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

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.

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.

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 1½ 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 1¼ or 1½-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 ¼ 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.