BITUMINOUS ROAD MATERIALS AND THEIR USE
Tars and asphaltic materials of various kinds are widely used for road
construction and maintenance, especially for road surfaces subjected
to motor traffic. Materials of this character that are employed in
highway work possess varying degrees of adhesiveness, and while they
may be semi-solid or viscous liquids at air temperature, they melt on
the application of heat and can be made sufficiently fluid to mix with
the mineral aggregates that may be used in the road surface. Upon
cooling, the bituminous materials return to the previous state and
impart a certain amount of plasticity to the mixture, at the same time
serving as a binding or cementing agent, which is sufficiently stable
for many classes of road construction.
=Classes of Bituminous Materials.=--Bituminous materials may be
classified, according to the source from which they are obtained, as
coal tars, water gas tars, native or natural asphalts and oil or
petroleum asphalts.
=Coal Tar.=--Coal tar is obtained as a by-product in the manufacture
of illuminating gas from coal. It is also obtained in the manufacture
of coke from coal. The tar thus obtained is manufactured into products
that are used for dust layers on gravel or macadam roads, binders for
macadam and gravel surfaces, fillers for brick, wood block and stone
block pavements and for expansion joints. These various materials
differ mainly in their consistency at air temperature. (They may
differ widely in chemical composition, but that need not be considered
herein.)
=Water Gas Tar.=--Water gas tar is obtained as a by-product in the
manufacture of illuminating gas from crude petroleum. It is used for
the same kinds of construction as coal tar, and the products utilized
for the several purposes, like the coal tars, differ mainly in
consistency.
=Natural Asphalt.=--Natural asphalt is found in deposits at many
places in the world, existing in beds or pools where it has exuded
from the earth or as veins in cavities in the rocks. It is of varying
composition and consistency, but those kinds in most general use are
solid or very viscous liquids at air temperature. Of the deposits that
have been developed on a commercial scale, the Trinidad lake in the
British West Indies and Bermudez deposit in Venezuela are best known.
Both of these materials are too hard in the natural state to be used
for road construction, and are softened, or fluxed as it is called,
with fluid petroleum oil before being used.
=Petroleum Asphalt.=--Petroleum asphalt is a residue remaining after
the fluid products have been distilled from petroleum. Residues of
this sort are not always suitable for road construction, but a number
of brands of road material are obtained from this source. Oil asphalts
are used for dust layers, for binders for macadam roads, for asphalt
cements for sheet pavement surfaces, and for fillers for block
pavements and expansion joints.
=Mixtures.=--Water gas tars and asphalts are sometimes mixed to
produce road materials, and likewise native asphalts and residues
obtained from petroleum are sometimes mixed to produce asphalt cements
for paving mixtures.
=Classification according to Consistency.=--The various bituminous
materials may be classified according to consistency in discussing the
various uses to which they may be put.
=Road Oils.=--Road oils are fluid petroleum oils of such consistency
that they may be applied cold or by heating slightly. They are used
as dust layers on earth, gravel and macadam surfaces. Their efficacy
depends upon the binding properties of the small amount of asphaltic
material that is contained in the oil.
=Liquid Asphalts.=--These are somewhat less fluid than the road oils,
and must always be heated before application, but are viscous liquids
at ordinary temperature. These materials are obtained from crude
petroleum or semi-solid native bitumens, in which case they are
usually called malthas. Both coal tars and water gas tars of
semi-solid consistency are also employed for the same class of
construction as the liquid asphalts.
These materials are used for carpeting mediums on macadam roads and as
cementing agents in the construction of hot-mixed macadam.
=Asphalt Cements.=--The solid asphaltic materials used for hot-mixed
types of construction are called asphalt cements. They may be
petroleum residues or native asphalts fluxed with petroleum oils. They
are solids at ordinary temperature and must be heated to a temperature
in excess of two hundred and fifty degrees before they are
sufficiently fluid to use. Asphalt cements are used for sheet asphalt
and asphaltic concrete construction and for hot-mixed bituminous
macadam.
=Fillers.=--Fillers are solid asphalts or tars that are used for
filling expansion joints in rigid pavements and for filling the spaces
between the blocks in brick, wood block and stone block pavements.
