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CHAPTER VIII
CEMENT CONCRETE ROADS
The cement concrete road is one of the later developments in highway construction, but the type has had sufficient use to show that it is one of the satisfactory types for heavy mixed traffic, and, where the proper materials are available, it is one of the economical types of construction.
Destructive Agencies.—It is well to have clearly in mind at the outset that the concrete in a road surface is subjected to certain destructive agencies not usually significant in connection with the use of concrete, and these are so often disregarded that the average serviceability of the concrete road surface is sometimes much lower than it would be if built with due regard for the effect of traffic on concrete surfaces. In most structural uses of concrete, its strength in compression only is utilized, and the factor of safety is such as to eliminate to some extent failures due to inferior materials or workmanship.
The concrete road surface is subjected to compression under wheel loads, to bending, causing tension in the concrete, to abrasion from wheels, and to tension and compression due to effect of temperature. The weight of the wheel loads may cause sufficient distortion of the road slab to produce rupture. The aggregates may be crushed under wheel loads if the material is too soft. Abrasion from steel tired vehicles wears away the concrete unless it is hard and durable. Changes in dimension due to the effect of change in temperature introduce tension or compression into the road slab and may result in cracks. Freezing and thawing in the subgrade subjects the slab to vertical movement and discontinuous support with the result that longitudinal and transverse cracks occur.
The foregoing indicates the importance of securing good concrete for road surfacing, and that is accomplished by using suitable aggregates, by proper design of the road surface and by following established construction methods.
Design.—The widths usually adopted for concrete roads are: for single track roads, 9 or 10 feet, and for double track roads, 18 or 20 feet. The thickness is 6 to 8 inches at the middle, varying with climatic conditions and with the kind of soil upon which the concrete is laid. The thickness at the edge is 1 inch less than at the middle except that 6-inch surfaces are usually of uniform thickness, the total crown being 2 inches. The thickness of the two course pavement is the same as would be used for a single course pavement in the same location. The surface of either width has a total crown of one or two inches to insure water running off the surface. The earth foundation is often flat, the crown being obtained by making the slab thicker at the middle than at the edge. Fig. 18 shows cross section for concrete roads.
In the state of California, concrete roads four or five inches thick and surfaced with a bituminous carpet mat have been successfully constructed. Similar designs have been used in a few other places, but for general practice it is unsafe to depend upon such a thin slab. Climatic and soil conditions probably account for the success of the thin roads in California.
Concrete Materials.—The coarse aggregate for the concrete may be broken stone or pebbles screened from natural gravel. Durability is necessary, but it is also important to have uniformity in the concrete so that the road surface will wear uniformly and consequently keep smooth. Supplies of broken stone are likely to contain a small percentage of soft pieces and such of these as are at the surface when the concrete is finished will crush under traffic, leaving a pit in the surface. Pebbles screened from gravel are also likely to be variable in durability and should be carefully inspected if they are to be used as aggregate for concrete roads. The harder limestones, some sandstones, pebbles from many of the gravel deposits and practically all of the igneous rocks make satisfactory aggregates for the concrete road.
Sometimes none of the coarse aggregates readily available are sufficiently durable or uniform for the wearing surface of the concrete road, but a suitable aggregate may be obtained at relatively high price by shipping considerable distances. In such cases what is known as the two course type of concrete road is employed. The wearing course usually is about 2 inches thick and is constructed with selected aggregates of good quality shipped in for the purpose. The lower course is constructed of aggregates which do not possess the desired qualities for a wearing course, but which are satisfactory for concrete not subjected to abrasion. The aggregates for the wearing course will be selected with the same regard for uniformity and durability that would be the case if they were for the one course pavement.
Bank run gravel, or run of the crusher stone, is generally not sufficiently uniform as regards proportion of fine and coarse material to produce uniformity in the concrete, and the use of aggregates of that character is not permissible for the wearing course, but under proper inspection they may be used for the lower course of two course pavements.
Fine Aggregate.—The fine aggregate is generally natural sand, but a mixture of natural sand and stone screenings is sometimes employed. The fine aggregate of whatever character must be clean, free from organic matter and sand, must contain no appreciable amount of mica, feldspar, alkali, shale or similar deleterious substances and not exceed two and one-half per cent of clay and silt. The sand is of such a range of sizes that all will pass the one-fourth-inch sieve and that not exceeding about five per cent will pass the 100-mesh sieve.
