Los Angeles herald., 1910-01-23

a moist condition. The action of high
speed tires has resulted in the forma-
ton of waves and chuckholes even
when dry macadams have been kept
damp. whether from natural rainfall
or by sprinkling. Therefore, experience
has demonstrated their unfitness under
modern conditions, and they are now
not considered the proper type of road
construction for roads on which much
motor car travel is expected.
The next step was the invention of
the protected macadam. This is con-
strueted in the same way as the dry
macadam, except that a top course of
from one to two inches was made of
sand or fine screenings saturated with
oil, coal tar, residuum from crude oil,
or some other substance which would
bond the surface together. 1t was
thought that this would overcome the
chuck-hole evil and would produce a
permanently smooth roadway, free
from waves and undulations. In this,
however, expectation has not been
realized. In the first place, it was diff
ficult to make a smooth roadway by
this method, since the dry macadam
below, no matter how well it might
have been rolled and settled before ap-
plying the dry course. would be more
or less disturbed during the process of
applying the top dressing. This resulted
in the formation of general waves in
the surface of any protected macadam,
as the rolling of the top course would
squeeze the fine material deeper into
the underlying matriX of rock in some
places than others. The rolling of the
top layer also had a tendency to dis
turb the macadam below to an extent
which made the formation of a smooth
surface practically impossible.
RlIt fbp traffic on gIIPb q road a ftp1.
it was constructed proved the most
serious of all objections. When the top
layer became plastic in warm weather,
the traffic would force it down in the
porous rock below, thus making the
roadway very uneven. High-speed
travel would also roll the underlying
rock up into hummocks and depres-
sions. The result has been that pro-
tected macadams are almost as un-
satisfactory as dry macadams. The
only difference in their favor being that
they are more free from dust and more
easily kept clean than dry macadams.
But so far as eVenness of surface is
concerned, the difference between the
two is slight after they have been in
use for few years.
Recent years have shown that there
is method of macadam road construc-
ton which is superior to the two al
fully overcome the objections l have
urged against the others. 1 refer to
what is known as bonded macadam, in
which the bond is no longer a mechani-
cal one from one rock fragment inter-
locking with another, but a cemented
bond produced by filling all the voids
in the broken stone with some sub
| stance possessing a high degree OF
cementing quality as well as elasticity
! The substance now generally used
for making bonded macadams are Vari-
Gus preparations made from coal tar
asphaltum and residuums from the
distillation of crude OIL
/ In California crude oil has also beer
suggested but so far experiments madt
Indicate that it will never be salts
| factory ingredient to be mixed will
rock for making bonded macadam
The reasons for this are that the
| heaviest oils are too light to fill the
interstices in the rock completely
without at the same time making tn,