=Bitumen.=--Bituminous materials are all soluble to a greater or
lesser extent in carbon disulphide and the soluble portion is called
bitumen. It is the bitumen that gives to the materials the cementing
properties utilized in road construction. Mixtures of mineral
aggregates and bituminous materials for various purposes are
proportioned with bitumen as a basis. Therefore, less of an asphalt
containing one hundred per cent bitumen will be used than of one
containing less than one hundred per cent of bitumen.
TABLE 8
PROPERTIES OF ASPHALTIC ROAD MATERIALS
(A) Material
(B) Specific Gravity
(C) Consistency
(D) Solubility in CS_2, Per Cent
(E) Solubility of Bitumen in CCl_4, Per Cent
(F) Solubility of Bitumen in 86 deg. Naphtha, Per Cent
(G) Fixed Carbon, Per Cent
(H) Flash Point
(I) Ductility
-------------------------+---------+----------+----------+------------
(A) | (B) | (C) | (D) | (E)
-------------------------+---------+----------+----------+------------
Mexican oil asphalts |1.03-1.05|As desired| 99.5-99.9|99.5-99.9
California oil asphalts |1.02-1.04|As desired| 99.9 | 99.9
Texas oil asphalts |1.01-1.03|As desired| 99.9 | 99.9
Bermudez natural asphalt|1.07 | 25 | 95 | 99+
Trinidad natural asphalt |1.40 | 7 | 56-57 | 100
Bermudez asphalt cement |1.04-1.06|Up to 135 | 95-97 |99.5 or more
-------------------------+---------+----------+----------+------------
-------------------------+---------+----------+----------+------------
(A) | (F) | (G) | (H) | (I)
-------------------------+---------+----------+----------+------------
Mexican oil asphalts | 70-80 | 13-16 | 200 deg.C. up| 60-100
California oil asphalts | 75-80 | 10-12 | 200 deg.C. up| 100+
Texas oil asphalts | 75-80 | 12-14 | 200 deg.C. up| 50-100
Bermudez natural asphalt| 68-70 | 13-14 | ... | ...
Trinidad natural asphalt | 64-65 | 10-11 | ... | ...
Bermudez asphalt cement | 77-80 | 11-12 | 175-200 | 25-50
-------------------------+---------+----------+----------+------------
=Specifications.=--Some properties of bituminous materials can be
varied in the process of manufacture, while others are inherent in the
material and cannot be changed in the process of manufacture.
Specifications must therefore be drawn with care to insure that the
requirements can be met by satisfactory materials. But certain
properties, such as specific gravity, may vary greatly among materials
equally satisfactory for construction purposes. One should not be
misled by apparent differences in the characteristics of materials,
because these may simply be natural peculiarities which have no
bearing on the usefulness of the material. There are given in Table 8
the properties of some of the commonly used bituminous materials and
the properties that can be varied in the process of manufacture are
indicated with an asterisk. A variation in these properties will
usually result in some change of other properties, but generally not a
great change.
SURFACES IN WHICH BITUMINOUS MATERIALS ARE UTILIZED
I. Surface Treatments
Attention has been directed to the rapid deterioration of water-bound
macadam when subjected to passenger automobile traffic.
In water-bound macadam the stones are held in place by a weak cement
composed of stone dust and water, and this cement is not sufficiently
strong to hold the stones in place when they are subjected to the
shear of automobile tires. In finishing the water-bound macadam
surface, the spaces between the stones are filled with screening and
in addition a layer about one-fourth inch thick is left on the
surface.
The automobile traffic first brushes aside all of the screenings and
smaller particles of rock, exposing the larger stones. These gradually
loosen as the road is used and are brushed aside. When this effect
begins, the road is said to be raveling. Various lengths of time may
elapse from the time the road is first finished until raveling begins,
depending upon the character of the stone, the weather and the amount
of motor traffic. During the period before raveling starts, it is
comparatively easy to restore the road surface at any time by the
addition of screenings or clay and sand. Usually there will be a few
small areas of the surface that, on account of faulty construction,
will ravel or become rutted much earlier than the remainder of the
surface. These can be repaired by the methods described in the chapter
on "Water-bound Macadam Construction." When the surface begins to
ravel seriously, maintenance becomes much more difficult and in order
to prevent raveling and the difficulties of maintenance thereafter,
the macadam surface is often coated with a bituminous material.