Proportions.—Various mixtures for the concrete are employed because these may properly vary to some extent with the exact character and grading of the aggregates. Experience seems to have shown that the concrete used for the wearing surface should have a crushing strength of at least 2500 pounds per square inch, and the mixture adopted is based on the requirements that will give the desired crushing strength. The common mixture for the one course pavement is one part cement, two parts sand and three and one-half parts coarse aggregate. For the wearing course of the two-course type of pavement, a mixture of the same kind is very often specified.
While these are perhaps the most widely adopted proportions, many others have been used, especially where the aggregates exhibit peculiarities or the traffic conditions are unusual. It is desired to emphasize that the purpose is to obtain concrete of the desired strength and there can be no such thing as "standard" proportions.
Measuring Materials.—In considering the methods employed for measuring aggregates, emphasis should be placed on the futility of rigid requirements for the aggregates, both as regards quality and range of sizes, if the materials are carelessly proportioned at the mixer. If even reasonably near uniform wearing qualities are to be secured throughout the entire area of the concrete road surface, successive batches of concrete must be alike, and to insure that, the aggregates including the water in each batch of concrete must be mixed in exactly the same proportions. The aggregates are measured in various ways, all essentially alike in that the intent is to insure exactly the same amount of each ingredient for each batch of concrete.
One method is to place bottomless boxes in wheelbarrows, fill the boxes level full and then lift off the box. Another is to use a wheelbarrow with a bed of such shape that the contents will be a multiple of 1 cubic foot when level full. For the larger jobs, the aggregates are hauled in industrial cars, each having sufficient capacity for a batch of concrete. The car body is provided with a partition so as to separate the fine and coarse material.
The water is measured in a tank which automatically refills to the same level each time it is emptied and when adjusted for a mixture will introduce the proper amount of water for each batch. It is highly important to use the least amount of water that will produce workable concrete.
Preparation of the Earth Foundation.—The concrete road is generally placed directly on the natural soil which has been brought to the proper cross section. Some engineers advocate that in preparing the subgrade, the earth be thoroughly rolled; others prefer not to roll the subgrade. If fills of considerable depth are constructed, they should either be rolled as built or else should be allowed to settle for some months before the concrete road is placed, preferably the latter.
Placing the Concrete.—The concrete is placed between substantial side forms of a height equal to the thickness of the concrete road slab at the edge, and is shaped roughly by means of shovels.
Various methods have been developed for striking the surface to the exact shape desired and smoothing it. If hand finishing methods are employed, a plank template is cut to the prescribed cross section and the concrete is shaped by drawing the template along the side forms. Sometimes the template is used as a tamper, being moved along very slowly accompanied by an up and down motion that tends to tamp the concrete. The template is then drawn along a second time to smooth the surface finally.
After the surface has been struck off by hand, it is finally smoothed, first by rolling crosswise with a slight hand roller about 8 inches in diameter and 30 inches long. The final finish is effected by dragging a piece of web belting back and forth across the surface.
Machines designed to tamp the concrete and strike it off to the required cross section are also employed for finishing. The machine is power operated and is carried on wheels that run on the side forms, and the machine moves slowly along as the tamping progresses. The concrete is tamped, struck off to shape and smoothed with the belt at one operation. This method of finishing produces denser and stronger concrete than can be produced by hand finishing methods.
Placing Concrete for Two-course Road.—The methods employed for the two-course concrete road are much the same as for the one-course road. The concrete for the lower course is placed and struck off by means of hand tools, and after that course has progressed a few feet, the upper course is placed and finished as has been described for the one-course road.
Curing the Concrete.—The setting action of cement is a chemical process, not merely a drying out of the water introduced in mixing the concrete. The chemical action is progressive for a long time, but is more rapid during the first few hours than during the later periods, and the concrete reaches about three-fourths of its maximum strength at the end of seven days. During the setting period and particularly during the first few days, plenty of water must be available to the cement.
To prevent too rapid loss of water from the concrete during the setting period, the surface must be protected from the wind and sun. This is accomplished by first covering with canvas as soon as the concrete has hardened sufficiently and by later covering with earth, to a depth of two inches. The earth covering is kept wet for about ten days and is left in place for about one month.
In some places the ponding method of curing is adopted. The surface is divided into sections by earthen dikes and the space inside the dikes filled with water to a depth of two or three inches. The water covering is maintained for two weeks or longer.
