Wausau pilot. [volume] (Wausau, Wis.) 1896-1940, 1917-09-18

ties. as possessed by the pebbles from
different gravel deposits. usuaify de-
penis on visual inspection. No; ;nfre-
quently very casual inspection will
reveal Whlcn deposit, among a great
number, contains the largest perren,
tage OF hard, durable pebbles. There
are comparatively few cases where
TINS point cannot be determined with
sufficient accuracy by sorting out the
peDDles 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 a 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 conn
try usually is composed largely of
trap-rock fragments and generally
ranks nigh as road material. While
limestone pebbles usually are less re-
slstant to wear than those of trap rock,
they possess the advantage of a rel
atively high cementing value and their
presence in gravel may aid greatly in
securing a well-bonded surface with
out an excess of clay.
The Binder.
No matter how durable may be the
pebbles contained in a given gravel
deposit, they cannot be used success
fully in 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 cemening 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 cementing
value.
The principal cementing agent, or
binder, present in most gravel depos-
its is clay, and in case of deposits
which do not carry sufficient binder,
clay usually is the material added to
correct the deficiency. While there are
many exceptions to this rule, it 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 q
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 q sand-clay surface also
should make a satisfactory binder for
gravel Surface, provided it is used
in proper proportion and is properly
mixed with sand
Grading and Proportions.
For gravel to make satisfactory
road surface, the stone particles
should be graded in size so that the
amount of binder required will be
reduced to q minimum. The reason
for this is that the binder usually is
much less resistant to wear than the
stone particles, and therefore it is de-
sirable that the latter form as large
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 it is desirable to incorporate
in road surface. Natural deposits
also not infrequently contain too large
a proportion of sand or clay to pro-
duce satisfactory results. 1t is desira-
BILE, therefore, that specifications COW
ering gravel for use in road construc-
ton should limit the proportions in
which the fine and coarse materials
shall be present and q maximum limit
ing size for the pebbles, as well as
definite requirements regarding the
quality of the pebbles and of the
binder.
In general, it has been found that
satisfactory gravel will conform the
following limits as to percentages of
fine and coarse material:
1. Material retained on a A-inch
sieve, 55 to Is per cent.
2. Material retained on %-inch
sieve, not less than Is per cent.
8. Material (clay) passing a 200
mesh sieve for the surface course, s to
Is per cent.
4. Material (clay) passing 200
mesh sieve for the foundation course,
10 to Is per cent.
The sand should be at least twice
as great as the clay content; and the
sand and clay, when thoroughly mixed,
should be sufficient to fill the voids be-
tween the larger gravel particles.
The percentages given above usually
will conform with this requirement.
A satisfactory mechanical analysis
of a gravel sample along the lines in
dicated above frequently may be made
in IKE field by first separating the
coarse material from the fine by means
of a N-inch mesh screen and then
washing the clay out of the fine ma
terial, as described in the discussion
of sand-clay roads. Both the coarse
and fine materials obtained from the
sample may be further separated into
different sizes by means of suitable
fF ic
sirable in making the test.
GRIT WILL AID INDIGESTION