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Compacting the Road

material, voids, stone, binding, rollers and surface


The materials may be compacted in a road either by placing them in position and allowing the traffic to pass over them or by rolling with a steam or horse roller.

The first method by itself is seldom practiced when it is possible to avoid it. It is hard upon the traffic, takes a long time to reduce the road to compact con.. dition, and a smooth surface is with difficulty pro duced. Where heavy horse rollers are employed they are clumsy and inconvenient to handle, and the work of rolling is slow as compared with the steam roller. In many instances, however, good results are obtained with them. They are not so expensive in first cost as steam rollers, and have not the disadvantage of fright ening horses.

Horse rollers are usually arranged so that the direc tion of motion may be reversed without turning the roller itself around, and also so that the weight may be changed by placing additional weight inside the roller or removing it. Horse rollers for this purpose usually bring a pressure of from 125 to 25o pounds per linear inch upon the road and weigh from 3 to 6 tons.

Steam rollers weighing from 8 to 15 tons are most commonly employed for compacting the road mate rials. They have the advantage of forcing the materials at once into a firm and compact mass and producing a smooth surface for the immediate use of travel: They admit also of the use of hard materials for binding. These rollers give a pressure under the drivers of from 40o to 65o pounds per linear inch.

The stone forming the body of the road should be placed and partially compacted before the addition of the small material, which may then be worked into the spaces between them.

The office of the binding material is to hold the stones in place and form a bearing for them, as well as to prevent the passage of water between them. It has no duty to perform in sustaining the loads. This is the objection to having the binding material mixed with the stones in advance, as would be the case when unscreened stone is used. A portion of the road stones

would be replaced by small material instead of having this material only in such voids as necessarily exist be tween the stones.

The quantity of binding to be used is that which will be barely sufficient to fill all the voids in the larger material. It has been contended that the lower por tion of the road should be porous in order to facilitate the escape of any water that may find its way through the surface, but the tendency of the best modern prac tice is in the direction of filling all the voids as nearly as possible, thus making the entire road practically one solid body, and it is now commonly agreed that the sur face of a properly constructed broken-stone road is very nearly impervious to water.

The voids in loose broken stone comprise about 40 to 50 per cent of the volume. In the stone when compacted in the road the voids are somewhat reduced, probably ranging from 30 to 40 per cent of the volume. The voids may be approximately determined in any case by filling a measure with the stone, shaken down as closely as possible, and then measuring the quantity of sand that can be added in the same manner.

In constructing a road with the use of a steam-roller, the road-stone is first put on to the required thickness and the roller passed over it to settle the stones into place and reduce the voids as much as possible. The binding material, representing a volume about equal to the voids in the stone, is then added, sprinkled, and rolled until the small material is washed and forced into the interstices, giving a smooth, hard surface. This is repeated for each layer of stone, or in some cases the small material is applied only to the top layer.

A thin coating of the binding material is then spread upon the surface and the road thrown open for travel.