No traffic is permitted on the surface for one month, and in cold weather traffic may be kept off the surface for a longer period.
Expansion Joints.—To permit the concrete slab to accommodate itself to changes in dimension due to temperature changes, expansion joints 1/2 inch wide are placed about every thirty feet. These consist of a sheet of some prepared bituminous material placed in position as the concrete is poured.
Experience seems to indicate that in spite of the expansion joints, the concrete will crack more or less and many engineers think it advisable to omit expansion joints in constructing the pavement and when cracks develop to pour bituminous material into them, thus forming expansion joints.
The prevailing practice in rural highway construction is to omit the expansion joints, but they are commonly adopted in city pavements.
Reinforcing.—To minimize the cracking, either bar or wire mesh reinforcing is used in the concrete. If bars are used they are placed in the concrete as it is poured so as to form a belt around each section about 15 feet square. If the mesh type is employed, a part of the layer of concrete is placed and smoothed off and a strip of the mesh laid in place. Additional concrete is then poured on top of the mesh to bring the slab to the required thickness.
Bituminous Coatings on Concrete Surfaces.—The concrete road surface is sometimes coated with a layer of bituminous material and stone chips or gravel pebbles. This is particularly advisable where no really satisfactory aggregates are available and the concrete surface would not possess sufficient durability. The bituminous material is applied hot to the surface and is then covered with stone chips or gravel pebbles, ranging in size from 3/4 inch down to 1/4 inch, the resulting coating being about 3/4 inch thick. Many failures of this type of surface have been recorded due to the difficulty of securing adhesion to the concrete. This seems to be due in part to inability to get the proper bituminous materials and in part to climatic effects. Considerable progress has been made in developing this type of surface and it may eventually become a satisfactory maintenance method.
Characteristics.—The concrete road is of a granular texture and is not slippery. It is of course rigid and noisy for steel tired vehicles. It is an excellent automobile road and its low tractive resistance makes it a desirable surface for horse drawn vehicles. It possesses a high degree of durability if properly constructed. It is likely to crack indiscriminately but as a general rule the cracks are not a serious defect.
Maintenance.—The cracks that appear in the concrete surface are filled once or twice a year, tar or asphalt being employed. The dust and detritus is cleaned out of the cracks and the hot filler poured in, with enough excess overflowing to protect the edges.
CHAPTER IX
VITRIFIED BRICK ROADS
Vitrified brick roads consist of a foundation course of Portland cement concrete, broken stone or slag macadam, or of brick laid flat, the first named being by far the most generally used, and a wearing course of vitrified brick.
Vitrified Brick.—Vitrified brick are made from clay of such a character that when heated to the required temperature they will fuse into a glassy texture. Brick roads are constructed on roads carrying the severest of traffic and the brick must therefore be tough and of high resistance to wear.
Not all of the clays from which brick may be manufactured will produce a product suitable for road construction, and paving brick, even though truly vitrified, are of different degrees of durability, depending upon the nature of the clay and the care exercised in the manufacture.
Paving brick are manufactured by the stiff mud process, which means that the clay is molded into form in a relatively dry condition. To accomplish this, considerable pressure is exerted in forcing the column of clay through the dies, which form the prism from which the brick are cut. If the clay is unsuitable in character or is not properly ground and mixed, the brick will possess planes of weakness between the various layers of clay which have been pressed together, and these planes, called laminations, are a source of weakness if too marked. It is usual to specify that the brick used for road surfaces shall be free from marked laminations.
If the brick is not properly burned it will be only partly vitrified and therefore not of maximum durability. It is customary to specify that the brick shall show a glassy fracture indicating complete vitrification.
Various defects of a minor nature occasionally develop in the brick during the successive steps in the manufacturing process. Check cracks resulting from the burning or from too rapid cooling are often encountered, but unless these are deep, that is 3/16 inch or more, they do not impair the wearing quality of the brick, nor indicate structural weakness. Kiln marks are formed on some of the brick due to the weight of the brick above in the kiln. These depressions are not objectionable unless the brick are so distorted that they will not lie evenly in the pavement.
Spacing lugs or raised letters are formed on one face of the brick to insure sufficient space between the brick for the filler. These lugs or letters are not less than 1/8 inch nor more than 1/4 inch high and of such design that they will not obstruct the free flow of filler into the joints between the brick.
