable than
otherwise, A condition of moistness
serves to prevent loss Of material worn
fine by the abrasive action Of the wind,
and the presence of thin film of wet
dust acts as a cushion to protect the
fragments of rock from the rude touch
and impact Of passing traffic, thus less
ening the wear and tear Of the surface.
In the same manner the moist cement
operates to distribute the bearing sur
faces of the broken stone and to reduce
the local intensity Of the friction be-
tween one rock and another, although
cement in this condition will tend to
allow a greater freedom Of movement
among the broken stones, and hence in
this way make the surface more yield
ing. Assuming that the loss through
increased abrasion Of material resulting
from moist cement between the rock
fragments is equal to the saving due to
the cushioning effect of small quanti-
ty Of moisture on the surface, there still
remains q saving to the road by the
protection afforded by preventing ex-
cessive loss through the action Of the
wind.
It is the custom in England to plant
hedgerows beside macadamized roads
in order to insure rapid drying Of the
road after a rain by the SUMS action.
In Germany it has been the practice to