The evening times., 1899-12-18

o. H. Dow. the chemist employed by the
District Government for the purpose
of testing the various brands Of
asphalt which are used in the District
for paving purposes. submitted report on
the mater to the Engineer Commissioner
recently, which shows the cause Of the fall
ure of the asphalt to remain in the condi-
tion in which it was laid.
The mixture Of asphalt with poor mate
rials or the improper handling of the same
will cause 1t to be absolutely worthless.
the chemist states. To those familiar Win
asphalt work the mode Of disintregation
can divided into three classes. First, dis
integration by cracking in long single
cracks that keep extending with time: Sec
ond. disintegration by rolling or showing.
the pavement becoming waxy; third, dis
integration in patches. where the pavement
will wear into hole which In time Will ex-
tend through to the base.
The most common Of these breaks occurs
in the first of these mentioned. In his re-
port the chemist says:
1T is evident to all who are familiar
with asphalt pavements that such cracks
are uue 10 tue COulIdStAou VI thS oSPN~~s
surface from cold. Some few cases have
been reported where the hydraulic cement
concrete under the pavement has cracked
as well as the surface. These contraction
cracks usually start near the gutter and
gradually extend across the street. 1hey
are also found radiating from iron sewer
traps and manholes set in the pavement
which rather indicate that the coemcient oil
expansion OF iron is greater than that Oh
the asphalt mixture.
They are often found at the joint be-
tween an old and new pavement. or at the
Joint made between one days work and an
other The latter would not occur II the
Joint had been properly made. These
cracks appear sooner and increase more
RAPIDLY on a street with but little or no
traffic. and are seldom found. unless the
material has been prepared with very poor
judgement, on streets with good amount
of travel.
The Mixing Of Asphalt.
when the pavement is subjected to
continuous traffic the asphalt surface.
which is more or less plastic at all tenn
peratures, is kept from cracking by the
constant kneading. so to speak. produced
by the traffic. And again, when an asphait
surface has but little or no trafhc, 1t be-
comes more porous owing to the fact that
expanding and contracting from heat and
cold without the compression due to traf-
fic, il becomes spongy, and as conse-
auence materially weakened. IL such
cracking occurs at all on a street with
fair amount of traffic, it Is evident that the
paving mixture used is at fault. either in
not being rich enough in bitumen or the
asphalt cement used was too hard
"As TO the proper quantity OF asphait
-inp riiy make
ii sufficiently rich: II is impossible to state
any fixed quantity for all cases, as so much
depends on the character Of the sand.
fine sand will hold more bitumen than
coarse one. and an angular one more than
rcrnd- grained one. The softness OF the
asphalt cement also has an influence on the
quantity that can be gotten into a mixture.
The softer the cement the less OF ii that
can be used.
"As to the relation that the character oil
the asphalt cement bears to the cracking Of
a pavement from contraction it is seh-evi-
dent that the harder or less plastic the as
phalt cement used the more liable the
payement is to crack. The general rule.
therefore. to follow in this mater is 10 use
as soft an asphalt cement as is possible
without having the pavement too SOFT ior
the conditions to which it will be subject
ed
The Liability to Crack.
After a revision of the causes which lead
to cracking cited above, Chemist Dow con-
cludes as follows:
The conditions that are hardest to meet
are where the pavement has little or no
traffic on street running north and south
with no shade, and in a climate that has
q big range of atmospheric temperatures.
such street will have practically no traffic
10 prevent it from cracking. as have ex-
plained above and will be exposed in sum-
mer TO the hottest rays of the sun. while
in winter it will be subjected TO the cold
est winds
The best asphalt surface mixture to
meet these conditions should be made Of
sharp sand, well graded from coarse to fine,
with the fine predominating. 1t should
have as much asphalt cement as the sand
will hold. The asphalt cement should be
as soft as is practicable to put in the mkt
ure without having it mark too badly liz
the hottest weather to which the pavement
will be subjected. Let US take this extreme
case and modify its different conditons-as.
for instance. if the street is. narrow and
runs east and west, so that il will be shaded
by the houses, or il the street Is at all
shaded so as to protect it more or less from
the hot summer sun, the asphalt cement
used can be softer. depending on the
amount of shade.
"If the traffic is increased to what might
be considered a moderate amount, the as
phalt surface mixture Just described will
do very well from the practical side, but
is richer in asphalt cement than necessary
so that it could be changed somewhat for
economy. If, however. We keep increasing
the traffic and the liability to crack lessons,
it will become more and more necessary to
alter the mixture to prevent it from rolling
and crowding out of shape.
STUDENTS AT CARLISLE.
Annual Report Of the Indian School
Superintendent.