Several varieties of paving brick are to be had, the difference being principally in the design or size.
Repressed Brick.—In this type of brick the spacing lugs are formed by pressing the green brick, after it has been cut to size, into a mold on one face of which are recessed letters or other devices into which the clay is pressed, thus forming the spacing lugs.
Vertical Fiber Brick.—These brick are designed to be laid with one wire-cut face up and spacing is provided by two or more beads on the side of the brick. Sometimes the vertical fiber brick has no spacing lug, it being contended that the irregularities of the brick are such as to provide all of the space required. In practice this does not always work out, as the brick are so regular in shape that when laid there is too little space between the brick to permit the introduction of a suitable filler. The use of brick without spacing lugs is just beginning and is not yet a generally accepted practice.
Wire-cut-lug Brick.—This is a type of non-repressed brick which has spacing lugs provided by cutting one face in a special manner which provided lugs for spacing. In this type the wire cut face is the one between the brick as they are laid in the pavement.
Tests for Quality.—The standard test for quality of paving brick is the rattler test. The brick rattler consists of a barrel of 14 sides 24 inches long, mounted so as to rotate at a speed between 29.5 and 30.5 revolutions per minute. The duration of a test is 1800 revolutions. Ten brick constitute a charge and these are placed in the rattler along with 300 lbs. of cast iron spheres. The spheres are of two sizes, the smaller being 1-7/8 inch in diameter when new, and the larger 3-3/4 inches in diameter when new. Ten of the larger spheres are used and the balance of the charge is made up of the small size.
When tested in the standard manner the loss allowable for the several classes of service are as follows:
- Maximum Loss Traffic Average Loss for any Brick - Heavy 20 per cent 24 per cent Medium 22 per cent 26 per cent Light 25 per cent 28 per cent -
Other Tests.—Sometimes the absorption test is specified for paving brick, but it is rarely a vitrified brick that will pass the rattler tests which fails to pass a reasonable absorption test. Absorption of water in an amount exceeding 4 per cent indicates incomplete vitrification and failure of such brick is almost certain during the rattler tests.
The cross breaking test is also sometimes employed, but generally only to check the general quality of the brick. Failure in service more frequently occurs from excessive wear than from any other cause and the cross breaking test has little significance, except for brick less than 3 inches thick, which are to be laid on a sand bedding course.
Foundation.—The foundation for brick roads is usually of Portland cement concrete, the thickness varying with the nature of the traffic and the kind of soil upon which the pavement is built. For well drained soils and normal highway traffic, 5 inches is the ordinary thickness of foundation. Under favorable conditions such as locations with sandy soils or in semi-arid or arid regions where the soil is always stable, the foundation may be four inches thick, and a considerable mileage of brick road has been built with concrete foundations less than four inches thick.
In other locations the soil and traffic conditions require a base six inches or more in thickness, and the proper thickness can be determined only after all of the factors involved are known and have been analyzed. It is impractical to adopt a standard thickness of foundation that will be equally economical for all locations and all kinds of traffic. As the brick pavement is essentially a heavy traffic type of surface, the design cannot be varied greatly with similar foundation conditions because the weight of individual loads is the significant factor and this does not vary so much as the volume of traffic. A variation in volume of traffic may be compensated for by a variation in the quality of the brick as already set forth.
The mixtures for the concrete foundation vary widely because of the variation in the aggregates employed. If the fine and coarse aggregate for the concrete are of good quality a mixture of one part cement, two and one-half parts sand and five parts of coarse aggregate would insure concrete of adequate strength. A somewhat leaner mixture is sometimes employed and would be satisfactory if the aggregates were of exceptional concrete making quality. Mixtures of sand and pebbles (unscreened gravel) may also be used if care is exercised to secure a mixture of adequate strength. The proportion will of necessity vary with each particular material and the discussion of the various considerations involved may be obtained from various standard works on concrete and concrete materials.
Broken stone macadam is sometimes utilized for the foundation course of the brick pavement and such foundations are constructed as water-bound, which is described in a previous chapter. The thickness, like that of the concrete foundation, varies with the soil conditions and the weight of the loads that are expected to use the road. The macadam is placed in a single layer and is rolled and bonded with screenings as described in the chapter dealing with water-bound macadam. Six inches is a common thickness for the macadam base. This type of foundation should be employed only where the soil is quite stable and where material costs are such as to insure that the macadam base is materially cheaper than one of concrete. This would usually be in locations where the cost of cement is high because of long hauls and where suitable macadam materials may be obtained close at hand.
