=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.
[Ill.u.s.tration: Fig. 18.--Cross Section for Concrete Highway]
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 coa.r.s.e 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 coa.r.s.e 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 coa.r.s.e 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 pa.s.s the one-fourth-inch sieve and that not exceeding about five per cent will pa.s.s 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 coa.r.s.e 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 part.i.tion so as to separate the fine and coa.r.s.e 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 s.p.a.ce 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 gla.s.sy 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 gla.s.sy 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.
s.p.a.cing lugs or raised letters are formed on one face of the brick to insure sufficient s.p.a.ce 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 princ.i.p.ally in the design or size.
=Repressed Brick.=--In this type of brick the s.p.a.cing 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 s.p.a.cing lugs.
=Vertical Fiber Brick.=--These brick are designed to be laid with one wire-cut face up and s.p.a.cing is provided by two or more beads on the side of the brick. Sometimes the vertical fiber brick has no s.p.a.cing lug, it being contended that the irregularities of the brick are such as to provide all of the s.p.a.ce required. In practice this does not always work out, as the brick are so regular in shape that when laid there is too little s.p.a.ce between the brick to permit the introduction of a suitable filler. The use of brick without s.p.a.cing lugs is just beginning and is not yet a generally accepted practice.