[Illustration: Fig. 20.--Oiling a Gravel Road]
If there is any dust or screenings on the road surface, the bituminous
material will not adhere to the stones and will soon flake off under
traffic. The surface of the macadam must therefore be thoroughly
cleaned before the bituminous material is applied. The usual practice
is to finish the road as water-bound macadam, and permit traffic on it
for a sufficient length of time to show any weak places in the surface
and at the same time thoroughly to season the surface. If any
defective places appear, they are repaired and when the surface
exhibits satisfactory stability, but before it begins to ravel, the
bituminous surface is applied. There will ordinarily be some stone
dust and some screenings remaining on the surface at the time
bituminous treatment is undertaken, and there may also be some caked
mud or other foreign material. All of this must be removed so as to
expose the stones throughout.
=Applying the Bituminous Binder.=--The bituminous binder may be
delivered in tank cars, which is desirable if the work is near a
railroad siding, or ample tank wagon service is available for long
hauls so that the tank will not be held up too long. Often it is
desirable to purchase the binder in barrels and haul these to the site
of the work in advance of beginning the construction of the surface.
The bituminous material may be applied by means of hand spreading cans
not unlike an ordinary garden watering pot, except that a slotted
nozzle is substituted for the ordinary perforated one. If hand methods
are employed for spreading, the bituminous material is heated in open
kettles and then spread on the surface, the quantity required usually
being about one-half gallon per square yard of surface. The
temperature of the binder should be great enough to insure fluidity
and the road should be dry at the time of the application. As soon as
the material has been spread, the surface is finished with a dressing
of chips.
=Finishing the Surface.=--For surface dressing the best material is
stone chips ranging in size from about 1 inch down to one-fourth inch.
But the chips must be of durable material, or they will quickly grind
into dust. They must be free from dust when applied, as the presence
of any considerable amount of dust interferes with the proper
finishing of the surface. The stone chips are rolled into the surface,
a sufficient quantity being used to just cover the surface.
=Patching.=--It almost always happens that some small areas will not
be properly cleaned or that for some unknown reason the coating peels
off the surface. Such places must be promptly patched to prevent them
enlarging under the action of traffic. This work is usually done by
patrolmen, who inspect the road at frequent intervals and make the
necessary repairs. The patrolman is equipped with a small heating
kettle, a spreading can and the necessary brushes, tampers and
miscellaneous tools needed for the repair work. The place to be
patched is carefully cleaned, coated with bituminous binder and stone
chips and tamped until dense and solid. Repairs made in this way are
exceedingly important in that they arrest deterioration in its early
stages and maintain a high degree of serviceability.
II. Penetration Macadam
A considerable mileage of macadam has been constructed in which an
attempt was made to eliminate the difficulties of maintenance by a
method of construction that involves applying a bituminous binder in
such a manner as to permit it to penetrate two inches or more into the
surface. It is expected that the binder will coat the stones to such
an extent as to increase materially the stability of the bituminous
macadam over the surface treated one. It is also expected that less
difficulty will be encountered in maintaining a surface of bituminous
material and stone chips on this type of road than on the water-bound
macadam. The extent to which these expectations have been realized has
varied to a marked degree and although some excellent surfaces have
been constructed by this method, the results have as a rule been
neither uniform nor entirely satisfactory. It seems to be apparent
that good results cannot be obtained unless the materials are entirely
suitable and the construction is carried out with unusual skill.
=Foundation.=--The foundation or lower course consists of a layer of
broken stone six inches thick placed on a well drained and thoroughly
rolled earth subgrade. In exceptional cases, the Telford type of
foundation might be employed.
The lower course of broken stone is finished in the same manner as
water-bound macadam, being bonded with stone screenings or with fine
gravel of high clay content.
Since this course is in reality the foundation of the surface, it is
necessary to secure stability by appropriate construction methods,
exactly as in constructing water-bound macadam.
