PARK AND CEMETERY. 
322 
a layer of stone chips not exceeding 
one-quarter of an inch in size from 
which the dust has been removed and 
then lightly rolled. The removal of 
dust from the layer of stone chips 
gives a grainy surface more like or- 
dinary macadam, which will give 
better foothold in slippery weather. 
Before the application of the bitumen 
the surface should be thoroughly 
cleansed of all dust, dirt and fine 
caked material. It should be thor- 
oughly dry and the weather should 
be warm. Because of the advantage 
of a gritty surface, the stone chips 
from which the dust has been re- 
moved are preferred to sand or 
screenings for the top coating. This 
provides a very agreeable drivewajf 
surface during at least a considerable 
portion of the year, but, as stated 
above, it is not durable. On the 
boulevards of the South Park System 
of Chicago that have been treated 
by this method, where the traffic 
varies from 6,000 to 7,000 vehicles 
per day, never more than one year’s 
reasonable service can be counted on 
from a treatment of this s«rt. Some- 
times it will last only a few months, 
and in some extreme cases the sur- 
face begins to break through in a 
few weeks. When these breaks occur, 
it is not a general breaking up of the 
surface. Repairs may be made by 
loosening up the worn spots and 
tamping in carefully any new stone 
required to level up, and recoating 
the places thus repaired. A second 
or third application may be made over 
the whole surface with benefit, after 
repairing the worn places as above. 
But if the breaks in the surface are 
numerous, or if the original coating 
has lost its consistency or life, the 
only repair that can successfully be 
done is to spike up the whole surface, 
harrow it to bring coarse stone on 
top, or add a layer of new stone, roll 
down again and then put on the new 
application of bitumen. 
The first application should be, as 
stated above, at the rate of about 
three-quarters of a gallon per square 
yard, but the second, if the surface is 
not spiked up, should not be at the rate 
of more than one-half of a gallon. As 
stated before, the temperature when 
the work is done,- conditions of mois- 
ture, dust, etc., affect very greatly 
the life and value of the treatment. 
These are conditions which cannot be 
foreseen or controlled. The season 
of the year when the greatest damage 
is done to driveways treated by the 
surface or penetration method is in 
the winter or spring when the frost 
and excessive moisture has left the 
macadam in a somewhat loosened and 
weakened condition and when the 
bitumen is more or less brittle from 
the cold. When in this state a road 
surface apparently in good condition 
will sometimes go to pieces badly 
within a few days. 
Another thing to be considered is 
the character of the vehicles using 
the roadway. On the Chicago boule- 
vards 75 per cent of all the vehicles 
are automobiles, and perhaps half 
the remainder are__rubber tired. Plain 
rubber tires are comparatively harm- 
less on any sort of bituminous sur- 
faced roadway where the particles 
are sufficiently bound together to pre- 
vent their being loosened, and where 
the protective coating of bitumen is 
not broken through. But the grinding 
and pounding of iron tired traffic has 
a much more serious effect where the 
layer of bitumen is comparatively thin, 
especially during the season of the 
year when it is brittle. This is evi- 
denced by the rapidity with which 
surfaces of this kind wear out on 
the intersections where streets carry- 
ing miscellaneous and mixed traffic 
cross the boulevards. 
The method of getting more pene- 
tration, alluded to in the early part 
of this paper, where the upper layer 
of macadam is left loose and un- 
bonded until the bitumen is poured, 
.gives, of course, a much better re- 
sult. The penetration into the loose 
stone is much greater and the finer 
grades of stone can be added and 
rolled in after the bitumen is ap- 
plied. The cost, however, is much 
greater, and the results are still by 
no means certain, owing to the crud- 
ity of the method. Too much 
bitumen may give a road surface' too 
soft in warm weather, which will rut 
and move under the wheels, and in 
other cases the sun will draw the 
bitumen to the surface making it 
sticky and disagreeable when warm 
and slippery when -cold. 
These conditions have occurred on 
work of this character done under the 
writer’s supervision and in one case 
on a stretch of boulevard 600 feet in 
length and 50 feet wide, as much as 
90 cubic yards of crushed stone was 
rolled in at various times during the 
two years following the treatment in 
the effort to overcome this unstable, 
soft condition. Part of this trouble 
was undoubtedly due to the condition 
of the macadam at the time of the 
application of the bitumen. It had too 
much fine material in it and was 
lacking in sufficient coarse, sound 
stone to make it stable and durable. 
The same trouble would probably not 
have occurred on a newly constructed 
road or on one where a heavy layer 
of new stone was used. But if a con- 
siderable amount of new stone is to 
be used it is the writer’s opinion that 
it is better and more economical to 
follow the method of mixing the 
bitumen with the layer of new stone 
before spreading, in the manner that 
will be described further on in this 
paper. 
The use of oil in the form of an 
emulsion, or of a light enough grade 
to be applied undiluted without heat, 
has been alluded to and will not be 
described further because all such are 
comparatively temporarj- in their ef- 
fects and do not supply much of a 
binding qualit}^ to the macadam. 
They should be classed as dust layers 
only. 
As the use of a driveway increases, 
and as the grade and character of the 
traffic passing over it improves, just 
in proportion does the objection in- 
crease to the repeated use of so dis- 
agreeable a substance as road oil. 
Like man}- other crude things of life 
it is tolerated when nothing better is 
known or nothing better can be af- 
forded. 
After several years of e.xperiment 
and experience with the surface or 
penetration methods described, the 
South Park Commissioners of Chi- 
cago were led to authorize the next 
step forward towards making per- 
manently better road surfaces by mix- 
ing the road material with the bitum- 
inous binder, spreading it on the sur- 
face -while hot and compacting it 
thoroughly by rolling. This, in its 
simplest form, is only a new appli- 
cation of old knowledge. What is 
commonly known as asphalt or sheet 
asphalt pavement is simply a mixture 
of asphalt with sand and finely crushed 
or ground stone, varied in occasional 
cases by the introduction of a pro- 
portion of coarse stone in the mix- 
ture. The pavement termed bitu- 
lithic, though advertised with much 
flourish of patent claims, is but the 
same old idea of mixing bitumen 
with stone and sand much after the 
manner of making concrete, the 
melted bitumen being substituted for 
the hydraulic cement and water. 
The new applications of this ’Knowl- 
edge consist, first, in brushing away 
the veil of mystery that has been 
thrown around the making of bitum- 
inous pavements, showing there is 
nothing difficult in mixing bitumen, 
sand and stone; and, second, in dem- 
onstrating that it can be done in a 
simple machine, portable and not 
much more difficult to handle than a 
