45 » ■ PARK AND CEMETERY. 
THE NEW MINERAL RUBBER PARK ROADWAY 
From a paper before the American Society of Municipal Improve- 
ments by Linn White, Engineer of South Park Commission, Chicago. 
The name mineral rubber, if it at- 
tracts attention at all, must excite in 
the mind of the investigator the ques- 
tions as to whether the qualities implied 
by the name are good and whether they 
are merited. Rubber implies that it is 
elastic, noiseless, dustless, non-slippery 
and impervious to water. Qualify it by 
the term mineral and we have added 
the ideas of permanency and strength. 
These are all requisites of the ideal 
pavement, of which I will venture to 
name three others ; Moderate cost, ease 
of repair, and a smooth sanitary surface 
of an agreeable dark color free from 
glare. Other requisite qualities of the 
ideal pavement might be enumerated, 
such as the materials of which it is 
composed must be plentiful and easily 
obtained, the machinery required must 
be simple and the labor needed should 
not require too high a training. These, 
however, are implied when we say it 
must be moderate in cost. There is one 
other minor quality of the ideal pave- 
ment wlrich should not be lost sight of, 
viz. : uniform consistency under a wide 
range of temperature. This, of course, 
is implied under permanency, but it is 
so important it should be emphasized. 
Granting that I have correctly named 
the essential qualities of the ideal pave- 
ment, the assertion is confidently made 
that the mineral rubber pavement em- 
bodies them to a large degree. How- 
ever, lest this assertion is called extrava- 
gant, I will say what is obvious, that 
no pavement can be so permanent and 
strong that it will sustain the heaviest 
loads indefinitely and never wear out, 
nor can it be so perfectly rubbery in 
character that the blows of a horse’s 
hoofs will not produce some noise. 
The mineral rubber pavement may be 
defined as an asphaltic concrete wear- 
ing surface, resting upon a base com- 
posed of cement concrete, macadam, 
bonded gravel, bituminous composition, 
or any other sort of permanent, stable 
base. The asphalt, or asphaltic cement, 
to give the rubbery qualities desired, 
must be comparatively soft, of rather 
low melting point, showing great range 
of temperature and permanency under 
extremes of exposure. The mineral ag- 
gregate is a well-graded crushed stone 
and sand mixture, which may contain a 
proportion of gravel, but the bulk of 
the larger particles must be angular 
enough to be compressed together into a 
well-bonded, stable mass without entire- 
ly depending upon the adhesiveness of 
the cement to give the bond. 
As to the demand for such a pave- 
ment, it is necessary to say that if it 
approaches the ideal the demand will 
be unlimited. It occupies a middle po- 
sition as to cost between the very high 
priced pavements usually laid on the 
expensive city streets, and the low cost 
methods of road construction which 
aspire to be classed among the so-called 
permanent pavements. It is elastic in 
more than one sense, because the char- 
acter of construction can be varied by 
making the wearing surface thicker or 
thinner, the foundation of a heavy con- 
crete or a more cheaply constructed 
macadam, and the stone aggregate of 
such a grade of stone as may be re- 
quired by the conditions of travel. An 
old macadam road forms an excellent 
foundation if made free of dust or mud 
and has a coarse, grainy surface. The 
wearing surface, having a stability of its 
own, does not creep or roll under traffic, 
and if the foundation proves somewhat 
weak, the pavement, by reason of its 
elastic and self-healing qualities, may 
be depended upon to adjust itself to the 
conditions. 
A pavement may have one prominent, 
excellent quality, such as great strength 
or durability under iron-tired traffic, but 
it may be so noisy, non-resilient or full 
of unsanitary joints as to be accepted 
only in ignorance of a better or as a 
last resort. On the other hand, it may 
commend itself only by its cheapness or 
its smoothly coated surface and be found 
to be only a temporary expedient or 
suited only to very infrequent travel. 
As the mineral rubber pavement has 
shown itself to occupy a conservative 
middle ground, capable of variation in 
strength and cost to suit the traffic con- 
ditions, always remaining an elastic, non- 
slippery, non-absorbent, non-dust and 
non-noise producing pavement, it would 
seem to have before it a demand as 
great and a field as large as can be 
opened up to such a product. 
The name mineral rubber pavement 
was given to it because, as recited above, 
its qualities seem to be well fitted by 
such a name, and as the result of an 
apt remark made by a city official after 
a section had been laid on a street over 
which he had charge. Observing its 
good qualities, he said ; “I always 
thought I should like to build a rubber 
road and here I believe we have it.” 
If I seem somewhat enthusiastic in 
enumerating the good qualities of this 
pavement, let the enthusiasm be credited 
to the pleasurable emotions of an engi- 
neer who believes he has found a solu- 
tion of some of the most vexing prob- 
lems he has met with in his practice. 
The mineral rubber pavement has been 
developed in Chicago. It is an out- 
growth of the conditions which existed 
on the streets and boulevards of that 
city, and is the result of a conscientious 
effort to find some means of bettering 
the driveway surfaces without excessive 
cost. The principal boulevards of Chi- 
cago under the control of the park com- 
missioners carry a large volume of traffic 
from which the heaviest teaming is 
mostly excluded. The boulevards in 
the downtown district, such as Jackson 
boulevard, running east and west 
through the center of the city, and 
Michigan avenue north of Twenty-sec- 
ond street have long been paved with 
standard sheet asphalt. But south of 
Twenty-second street and north of Chi- 
cago avenue up to within the last two 
or three years the best efforts of the 
park management have been directed to 
trying to maintain macadam road sur- 
faces. Before the advent of the auto- 
mobile in such large numbers, and, I 
may say, before quite so high a stand- 
ard of road and street construction had 
been set up, it was possible to do so. 
Well-crowned and well-drained ma- 
cadam roadways 12 inches thick, the 
upper 4 inches of granite, bonded with 
limestone screenings or a natural ce- 
mentitious gravel, were considered the 
acme of good road construction for 
driveways from which traffic teaming is 
excluded. But by, say, about 1905 it 
had been thoroughly impressed upon the 
minds of the commissioners that water 
sprinkled macadam roadways could not 
be maintained in a satisfactory Condi- 
tion with the expenditure of any amount 
of labor or money. The constant in- 
terruption of travel for repairs was one 
of the difficulties which had developed 
under existing conditions. So, in 1905, 
the South Park Commissioners took the 
lead in the experiments with various 
oils and tar preparations as dust layers 
and road preservatives. At first it was 
thought if the dust could be allayed, or 
its formation prevented, the raveling of 
the macadam would be stopped. Soon 
it was found that the lighter oil or tar 
liquids, though reasonably effective in 
preventing dust, added so little material 
of a binding quality to the road sur- 
face that the raveling continued. It was 
then realized that to get any real last- 
ing results at least the upper layer of 
macadam must be penetrated and bond- 
