PARK AND CEMETERY. 
150 
TAR FOR THE SURFACE TREATMENT OF ROADS 
Read by Hervey J. Skinner, Engineering 
Chemist, Before American Gas Institute 
The problem of preserving the sur- 
face of roads and highways and the 
prevention of dust has been given at- 
tention by highway engineers for many 
years, but not until the year 1905 was 
the question seriously considered in the 
United States. The increasing use of 
automobiles and motor vehicles about 
this time introduced a new condition to 
be considered in the building and pres- 
ervation of roads. Reports from vari- 
ous parts of the United States, Eng- 
land, France, and wherever the use of 
motor vehicles had become common, 
showed that the automobile was exert- 
ing an extremely destructive effect upon 
road surfaces. 
The advent of the automobile has 
changed very materially the character of 
the traffic. The action of the rubber 
tires is very different from that of iron 
tires and practically no dust is worn 
off to replace that removed by ordinary 
wear. The great tractive force or shear 
exerted by the driving wheels of motor 
vehicles is the main cause of this in- 
jurious effect. It has been demonstrated 
by a series of experiments in which 
separate speedometers were connected to 
the front and rear wheels of automo- 
biles ■ that there is a very appreciable 
amount of slipping of the driving 
wheels on the surface of the road. This 
slipping effect throws into the air large 
quantities of the fine surface material, 
which is caught by the air currents 
generated by the car body and subse- 
quently removed from the road by the 
wind. 
The increasing number of automo- 
biles each year emphasizes their import- 
ance as a factor to be taken into consid- 
eration; and consequently highway engi- 
neers have given careful study to the 
necessity of making a change in the 
present practice of road construction, 
as well as evolving some new method 
of preserving the surface of the enor- 
mous mileage of roads already built. 
Coal tar has been employed in road 
building for many years. Tar macadam 
roadways were constructed in Notting- 
ham, England, as early as the year 1840. 
The application of tar to the surface 
of a completed macadam road, while 
comparatively new in this country, was 
tried in France about forty years ago. 
In the United States, the first experi- 
ment which attracted the attention of 
road engineers was in the summer of 
1905, when a series of careful experi- 
ments were made at Jackson, Tenn., by 
the United States office of public roads 
in co-operation with the city engineer 
of Jackson, to determine the value of 
coal tar in the treatment of broken 
stone roads. 
The methods of applying the tar have 
varied considerably, but perhaps the one 
more generally used, especially in., the 
earlier experiments, is to remove the 
dust and all loose particles by thorough- 
ly sweeping the surface of the road 
and then applying the tar from an open 
kettle mounted on wheels and fitted 
with a portable fire box. The tar is 
brushed over and into the surface with 
stiff brooms such as are ordinarily used 
for street work. The kettle is kept in 
advance of the workmen and by using 
two kettles and heating one while the 
other is in use, the process is made 
continuous. After allowing the tar to 
soak into the surface for at least ten 
hours, it is covered with a layer of 
sand or fine stone screenings. When 
several hours have elapsed, the road is 
completed by rolling with an ordinary 
steam road-roller. 
In some places, and particularly in 
Europe, mechanical means have been 
employed for applying tar. Some years 
ago, the Road Improvement Association 
held in England a competitive trial in 
which many of the machines were fitted 
with ingenious devices. Nearly all of 
them provided for heating the tar and 
applying it under pressure by means of 
compressed air. Some of them were 
designed for carrying on the whole op- 
eration with one passage of the vehicle. 
The dust and loose particles were first 
removed by suction and drawn up into 
a receiver. The tar was then applied 
by compressed air and spread over the 
surface of the road by means of auto- 
matic brushes. The fine material pre- 
viously removed was again distributed 
over the surface and rolled by the 
steam-heated wheels of the machine. 
Some of these machines were drawn by 
horses, and others, the larger ones es- 
pecially, were self-propelled. 
The method most commonly used at 
the present time is a compromise be- 
tween strictly surface application and 
tar macadam construction and is known 
as the penetration method. In this 
method the surface of the road is brok- 
en up by means of a scarifier, new ma- 
terial added to fill ruts or other de- 
pressions, and the road reshaped. After 
a slight rolling without the addition of 
water, the tar is applied and allowed 
to stand as in the previous method. A 
dressing of fine material is then spread 
over the surface and the road well 
rolled, with the result that a surface is 
formed in which all of the spaces be- 
tween the hard stone are filled with a 
minimum amount of tar. A road treated 
in this manner is similar to one built 
by the tar macadam process, in which 
the stone and tar are mixed before be- 
ing laid. 
The character of the tar is of almost 
equal importance as the condition of the 
road. Tars vary widely in composition 
even when produced by the same pro- 
cess. The character of the coal used, 
the method of carbonization and the 
temperature of distribution all have a 
decided influence on the composition of 
the tar. The value of coal tar in the 
surface treatment of roads depends al- 
most entirely upon the binding power 
of its heavy bitumens. Besides these 
bitumens,, there are present other sub- 
stances such as water, ammoniacal 
liquor, oily constituents including the 
light oils and the creosote or “dead” 
oil, napthalene, anthracene and similar 
compounds, and free carbon, the pro- 
portion of which varies according to 
the manner in which the distillation 
process has been carried out. 
The presence of water in coal tar has 
a similar effect as moisture in the road. 
If a tar containing water is applied to 
a dry road, the latter absorbs the water 
more readily than the tar, producing the 
tar-proof effect to which reference has 
already been made. Difficulty is also 
experienced in handling tar containing 
an appreciable amount of water since 
the water causes foaming, and if the 
vessel is heated by direct fire the dan- 
ger of the tar going over the side of 
the vessel and taking fire is great. Am- 
monia is another undesirable constitu- 
ent. Napthalene and anthracene, while 
they exert no particular harmful effect 
as in the cases of water and ammonia, 
have no binding power and their pres- 
ence simply reduces the amount of bi- 
tumens which may be present. Free car- 
bon, like napthalene, has no detrimental 
effect, but, on the other hand, it is a 
useless constituent. The oily constitu- 
ents of the tar are valuable to some 
extent since they act as diluents. 
The amount of tar is another import- 
ant detail and if more is applied than 
the road can properly absorb, it will re- 
main on the surface and be taken up by 
the top dressing with the resulting for- 
mation of a crust. Another objection 
to an excessive amount of tar is that 
it has a tendency to become sticky in 