Old macadam roads are sometimes utilized for the foundation for the brick surface, but the instances where this is permissible are comparatively few in number. When an old macadam is to be used it is reshaped to the proper cross section and re-rolled and bonded so as to afford a stable foundation of the proper cross slope.
BEDDING COURSE FOR BRICK SURFACES
In order to equalize the variations in size and shape of the brick, they are laid on a bedding course composed of material into which the brick may be forced by rolling. In this way the upper surfaces of all brick can be brought to the proper elevation to insure smoothness and easy riding qualities. Several kinds of bedding course are now employed.
Sand Bedding Course.—The sand bedding course has been referred to as a sand cushion, but as a matter of experience the cushion effect is slight, although sometimes pavements have become uneven because the brick have pushed down into the sand after the pavement was used for a time. The sand for the bedding course should preferably be fine grained, all particles passing the eight mesh sieve, but ordinary concrete sand is satisfactory. The sand need not be clean, as a comparatively large percentage of silt or clay does not impair the usefulness of the material.
Sand Mortar Bedding Course.—In order to eliminate the tendency for the straight sand bedding course to shift because of the impact of traffic on the brick, a lean cement mortar is sometimes employed rather than the straight sand. Sand and cement in the ratio of one part cement to four or five parts of sand are mixed dry, and after the brick have been rolled, is moistened to furnish water to hydrate the cement. The sand employed is ordinary clean concrete sand.
Green Concrete Bedding Course.—In the monolithic type of brick road construction, the brick are laid directly on the green concrete base before the concrete has taken a set and the irregularities of the brick are taken up by rolling them until bedded in concrete.
FILLERS FOR BRICK SURFACES
The spaces between the brick are filled with some material that will prevent the brick from being displaced and prevent water getting to the bedding course. A suitable filler must adhere to the brick and fill completely the spaces between them. It must withstand traffic so as to remain intact in the joints and when in place it must be rigid enough to prevent displacement of the brick.
Cement Grout Filler.—One of the most commonly used fillers for brick pavements consists of a grout composed of Portland cement and fine sand. When properly mixed and applied the grout filler meets all requirements for a filler except that it is non-elastic and some means must be adopted for caring for pavement expansion.
Bituminous Fillers.—Asphaltic materials and tars are widely used as fillers for brick pavements. Such fillers are of high melting point and consequently solid at ordinary temperature. They are poured into the joints hot and when they cool are firm enough to comply with the requirements for a filler. In addition, they have enough ductility to accommodate the expansion of the pavement due to temperature changes.
Mastic Fillers.—Mastic consists of a mixture of about equal volumes of fine sand and a solid bituminous material. The mixture is prepared at high temperature and is worked into the joints between the brick while hot. When cool it resembles the straight bituminous filler except that the mastic is somewhat more resistant to wear than the straight bituminous filler.
EXPANSION JOINTS
It is recognized that brick will expand and contract with changes in temperature. When a bituminous or mastic filler is employed there is sufficient yield to the filler to accommodate the change in dimension in the brick, but when the grout filler is used either the expansion joint must be provided or the pavement must be designed to withstand the compression due to expansion of the brick. Expansion joints may consist of a sheet of bituminous mastic prepared for the purpose and set in place in the pavement. The sheet of joint material is simply inserted between courses of brick at the proper place.
Another method of forming an expansion joint consists in placing a strip of wood between courses of brick at the place where a joint is required. After the pavement has been grouted, the wooden strip is pulled out and the joint is filled with a suitable bituminous filler.
Marginal Curb.—If the sand bedding course is employed, it is necessary to provide curbing along the sides of the brick to hold the bedding course in place. The curb is usually constructed integral with the base and of concrete of the same mixture as the base. The width of the curb is usually six inches and the top of the curb is at the same elevation as the edge of brick surface.
CONSTRUCTION OF THE SURFACE
Before the construction of a brick surface should be undertaken on a road, the drainage should be provided for even more completely than for a less costly type of surface since it does not pay to jeopardize the stability of the pavement by failure to provide adequately for the stability of the supporting soil. Grades should also be reduced to the economical limit.
