Part 9 (1/2)
=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 soard for the effect of traffic on concrete surfaces In th in compression only is utilized, and the factor of safety is such as to eliminate to some extent failures due to inferior materials or workmanshi+p
The concrete road surface is subjected to co tension in the concrete, to abrasion from wheels, and to tension and coht 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 die in temperature introduce tension or compression into the road slab and rade subjects the slab to vertical itudinal and transverse cracks occur
The foregoing indicates the i, and that is accon of the road surface and by following established construction n=--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 thewith 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 unifor 2 inches The thickness of the two course pavele 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 by18 shows cross section for concrete roads
[Illustration: 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 ns 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 ravel Durability is necessary, but it is also important to have uniformity in the concrete so that the road surface ear uniformly and consequently keep smooth Supplies of broken stone are likely to contain a se 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 froravel 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 liravel deposits and practically all of the igneous rocks ates for the concrete road
Soates readily available are sufficiently durable or unifor surface of the concrete road, but a suitable aggregateconsiderable distances In such cases what is known as the two course type of concrete road is e course usually is about 2 inches thick and is constructed with selected aggregates of good quality shi+pped 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 saard for uniformity and durability that would be the case if they were for the one course paveenerally not sufficiently uniforards proportion of fine and coarse material to produce uniforates of that character is not per course, but under proper inspection they may be used for the lower course of two course paveenerally natural sand, but a s is soate of whatever character anic 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 soates Experience see surface should have a crushi+ng strength of at least 2500 pounds per square inch, and the ive the desired crushi+ng strength The common mixture for the one course pavement is one part cegregate
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, ates 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 ates, eid requiree of sizes, if the materials are carelessly proportioned at thequalities are to be secured throughout the entire area of the concrete road surface, successive batches of concretethe water in each batch of concrete ates are measured in various ways, all essentially alike in that the intent is to insure exactly the saredient 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 wheelbarroith a bed of such shape that the contents will be a er 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 ahly 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 Sorade, 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 forht equal to the thickness of the concrete road slab at the edge, and is shaped roughly by means of shovels
Variousthe surface to the exact shape desired and s methods are employed, a plank template is cut to the prescribed cross section and the concrete is shaped by drawing the te the side forvery slowly accompanied by an up and down motion that tends to ta a second time to smooth the surface finally
After the surface has been struck off by hand, it is finally sht hand roller about 8 inches in diaging 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 e The machine is power operated and is carried on wheels that run on the side forresses The concrete is tamped, struck off to shape and smoothed with the belt at one operation This er concrete than can be produced by hand finishi+ngConcrete for Two-course Road=--The methods employed for the two-course concrete road are much the same as for the one-course road