Part 8 (2/2)
It is not uncommon to use the same size of stone for both upper and lower course and yet in many instances stone up to that which will just pass a 3-1/2-inch screen is used for the lower course Stone much smaller in size may also be used successfully, but if the stone is broken to a smaller size than is required, unnecessary expense is incurred
The bonding material is the finer portion of the product of the crusher, which is called screenings This material may be so finely crushed as to pass a one-fourth inch screen, or may be so coarse as to just pass a one-half inch screen, but in any case must contain all of the dust and fine material produced by the crusher
Where the soil and drainage conditions demand an especially stable foundation course, the Telford type is used The Telford foundation consists of a layer of stones of various diive a thickness of 8 inches These large stones are placed by hand and therefore the size require one dimension about 8 inches and the others not over 10 or 12 inches are satisfactory
=Earth Work=--A thoroughly drained and stable earth foundation is essential to success with thethe stone, the road rade reduction work completed Preferably heavy fills should have a year to settle before the macadam surface is placed Side ditches, necessary culverts and tile drains should be constructed as required for drainage The earth work is often carried out in connection with the construction of theIn that case, the fills must be carefully rolled as they are placed The road bed may be shaped in connection with the other earthwork If the road has been brought to a satisfactory grade so the macadam, the road bed for the broken stone will be prepared as needed for placing the stone
=Foundation for the Macadaenerally placed in a trench as described in the trench ravel It is an almost universal practice to co with an 8- or 10-ton power roller, and if the stone is not held between substantial earth ber merely serves to spread the stone out over the road bed instead of co it If an attempt isfoundation, no benefit results, but on the contrary the stone is likely to be forced down into the soil To insure that the layer of broken stone can be co, it is first necessary to roll the earth foundation until it beco places appear during the rolling these should be corrected by tile drains or by re with material that will compact when rolled
It is not always easy to determine why these soft places exist in what appears to be a well drained roadway, especially since they are as likely to be found on fills as anywhere else Apparently they are due to local pockets of porous soil held by denser soil so that the water does not readily drain away It is usually true that such places are observed during the season of frequent precipitationother seasons of the year
In dry cli suitable foundations for the broken stone road are largely eliminated, but it may be observed that this type of surface is not suitable for such climates unless some sort of bituminous binder is e power of the stone dust is inadequate when the surface is continually dry
[Illustration: Fig 17--Cross Section for Macadam]
=Telford Foundation=--When the Telford type of foundation is erade is prepared and then the Telford stone placed carefully by hand The spaces between the large stones are filled with the spalls broken fro them in place When completed the base is rolled with a heavy roller to secure a firht inches Any fairly sound stonethe Broken Stone=--It has been found ireater thickness than about 5 or 6 inches of loose stone, therefore, the stone for the macadam surface is usually placed in two layers, the first or lower layer being rolled before the next layer is placed The stone is hauled in duons, trucks or dump cars, dumped on the road bed and spread by hand rakes or by rader and is then rolled To insure the proper thickness the loads are accurately spaced to spread to the proper thickness
=Rolling=--A three-wheeled or ”macadam” type of roller, of the self-propelled type, is best for coht to fifteen tons, but for most conditions the ten or twelve ton size seems to be preferable On Telford base construction, a heavier machine is desirable and for very hard stone it may be successfully e the edge of the stone and each successive trip is made a little nearer the middle until finally one half of the strip of stone has been rolled The roller is then taken to the opposite side of the roadway and the operation repeated on the other half The rolling is continued until the stone is thoroughly compacted, which is evidenced by the fact that the roller ht track in the surface
The second layer of stone is then placed and rolled in the sas=--After the upper course has been rolled, the screenings are spread on it fro used to fill the voids in the layer of stone and furnish a slight excess As the screenings are spread they are rolled to work them into the voids When these are filled, the surface is sprinkled thoroughly by ain rolled In this way the dust and water are mixed into a mortar which fills the crevices between the stones Thisa bond that is weak, but sufficient for the purpose if the traffic is not too heavy A broken stone road finished in this way is called a water-bound macadam, and is ready for traffic in three or four days after completion
=Bituminous Surfaces=--On account of the inadequacy of the water-bound macadam when subjected to motor traffic and to obviate the tendency of broken stone surfaces to loosen in dry weather, there has been developed athe surface with a bituminous material such as tar or asphalt This will be described in detail in a later chapter
=Maintenance=--Even under favorable conditions as regards kind and amount of traffic the macadam road requires constant maintenance The first effect of traffic will be to brush away the fine er stones in such a way that they are rather easily loosened and removed from the surface by wheels and the hoofs of animals This finer material must be replaced as fast as it is removed so as to protect the surface Either stone dust or clayey sand may be used, but clay if used alone is likely to be sticky et and prove to be worse than the condition it was expected to correct In tiradual effect of wear, which will inevitably effect some portions of the surface more than others, or on account of subsidence of the foundation Uneven places are repaired by first loosening the stone, then restoring the cross section by adding newit in place
If a bitu has been applied, it will eventually peel off in places and these places must be recoated as soon as practicable
Eventually the surface will be worn to such an extent that an entirely neearing surfacethe entire surface to a depth of 3 or 4 inches and then adding a new layer of broken stone The loosening is sometimes accomplished by means of heavy spikes inserted in the roller wheels, and at others by means of a special tool known as a scarifier
The new surface is placed and rolled in precisely the sainal construction, but the layercourse A new course will not bond to the old surface unless the old hly broken up first
=Characteristics=--The water-bound h surface of low durability for rubber tired vehicles It has long been the standard rural highway for steel tired vehicles, but cannot carry any considerable amount of motor traffic It is easily repaired When finished with a bitureatly increased and the dust is eliid for truck traffic, unless placed on exceptionally good foundation
CHAPTER VIII
CEMENT CONCRETE ROADS
The cehway 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