The earth subgrade is brought to the proper elevation and cross section and is thoroughly rolled. If there are places where the soil will not compact properly under rolling, these places are corrected by taking out the material and back filling with new material that will properly compact under the roller.
The aggregates for the concrete may be distributed along on the prepared subgrade or may be stored in stock piles or bins at convenient points. If stored on the subgrade, a traction mixer is employed which is drawn along the road as the work progresses, the materials being placed directly in the mixer. If stored at a central point, they may be transported to the mixer on the road and dumped directly into the mixer, or the mixer may be set up at the storage piles and the concrete hauled in trucks to the road where it is deposited and shaped.
The concrete is spread to the proper thickness and tamped either by hand or by machinery. If the marginal curb is to be employed, it is constructed immediately after the concrete for the base has been finished but before the cement begins to set.
After the foundation concrete has set, the bedding course is spread and struck off to the proper thickness. When the bedding course consists of sand-cement mortar, the sand and cement are mixed dry and spread to prescribed thickness. It is considered to be desirable to roll the sand bedding course with a light hand roller before the brick are placed, but the sand-cement bedding course is not rolled. The bedding course must be carefully shaped by means of a templet or strike board before the brick are placed.
The brick are laid in straight courses across the pavement, with the spacing lugs all in the same direction if brick with spacing lugs are employed, and with the lugs in contact with the brick of adjoining courses. If brick without spacing lugs are used they are laid loosely so that there will be room for the filler between the brick of adjoining courses.
After the brick have been laid they are rolled to bed them in the sand or sand-mortar bedding course and thus secure a smooth surface. For this purpose a light, power driven, tandem roller is used and the rolling is continued until the brick are thoroughly bedded. Any defective brick that are noted are removed and replaced with good brick and after this culling has been completed the surface is once more thoroughly rolled. If a cement-sand bedding course is employed, the surface is sprinkled just after the final rolling so that water will flow down between the brick and moisten the bedding course sufficiently to cause the cement to set. In some cases, the sand-cement bedding course is sprinkled just before the brick are laid but in warm weather the setting would take place before the brick could be rolled if that were done. In cool weather the setting is sufficiently slow to permit rolling before the bedding course hardens.
The filler is applied to the surface after the rolling. If the bituminous type of filler is employed, the hot filler is poured onto the surface and worked into the joints by means of squeegees, with comparatively little material left on the surface. In some instances cone-shaped pouring pots are employed and the material is poured directly into the joints.
The cement grout filler is applied in the same general manner as the bituminous filler. The grout, consisting of equal parts of sand and cement, is mixed to a thin consistency and poured onto the surface and is then worked into the joints with squeegees. Two or more applications are usually required to effect a complete filling of the joints. The surface should be covered with sand and be kept moist until the cement grout has set.
CHAPTER X
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 CS2, Per Cent (E) Solubility of Bitumen in CCl4, 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.
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 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.
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 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 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
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 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.
INDEX
Administration county; 15 federal; 17 highway; 13 state; 16 township; 13
Aesthetics; 62
Aggregate, fine; 101
Aggregate, coarse; 100
Air resistance; 51
Alignment; 46
Applying bituminous binder; 122
Asphaltic concrete; 128
Asphalt, natural; 117 liquid; 118 petroleum; 117
Assessments, special; 19 zone method; 20
