The Red Cloud chief., 1917-09-13

to which pebbles possess these quail
ties depends very largely on the char
acter Of the parent stone from which
they were originally produced and ac-
cordingly varies over a wide range.
Since nearly all gravel deposits con-
tain pebbles which have been formed
from many different kinds of stone,
II has not been considered practicable
to apply the ordinary laboratory tests
for determining hardness, toughness,
and per cent Of to gravel, and
the matter Of comparing these quail
ties, as possessed by the pebbles from
different gravel deposits, usually de-
ponds on visual inspection. Not infre-
Quently a very casual inspection will
reveal which deposit, among a great
number, contains the largest percen-
tage of hard, durable pebbles. There
are comparatively few cases where
this point cannot be determined with
sufficient accuracy by sorting out the
pebbles contained In representative
samples from the different deposits,
testing the various kinds with a hand
hammer, and determining the relative
proportions In which the more durable
materials are present in the sample by
means of n weighing device.
Ordinarily the most durable pebbles
which occur in gravel deposits are
those composed Of either quartzite or
ebert, though many deposits contain
fragments of limestone or trap rock,
either of which may make an excellent
road-surfacing material. For Exam
pie, the so-called "blue gravel" which
occurs in certain sections of the coun-
try usually Is composed largely of
trap-rock fragments and generally
ranks high as a road material. While
limestone pebbles usually are less re-
slstant to wear than those of trap rock,
they possess the advantage of IA rel
atively high cementing value and their
presence in gravel may aid greatly In
securing q wcll-bonded surface with
out an excess of clay.
Tho Binder.
No matter how durable may be the
pebbles contained in a given gravel
deposit, they cannot be used success
fully in a road surface unless they can
be well bonded together so as to pre-
sent a combined resistance to the dis
turbing action of traffic. To accom-
plish this bond requires that the gravel
contain some cementing or binding
agent such as iron oxide, carbonate
of lime, or clay. AS already explained,
certain pebbles, such as those COM
posed of limestone, possess the prop
erty of becoming firmly bonded togeth-
er by virtue of their own cementfng
value.
The principal cementing agent, or
binder, present in most gravel depOs-
its is clay, and in case Of deposits
which do mo- carry sufficient binder,
clay usually is the material added to
correct the deficiency. While there are
many exceptions to this rule, 1t is suf.
ficiently general to warrant treating
as special cases those deposits In
which the binder Is not clay.
The suitability of clay for use as a
binder for a gravel road surface de.
ponds on exactly the same character.
istics as its suitability for use in sand.
clay construction. That is, a quality
of clay which could be used satisfac-
torily in Il sand-clay surface also
should make a satisfactory binder for
a gravel surface, provided it is used
in proper proportion and Is properly
mixed with sand.
Grading and Proportions.
For gravel to make a satisfactory
road surface, the stone particles
should be graded in size so that the
amount of binder required will be
reduced to a minimum. The reason
for this Is that the binder usually Is
much less resistant to wear than the
stone particles, and therefore 1t Is de-
sirable that the latter form as large
a part Of the wearing surface as Is
practicable. Most gravel deposits as
they occur in nature satisfy this re
quirement in so far as grading Of the
pebbles is concerned, but they nearly
always contain pebbles of a size larg
er than ii is desirable to incorporate
in a road surface. Natural deposits
also not in~freQuently contain too large
n proportion of sand or clay to pro
duce satisfactory results. 1t is desira'
bic, therefore, that specifications GOV
ering gravel for use in road construc
ton should limit the proportions In
which the fine and coarse materials
shall he present and a maximum limit
ing size for the pebbles, as well as
definite requirements regarding The
quality Of the pebbles and of the
binder.
In general, 1t has been found tina
satisfactory gravel will conform the
following limits as to percentages OF
fine and coarse material:
1. Material retained on a K-incf
sieve. 55 to rs per cent.
2. Material retained on a A-incf
sieve, not less than Is per cent.
3. Material (clay) passing a 200
mesh sieve for the surface course, 8 t.
10 per cent.
a. Material (clay) passing n 200