[Illustration: Fig. 21.--Type of Roller used on Gravel and Macadam
Roads]
=Upper or Wearing Course.=--The wearing course consists of a layer of
stone about two and one-half inches thick. The stone is placed and
rolled and the spaces between the stones partially filled with some
suitable bituminous material. The bituminous material is usually
applied by means of a mechanical spreading device connected to a tank
wagon. The bituminous materials employed for this class of
construction are semi-solid in character and must be heated to give
them sufficient fluidity for application. They may be heated in the
tank wagon which is used for the application or they may be heated in
separate tanks and transferred to the distributing wagon for
spreading. Some kind of a nozzle or group of nozzles is employed for
spreading the material so that it can be delivered in the form of a
spray or at least in a thin fan-shaped stream and can be distributed
in a fairly uniform layer over the stone. The binder will cool rather
rapidly after it is applied, but meanwhile will flow into the openings
between the stones and will form over the surface stones a coating of
slight thickness.
The surface of the macadam is next covered with a layer of chips of
tough rock, similar to the material used for the final dressing in
surface treatments. These are carefully brushed into the openings
between the larger stones by means of heavy brush brooms. This is an
exceedingly important part of the work and often a much neglected part
of the construction.
The surface is then covered with a second application of bituminous
material, somewhat less in quantity than required for the first
treatment and the surface again covered with stone chips and brushed.
The surface is then thoroughly rolled and is ready for traffic.
=Patching.=--As in the case of surface treatments, there are likely to
be places that, on account of defects in the construction, will fail
soon after the road is placed under traffic. These will quickly
enlarge unless they are repaired promptly. The repairs are made by
loosening the stone in the area affected and adding new stone as
needed and then pouring on the necessary amount of bituminous material
to coat the stones. Allowance must be made for the compression of the
material by tamping so that a depression does not result. The stones
are carefully tamped to place and covered with chips which are also
tamped.
=Characteristics.=--The penetration macadam is a surface well adapted
to motor traffic if the individual vehicles are not too heavy. It is
likely to squeeze out of shape under motor truck traffic, becoming
seriously uneven and uncomfortable for traffic. Its durability is
materially affected by the construction methods followed.
III. Hot Mixed Macadam
The wearing course of the mixed macadam is composed of graded broken
stone or gravel and a bituminous binder. Usually the bituminous
material only is heated prior to the mixing, but sometimes the stone
is also heated.
=Foundation.=--The lower course, which serves as the foundation, is
either broken stone macadam, gravel or concrete.
Where a foundation of broken stone is used, it is constructed of the
materials and in the manner described for the foundation of the
penetration macadam. Quite often a badly worn macadam or gravel road
is used for the foundation and a new wearing course provided by adding
a mixed macadam surface. If such is the case, the old surface is
worked over so as to restore the shape sufficiently and to insure that
it is everywhere of sufficient thickness.
Portland cement concrete is sometimes used as a foundation for the
mixed macadam, but not often. Usually if the traffic is of a character
requiring a concrete foundation, it is desirable to use a better
wearing course than the mixed macadam, and the asphaltic concrete or
sheet asphalt type of surface is employed. It is necessary to finish
the surface of the concrete base with some device that will leave the
surface rough to prevent the macadam from creeping. A knobbed tamper
which leaves numerous irregular depressions about 2 inches in diameter
and three-fourths inch deep is often employed.
=Sizes of Stone.=--For the wearing surface, stone ranging in size from
2 inches down to one-fourth inch is usually employed. If the stone is
of good quality the maximum size may be but 1-1/2 inches, but soft or
even medium stone of that size are likely to crush under traffic. The
stone for the base course should preferably be from 3 inches down, but
any available size will be satisfactory if the layer is well rolled
and bonded. The base course is constructed in the same manner as
water-bound macadam and any material satisfactory for the base course
of macadam will serve for the base course of mixed macadam. Screenings
having good bonding properties will also be required for the base
course.
=Mixing and Wearing Surface.=--Several methods are employed in mixing
the wearing surface. The simplest is to mix by hand with shovels. The
aggregates are heated in improvised heaters which may consist of
nothing more than a metal pipe two or three feet in diameter, around
which the stone is piled. The mixing platform is usually a metal plate
sometimes arranged so that it can be heated by means of a fire
underneath. The bituminous material is heated in kettles. For some
mixtures, the stone is not heated, but the bituminous material is
always heated. The batch of stone is placed on the mixing platform,
the bituminous material added and the materials mixed by hand.
