Tensas gazette. [volume] (St. Joseph, La.) 1886-current, 1917-10-05

road surface are hardness, toughness,
and resistance to wear The extent
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 wide range.
Since nearly all gravel deposits con-
tain pebbles which have been formed
from many different kinds of stone,
it has not been considered practicable
to apply the ordinary laboratory tests
for determining hardness, tOughness,
and per cent of wear 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 very casual inspection will
reveal which deposit, among 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 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 coun-
try usually is composed largely of
trap-rOck fragments and generally
ranks high as road material. While
limestone pebbles usually are less re-
sistant to wear than those Of trap rock,
they possess the advantage of rel
atively high cementing value and the.e
presence in gravel may aid greatly IN
securing a well-bOnded surface with
out an excess of clay.
Tho Binder.
No matter how durable may be the
pebbles contained in q given gravel
deposit, they cannot be used success
fully in road surface unless they can
be we'll 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 cementine
l value.
The principal cementing agent, OF
binder, present in most gravel depos
!. its is clay, and In case Of deposit:
which do not carry sufficient binder
clay usually is the material added t.
correct the deficiency. While there arr
, many exceptions to this rule. it is suf
ficiently general to warrant treatin:
as special cases those deposits IL
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
i istics as its suitability for use in sand
clay construction. That is, qualit
of clay which could be used satisfaC
- torily in a sand-clay surface ais
should make a satisfactory binder TO
:, gravel surface, provided it is use
in proper proportion and Is properl
mixed with sand.
Grading and Proportions.
:. For gravel to make a satiSfactor
= road surface, the stone particle
n should be graded in size so that tn
y amount Of binder required will b
D reduced to a minimum. The reaso
for this is that the binder usually
much less resistant to wear than tn
III stone particles, and therefore 1t is d'
III sirable that the latter form as larg
e. a part of the wearing surface as
er practicable. Most gravel deposits 9
or they occur in nature satisfy this r
w Guirement in so far as grading OF tn
III pebbles is concerned, but they nearl
re always contain pebbles Of size lar
Il err than it is desirable to incorpora1
n In a road surface. Natural deposit
e. also not infrequently contain too lars
TX a proportion Of sand or clay to pr
n duce satisfactory results. 1t is desir
1T BILE. therefore, that specifications co
he ering gravel for use in road cOnstru
Sc ton should limit the proportions
IS which the fine and coarse materia
us shall be present and a maximum limi
ed ing size for the pebbles, as we'll {
definite requirements regarding Il
as quality of the pebbles and Of II
lie binder.
ng In general, it has been found tn
satisfactory gravel will conform t
he following limits as to percentages
he fine and coarse material:
1. Material retained on a Kim
ue sieve. So to IS per cent
ITS 2. Material retained on a A-in
sieve, not less than Is per cent.
ue 8. Material (clay) passing a q
IN mesh sieve for the surface course, 8
ar ID per cent.
ng 4. Material (clay) passing a 2
mesh sieve for the foundation cour
is 10 to Is per cent.
its The sand should be Mt least twi
'a1 as great as the clay content; and
sand and clay, when thoroughly mix
or should be sufficient to fill the vOids