Evening star., 1936-10-03

Il
process, will collect much additiona,
dust. II the stones are soft. This will
pass into the mixture and will tend
to upset the very delicate formula
devised by the engineer in charge, as
being the correct mixture.
The absorption test is a simple one
and is self-explanatory. On Its re-
suit hinges the speed Of the job. L.
the stone is apt to absorb too much
humidity. the length Of time require
to dry 1t is increased. Only complete
ly dry stone and sand can be used TO
mixing. otherwise the humidity woul
increase the weight of the materis
and again would cause q variation i:
the fixed mixture
These few illustrations will serve
give an idea of the minute care tha
is taken in order to attain a mixtuI
to comply with the specifications
the engineer in charge. His calculs
tions as to quality, quantity and sE
Of stone, amount Of dust and asphal
must be followed with an exactne
comparable only to the care taken (
a druggist in filling a prescription.
INACCURATE or careless calcula
tions in weighing the differen
materials that go into a batch C,
mixture" destined later to be dumped
on the street or highway, may cause
the cracks which you and ! see whe
the road has been finished, or perhap
those greasy" spots that are not onl,
an eyesore, but reveal poor workman
ship in construction, and will lead to
further defects when cold or hot
weather sets in.
How does this engineer arrive a.
his precise formula, adaptable to {
particular need?
He has the qualities of the mat.
rials, as revealed in the chemic;
laboratory; he knows exactly the tyi
and capabilities of the asphalt at hi
disposal. In the field he has studiec.
the traffic, also the surrounding
countryside as TO drainage and sub
soil. He knows whether the soil is
spongy, hard, rocky. or perhaps the
gumbo type. which is found in some
sections of Southern United States.
If the country is usually very hot
in the Summer season and the Win
ter Is characterized by much rain, he
not only must make an important de-
cfsion as to the type of asphalt used
In the pavement, but also the amount
Of fine dust and sand to be used in
the layer Of finishing surface which
covers the more coarse under surface.
In that case low penetration asphalt
would be used, that is, hard asphalt.
Depending on the fineness of the dust,
this, in turn, is increased or decreased
In quantity. In cold climates the con-
trary would perhaps be desirable. We
say perhaps" for in some countries
it is extremely hot in Summer and
again extremely cold in Winter. which
calls for q different treatment of the
materials and in the manner of lay
ing the surface.
The scales with which the mixing
machines are equipped are an object
Of constant scrutiny by the engineer
In charge. The man who must weigh
each measure Of stone, sand. dust and
asphalt that goes to form q batch of
mixture, which will be released into
waiting truck and carried to the
project, has q very responsible and
exacting job. In the Summer months
that man Is usllallv rroyided with a
gas mask. in order to prevent in
haling the fine dust that escapes from
the bins. The heat, near the drying
material, is always terrific, since the
mixture generally leaves the plant for
street or highway at temperatures
carrying from $50 to 400 degrees Fab
renheit, depending on the distance
from the construction. Generally
higher temperatures than these pre-
vail in the drying drums, in which
sand. dust and stone are kept re-