Machine mixing is practiced much more extensively than hand mixing,
being both more rapid and cheaper. The mixer is similar to a concrete
mixer except that the drum is arranged so that it can be heated. The
hot stone and the bituminous binder are put into the drum and mixed
for the requisite length of time. Sometimes the stone is mixed cold,
the bituminous material only being heated.
=Placing the Wearing Surface.=--The hot mixture is carted to the road
and spread to such thickness that after rolling the wearing surface
will be not less than two inches thick. The hot mixture is dumped and
then spread by means of shovels to the approximate thickness and the
spreading completed by means of rakes. The surface is then rolled
either with a tandem or a three-wheeled roller until thoroughly
compressed.
=Seal Coat.=--After the rolling has been completed, the surface is
covered with hot bituminous cement and dressed with pea gravel or
stone chips and again rolled. Traffic may be permitted in twenty-four
hours.
=Characteristics.=--The mixed macadam is a somewhat resilient surface
of excellent riding qualities and considerable durability for medium
traffic. It is likely to creep and become uneven when subjected to
heavy loads. The seal coat will wear off in two or three years and
will require replacing.
IV. Asphaltic Concrete
Asphaltic concrete is a name given to a road surface mixture which is
composed of graded stone, graded sand and asphalt cement. This type is
designated as asphaltic concrete because of the analogy of the mixture
to Portland cement concrete.
Asphaltic concrete is of two general types known as bitulithic, or
Warrenite, and Topeka asphaltic concrete, respectively, the
differences being in the nature of the mixture.
=Bitulithic or Warrenite.=--The stone employed for these types is
graded down from a size about equal to one-half of the thickness of
the wearing course, and stone passing a 1-1/4 or 1-1/2-inch screen is
usually specified. From the maximum size the stone is graded down to
the finest particles produced by the crusher. The range of sizes of
stone will vary with the source of the supply, and in order to secure
the desired density in the mixture, varying amounts of graded sand and
mineral dust, such as ground limestone or Portland cement, are added
to the broken stone. Usually the resulting mixture contains less than
fifteen per cent of voids, and to this carefully graded mineral
aggregate there is added enough asphalt cement to bind together the
particles.
=Topeka Asphaltic Concrete.=--In this type of asphaltic concrete, the
mineral aggregate consists of a mixture of carefully graded sand and
of broken stone of such size that all will pass a one-half-inch screen
and graded down to the fine dust produced by the crusher. To this
mixture is added about nine per cent of Portland cement or limestone
dust. The voids in the mixture are usually about twenty-five per
cent.
It will be seen that the essential differences between the Bitulithic
and Topeka types are these: the Topeka type contains a larger
percentage of voids and stone of a smaller maximum size than the
Bitulithic. Both types have been extensively employed for city paving,
but the Bitulithic and Warrenite types have also been used to some
extent for rural highways. The Topeka type has been used but little
for rural highways.
=Foundation.=--The foundation for the asphaltic concrete may be an old
macadam road, a base course constructed of broken stone or Portland
cement concrete, the latter being used much more extensively than
either of the other types.
Sometimes asphaltic concrete is used for resurfacing water-bound
macadam or gravel roads when the traffic has increased to the point
where the cost of maintenance of the water-bound macadam has become
excessive. The existing surface is repaired and the cross section is
restored, or possibly flattened somewhat.
=Placing the Surface.=--The stone, sand and asphalt cement are heated
to the required temperature and combined in the proper proportions and
are then thoroughly mixed by a mechanical mixer. The mixture is hauled
directly to the road and is dumped and spread by means of rakes. It is
then rolled thoroughly while still hot, a three-wheeled roller being
most satisfactory. After rolling, a seal coat of hot asphalt cement is
spread over the surface and covered with hot stone chips about 1/4
inch in size. The surface can be opened to traffic immediately after
the surface has been completed.
=Characteristics.=--The asphaltic concrete surface is of excellent
riding properties, is easily repaired and of moderate durability. It
is a particularly desirable surface for pleasure automobile riding and
for horse drawn traffic.