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Surface Course

asphalt, material, rock, binder, rolling and roller


Transportation. The materials for the binder and surface of asphalt pavements must be carried from the mixing plant to the street in some form of truck or wagon which will admit of the materials being delivered with small loss of temperature. Some form of dump wagon is commonly employed for this purpose, carrying from 2 to 4 tons of the materials at a load. The loss of heat is not rapid when the material is carefully handled and properly protected by tarpaulins, and the temperature of the mass should not be reduced more than about 10 degrees, where transportation to the street takes 2 or 3 hours.

Placing. As soon as the rolling of the binder course has been completed, it is ready for the surface layer. This is usually If to i inches thick where a binder course is used, or 2 to 2i inches in single course work. The surface material is distributed by hot shovels from the piles into which it is dumped from the wagons, all the material being handled over as in the case of the binder. It is then spread into a smooth layer of proper thickness, with hot rakes, all lumps being broken and the material loosened up so that under the roller it may compact to a uniform density. After raking smooth, the surface is rolled with a steam roller. A light roller (2 to 4 tons) is commonly used for the first rolling until the material is sufficiently compact to bear the heavier one (usually weighing 6 to 8 tons), which completes the shaping of the pavement. A coating of dust, usually hydraulic cement, is given to the surface before the final rolling. This gives proper color to the surface.

The handling of the material necessarily varies some what with its character and requires, for good results, skill and experience on the part of the men in charge of the work. It is highly important that the material be so evenly distributed as to give a surface of uniform density; otherwise the surface may compress unequally under the traffic, becoming uneven and wavy. It is

also necessary that the rolling be carefully done in order to properly compress the asphalt and bring the surface to the required form. When the surface is rolled out of shape through careless handling, it is difficult to bring it back again. The roller must be so balanced as to distribute the weight uniformly, a pressure of 200 to 30o pounds per linear inch of tire being required for the ultimate compression of the asphalt surface.

Rock asphalt. Pavements of rock asphalt are con structed in the same manner as those from free bitumen. The rock asphalt makes a harder surface and is more slippery than that made from free bitumen. It has never come so extensively into use in the .United States. In Europe, where rock asphalt is very exten sively used, pavements made from free bitumens mixed with sand are frequently denominated artificial asphalt as distinguished from asphalt or natural asphalt, by which is meant the rock found impregnated with bitumen.

In the European rock asphalt pavements the binder course is not so commonly employed as in the United States, and in many cases the finishing of pavement is by means of tampers and smoothing irons instead of rollers, the compression given to the surface not being so great, ultimate compacting being accomplished by the traffic. At the edges of the pavement and in places which cannot be reached by the roller, small hand tools such as hot smoothing irons and tampers are employed for finishing the surface.' Sometimes also where the rolling has failed to compress the pavement into proper surface, it may be necessary to soften the surface with smoothing irons in order to reduce it to the required form.