Fig. 21.—Type of Roller used on Gravel and Macadam RoadsFig. 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 theform 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.
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 constructedin 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.
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.
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.
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 stateauthority. 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 assigned 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 shovel1 spade1 stone rake1 pick1 scythe1 tamper1 or more road drags1 mowing machine for cutting weeds (sometimes)1 wheelbarrow1 small kit carpenter's tools1 light truck
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 resurfacingis 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.
Fig. 22.—Scarafier used in Gravel Road MaintenanceFig. 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.
On brick and concrete roads, the principal 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.
Administration county;15federal;17highway;13state;16township;13Aesthetics;62Aggregate, fine;101Aggregate, coarse;100Air resistance;51Alignment;46Applying bituminous binder;122Asphaltic concrete;128Asphalt, natural;117liquid;118petroleum;117Assessments, special;19zone method;20Bedding course, green mortar;111sand mortar;111sand bedding mortar;111Binder for gravel;75Bitulithic or warrenite;128Bitumen;118Bituminous coatings on concrete;105Bituminous fillers;112Bituminous road materials and their use;116Bituminous surfaces;96, 120Blade grader;69Bonding;87Bonds, annuity;26serial;27sinking fund;25Box culverts;39Brick roads;113Brick, repressed;107tests of 108;vertical fiber;107vitrified;106wire-cut-lug;108Broken stone road surfaces;89Cement, asphaltic;118Cement concrete roads;98Cement grout filler;112Characteristics, asphaltic concrete;129bituminous macadam;125broken stone;97concrete;105mixed macadam;128sand clay;78Classes of bituminous materials;116Classification according to consistency;117Clay and cement concrete pipe;39Coal tar;116Concrete, asphaltic;128Concrete materials;100Concrete pipe;39Control of erosion;61Costs;70County administration;15Cross sections;60,65Culverts;56Curing concrete;103Design, broken stone roads;89concrete roads;99earth roads;42Desirability of road bonds;27Development of traffic;2Drainage, necessity of;29Drainage of roads;29Earth roads, in arid regions;72humid regions;65value of;73Earth works;92Education, rural;6Effect of grades;54Elevating grader;66Elevating grader work;68End walls for culverts;39Energy loss on account of grades;57Entrances, farm;37,61Expansion joints;104Farm entrance culverts;37Federal administration;17Fillers;118Finance, highway;19Fine aggregate;101Finishing surface of concrete;122Foundation, asphaltic concrete;129brick;109macadam;93mixed macadam;126penetration macadam;123Telford;94Gang maintenance;131Grader, Maney;67use of;69Gravel, ideal;81natural;83roads;74General taxation;24Good roads and commerce;7Green concrete bedding course;111Highway administration;13Highway finance;19maintenance;130Importance of design;30Ideal road gravel;81Inter-city traffic;5Inter-county and inter-state traffic;5Internal resistance;50Intersections;46Laying tile;35Length of culvert;37Liquid asphalt;118Local farm to market traffic;4Macadam;89Maintenance, concrete;105earth roads;70general;131gravel roads;88macadam;96of highways;130patrol;131Maney grader;67Marginal curb;113Measuring materials;101Metal pipe;38Mixing wearing surface;127Mixtures;117Natural asphalt;117gravel;79Necessity for planning;42drainage;29Patching;122, 125Patrol maintenance;131Pebbles, size of;80Petroleum asphalt;117Placing asphaltic concrete;129Placing broken stone;94Placing concrete;102, 103mixed macadam;127Plans for roads;43Preliminary investigation;44Preparation of earth foundation;102of road;85Private entrances;61Properties of stone;90Proportions for concrete roads;101Purpose of highways;1Reinforced concrete box culverts;39Reinforcing;104Repressed brick;107Road oils;117Road plans;43Rocks, kind of, for macadam;91Rolling, macadam;95Rolling resistance;50Run-off;31Rural education;6Rural social life;7Safety consideration;58Sand bedding course;111Sand clay and gravel road;74Sand mortar bedding course;111Seal coat;127Serial bonds;27Sinking fund bond;25Slip scraper;67Special assessments;19Specifications;119Spreading screenings;95State administration;16Stone, use of;92Surface drainage;30Surfaces, bituminous;120Surface method;87Superelevation;47Tests, brick;108Tile drains;35Topeka asphaltic concrete;128Tractive resistance;52Trench method;85Truck operation costs;9Types of culverts;38Underground water;34Undulating roads;58Use of blade grader;69Utilizing natural gravels;83Value of earth roads;73Variation in rainfall;64Variation in soils;63Vehicle taxes;24Vertical fiber brick;107Vitrified brick roads;106Vitrified brick;106Water gas tar;117Width of roadway;59Wire-cut-lug brick;108Zone method of assessing;20