The Arvada enterprise., 1917-09-13

samples from the Qlnerent deposits,
testing the various kinds with 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 quartzlte 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-
sistant 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 combined resistance to the dis
turbing action Of traffic. To accom-
plish this bond requires that the gravel
contain some cemehting 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, 1t is SUI
ficiently general to warrant treating
ms special cases those deposits In
which the binder Is not clay.
The suitability Of clay for use as
binder for a gravel road surface de-
ponds on exactly the same character
istics rs its suitability for use In sand
clay construction. That is, q quality
Of clay which could be used satlsfac-
tOrily in sand-clay surface also
should make n satisfactory binder for
gravel surface, provided 1t 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 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-
sirnble 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 no far as grading of the
pebbles Ii concerned. but they nearly
always contain pebbles Of size larg-
er than 1t Is desirable to incorporate
In road surface. Natural deposits
also not infrequently contain too large
n proportion Of sand or clay to pro-
duce satisfactory results. 1t 10 deslra-
BILE. therefore. TINT 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 maximum limit
Ing size for the pebbles, as well a.
definite requirements regarding the
quality Of the pebbles and Of the
binder.
In general. II has been found TINT
satisfactory gravel will conform the
following limits rs to percentages Of
fine and coarse material:
1. Material retained on a k.inch
sieve. 55 to Is per cent.
2. Material retained ON q x.lnch
sieve. not less than Is per cent.
g. Material (clay) passing a 200
mesh sieve for the surface course, 8 to
Is per cent.
4. Material (cluy) passing a 100
mesh sieve for the Foundation course.
10 to is per cent.
The sand should be at least twice
ms great as the clny content: and the
snnd md clay, when thoroughly mixed.
should be sufficient to fil the v0ids he.
tween the larger gravel particles.
The percentages given above usually
will conform with this requirement.
A satisfactory mechanical analysis
Of gravel simple nlong the lines In.
dicatcd above frequently may be made
in the field by first separating the
coarse material from the nnc by means
Of x.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 nns materials obtained from the
sample may be further separated Into
different sizes BY means Of suitable
screens II. for any reason, this 10 de.
afable In making the test.