PARK AND CEMETERY. 
398 
from one-half to two inches, depending 
on the thickness of the layer of stone 
it is desired to bond with the bitumen. 
Generally this layer should not be less 
than two inches thick, as the bitumen 
should penetrate to that depth to make 
a safe, stable road surface under aver- 
age conditions of traffic. 
After lightly rolling, the hot bitumen 
should be poured on until the voids in 
the stone are all evenly filled. Over the 
surface is then spread a layer of screen- 
ings and fine crushed stone, none of 
which should exceed one-half inch in 
size. It is then thoroughly rolled un- 
til the fine material is forced into the 
interstices of the large stone, complete- 
ly filling them, with a thin layer of 
fine stone left on the surface. 
If during the rolling process any of 
the bitumen is forced through to the 
surface, more screenings should be 
spread until the whole surface is well 
covered. 
In some cases a second light coat of 
bitumen is poured over the surface, with 
a second dressing of stone chips or 
screenings. This, however, should not 
be done with any but the lighter bi- 
tumens, either tar or oil. Anything 
heavy enough to be termed asphalt and 
to produce a safe and stable binder will 
not penertate through the layer of fine 
stone or screenings, and consequently 
will lie on the surface as a thin coating 
not bonded to the rest of the pavement. 
The best results may be expected from 
the use of the coarser stone and heavier 
binder, so long as it is not of a qual- 
ity that will be hard and brittle in cold 
weather. For this reason, a compara- 
tively heavier grade of asphalt may be 
used than of tar. 
The quantity of bitumen, either tar 
or asphalt, used in poured macadam 
should be from two to three gallons per 
square yard. Of the lighter grades of 
bitumen the less quantity should be 
used, otherwise a shifting, rutting sur- 
face will be gotten. 
The drawbacks to the use of poured 
macadam are the irregular and some- 
what uncertain results obtained, and the 
limited season in which the work can 
be done. Weather conditions must be 
just right, neither cold nor damp, and 
the stone must be clean and free from 
dust. Old macadam cannot be poured. 
The stone must be fresh and clean. 
The greatest obstacles to successful 
work in poured macadam may be set 
down as moisture and dust. 
We come now to another form of bi- 
tuminous composition, specially adapted 
to use on macadam base, which may 
most properly be termed bituminous 
concrete. This includes the bithulithic 
or tar concrete, and the asphaltic con- 
crete. 
This kind of pavement is mixed like 
concrete and laid hot. The tar con- 
crete, when well laid, makes a pavement 
of great strength and good qualities, 
when the temperature is just right. 
(Tar, howe\-er, is much more susceptible 
to the influence of heat and cold than 
asphalt and is hence a more dangerous 
material to use.) 
A bituminous concrete should be laid 
hot and freshly mixed for the same rea- 
son that a sheet asphalt or sand asphalt 
pavement should be laid hot. There is, 
however, an additional reason why the 
tar or asphaltic concrete should be 
handled as quickly as possible. If it is 
hauled long distances while the bitumen 
is in a liquid or semi-liquid state, it is 
liable to settle down to the bottom of 
the load and become partially separated 
from the coarse aggregate. In the case 
of the sand asphalt paving mixture, the 
finer aggregate serves to hold the melted 
bitumen better. 
In the South Park system of Chicago 
there have been laid during the season 
of 1909 about 150,000 square yards of 
this asphaltic concrete pavement on a 
macadam base such as I have attempted 
to describe. All of it has been mixed 
in two portable asphalt mixing machines, 
which were designed and built especially 
for the South Park commisisoners. 
These machines are complete plants in 
themselves, drying and combining the 
stone and sand, heating the asphalt, au- 
tomatically proportioning the materials, 
and delivering the paving composition 
ready mixed for laying. The machines 
are moved along the street, about 300 
feet at a time, similar to portable con- 
crete mixers. They are strictly portable. 
MOVING THE PORTABLE MIXING' PLANT. 
SPREADING ASPHALTIC CONCRETE MIXTURE ON OLD 
MACADAM BASE. 