Bedding course, green mortar; 111 sand mortar; 111 sand bedding mortar; 111
Binder for gravel; 75
Bitulithic or warrenite; 128
Bitumen; 118
Bituminous coatings on concrete; 105
Bituminous fillers; 112
Bituminous road materials and their use; 116
Bituminous surfaces; 96, 120
Blade grader; 69
Bonding; 87
Bonds, annuity; 26 serial; 27 sinking fund; 25
Box culverts; 39
Brick roads; 113
Brick, repressed; 107 tests of 108; vertical fiber; 107 vitrified; 106 wire-cut-lug; 108
Broken stone road surfaces; 89
Cement, asphaltic; 118
Cement concrete roads; 98
Cement grout filler; 112
Characteristics, asphaltic concrete; 129 bituminous macadam; 125 broken stone; 97 concrete; 105 mixed macadam; 128 sand clay; 78
Classes of bituminous materials; 116
Classification according to consistency; 117
Clay and cement concrete pipe; 39
Coal tar; 116
Concrete, asphaltic; 128
Concrete materials; 100
Concrete pipe; 39
Control of erosion; 61
Costs; 70
County administration; 15
Cross sections; 60, 65
Culverts; 56
Curing concrete; 103
Design, broken stone roads; 89 concrete roads; 99 earth roads; 42
Desirability of road bonds; 27
Development of traffic; 2
Drainage, necessity of; 29
Drainage of roads; 29
Earth roads, in arid regions; 72 humid regions; 65 value of; 73
Earth works; 92
Education, rural; 6
Effect of grades; 54
Elevating grader; 66
Elevating grader work; 68
End walls for culverts; 39
Energy loss on account of grades; 57
Entrances, farm; 37, 61
Expansion joints; 104
Farm entrance culverts; 37
Federal administration; 17
Fillers; 118
Finance, highway; 19
Fine aggregate; 101
Finishing surface of concrete; 122
Foundation, asphaltic concrete; 129 brick; 109 macadam; 93 mixed macadam; 126 penetration macadam; 123 Telford; 94
Gang maintenance; 131
Grader, Maney; 67 use of; 69
Gravel, ideal; 81 natural; 83 roads; 74
General taxation; 24
Good roads and commerce; 7
Green concrete bedding course; 111
Highway administration; 13
Highway finance; 19 maintenance; 130
Importance of design; 30
Ideal road gravel; 81
Inter-city traffic; 5
Inter-county and inter-state traffic; 5
Internal resistance; 50
Intersections; 46
Laying tile; 35
Length of culvert; 37
Liquid asphalt; 118
Local farm to market traffic; 4
Macadam; 89
Maintenance, concrete; 105 earth roads; 70 general; 131 gravel roads; 88 macadam; 96 of highways; 130 patrol; 131
Maney grader; 67
Marginal curb; 113
Measuring materials; 101
Metal pipe; 38
Mixing wearing surface; 127
Mixtures; 117
Natural asphalt; 117 gravel; 79
Necessity for planning; 42 drainage; 29
Patching; 122, 125
Patrol maintenance; 131
Pebbles, size of; 80
Petroleum asphalt; 117
Placing asphaltic concrete; 129
Placing broken stone; 94
Placing concrete; 102, 103 mixed macadam; 127
Plans for roads; 43
Preliminary investigation; 44
Preparation of earth foundation; 102 of road; 85
Private entrances; 61
Properties of stone; 90
Proportions for concrete roads; 101
Purpose of highways; 1
Reinforced concrete box culverts; 39
Reinforcing; 104
Repressed brick; 107
Road oils; 117
Road plans; 43
Rocks, kind of, for macadam; 91
Rolling, macadam; 95
Rolling resistance; 50
Run-off; 31
Rural education; 6
Rural social life; 7
Safety consideration; 58
Sand bedding course; 111
Sand clay and gravel road; 74
Sand mortar bedding course; 111
Seal coat; 127
Serial bonds; 27
Sinking fund bond; 25
Slip scraper; 67
Special assessments; 19
Specifications; 119
Spreading screenings; 95
State administration; 16
Stone, use of; 92
Surface drainage; 30
Surfaces, bituminous; 120
Surface method; 87
Superelevation; 47
Tests, brick; 108
Tile drains; 35
Topeka asphaltic concrete; 128
Tractive resistance; 52
Trench method; 85
Truck operation costs; 9
Types of culverts; 38
Underground water; 34
Undulating roads; 58
Use of blade grader; 69
Utilizing natural gravels; 83
Value of earth roads; 73
Variation in rainfall; 64
Variation in soils; 63
Vehicle taxes; 24
Vertical fiber brick; 107
Vitrified brick roads; 106
Vitrified brick; 106
Water gas tar; 117
Width of roadway; 59
Wire-cut-lug brick; 108
Zone method of assessing; 20
* * * * *
[Transcriber's Notes:
The transcriber made these changes to the text to correct obvious errors:
1. p. 5, accomodate —> accommodate 2. p. 39, guage —> gauge 3. p. 46, enbankment —> embankment 4. p. 63, tought —> tough 5. p. 68, absorbant —> absorbent 6. p. 73, persistant —> persistent 7. p. 77, indispensible —> indispensable 8. p. 119, aspealt —> asphalt 9. p. 127, repaid —> rapid 10. p. 130, Vetrified brick; 105 —> Vitrified brick; 106 11. p. 130, Virtical —> Vertical
End of Transcriber's Notes] |
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