American Rural Highways Part 12

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 a.n.a.logy 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 pa.s.sing 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 pa.s.s 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.

CHAPTER XI

MAINTENANCE OF HIGHWAYS

Proper maintenance of highways is equally important with proper construction. With nearly all types of road construction, the need for maintenance arises soon after the surface is placed under traffic and is continuous thereafter. The nature and amount of maintenance work varies greatly among the several types of surface and the organization suitable for a system of highways will depend to a considerable extent upon the kinds of surfaces that are to be maintained.

The upkeep of a road may be conveniently considered as of two kinds, viz., (1) that which has to do with the wearing surface and earth shoulders or berms upon which there is some traffic and (2) that which has to do with the side ditches and drainage structures and keeping the roadside in presentable condition. Both kinds of work are usually carried out by the same organization, but whereas the nature of the work indicated under (1) will vary with the type of wearing surface and with all variations in traffic, that which is indicated under (2) will be nearly constant in any locality.

ORGANIZATION FOR MAINTENANCE

Maintenance of highways is preferably under the administration of the same authority as construction and when an improvement is undertaken under the jurisdiction of a State Highway Department, the completed improvement is ordinarily maintained under the state authority. If the improvement is made by county authorities, the maintenance is also carried out under county authority.

The nature of the organization of maintenance forces is dependent upon the kind of roads to be cared for and must of necessity be varied in any instance as conditions demand. In general, either maintenance gangs or patrolmen are employed and often both are used on the same road system.

=Patrol Maintenance.=--Where this system is in operation, the highway system is divided into patrol districts of from six to eighteen miles of highway and a single patrolman is placed in charge of each district. He is provided with all of the necessary tools and materials required in his district and performs all of the work required in the ordinary upkeep of the highway. He should work under the direction of the county engineer or the district engineer for the state highway department, because his work involves the use of materials and processes requiring technical supervision.

=Gang Maintenance.=--The maintenance gang may be employed for some types of road surface in lieu of the patrolman or with other types of surface may be employed to supplement the work of the patrolman. The maintenance gang consists of three to ten men and is furnished all of the tools and materials required for the particular kind of work they do. Ordinarily the gang goes over the roads a.s.signed to it once each season and performs those repair operations requiring more work than the patrolman can find time for. The work of the maintenance gang like that of the patrolman should be under engineering supervision.

=Maintenance of Earth, Sand-clay, Gravel and Macadam Roads.=--The ordinary upkeep of earth, sand-clay, gravel and macadam surfaces is most readily accomplished by the patrol method, since constant care is required to keep the roads in a condition of maximum service ability.

The tools required for each patrolman may include the following:

1 shovel 1 spade 1 stone rake 1 pick 1 scythe 1 tamper 1 or more road drags 1 mowing machine for cutting weeds 1 wheelbarrow (sometimes) 1 light truck 1 small kit carpenter's tools

The work of the patrolman consists in keeping the surface of the road smooth by dragging, repairing chuck holes by tamping in fresh material of the appropriate kind, keeping the ditches and culverts free from obstruction, cutting weeds and repairing bridge floors if they are of plank construction. Removal of snow drifts is sometimes a part of the patrolman's duty, but more often that is done by special gangs.

Usually the patrolman is authorized to hire teams for dragging and cutting weeds.

When an earth road requires to be re-graded so as to restore the cross-section and deepen the ditches, a gang is sent in to perform that work, as it is obviously impossible for the patrolman to perform work, of that kind.

If the gravel road is being maintained with a bituminous carpet coat, the patrolman will be furnished the necessary tools to enable him to patch the surface with bituminous material as necessity requires.

When the surface deteriorates to such an extent that a new carpet coat is required, the gang system is employed for all work connected with resurfacing, instead of attempting to have the work done by patrolmen.

The maintenance of the macadam road is carried out in much the same manner as that of the gravel road. The binder of stone dust or clayey sand is renewed as often as it is swept off by traffic. Depressions or ruts are repaired by first loosening the surface with a pick and then adding broken stone and screenings to restore the surface.

When the macadam reaches the stage where entire resurfacing is needed, the work is performed by gangs organized and equipped for the purpose; and likewise when the surface is being maintained with a bituminous carpet, the renewal of the carpet coat is performed by special gangs, but the ordinary upkeep of the surface by patching is handled by a patrolman.

MAINTENANCE OF MIXED BITUMINOUS SURFACES

[Ill.u.s.tration: Fig. 22.--Scarafier used in Gravel Road Maintenance]

These types of surface can be kept in satisfactory condition if they are carefully repaired once or twice each season. This work requires considerable experience and some special equipment, not ordinarily supplied to patrolmen. A gang is organized for the work and supplied with the proper equipment. They go over the roads and patch all worn places, generally first removing the wearing surface entirely in the area affected.

The wearing surface mixture is then prepared and tamped or rolled into place. If the area affected is small, tamping is satisfactory, and when the area is considerable, rolling is employed. The upkeep of the side roads may be accomplished by the same gang but is preferably taken care of by patrolmen, who do not attempt any but minor repairs to the wearing surface.

MAINTENANCE OF BRICK AND CONCRETE ROADS

On brick and concrete roads, the princ.i.p.al work on the wearing surface consists in filling the cracks with a suitable bituminous material.

This work is done by patrolmen or by special gangs and generally will be done once each year. The upkeep of the side roads is cared for by patrolmen who drag the side roads and cut the weeds as occasion requires.