Binding the Road for Broken-Stone

The binder is applied by spreading a layer of " fines " about half an inch thick over the partially rolled surface. The filler should be dumped upon a board platform, and not directly upon the road surface; and should be distributed evenly over the stone with a shovel. Under no consideration should loam or vegetable matter be allowed to contaminate the stone screenings. After the binding material has been evenly distributed, the surface is then sprinkled and rolled. The sprinkler should have many fine openings, the object being to give a gentle shower rather than a violent flooding. The water washes the fine material into the cavities below, and the roller crushes the small fragments and makes more dust. The rolling also aids in working the binder into the mass; in fact, the binder can be worked in to a considerable extent by dry rolling, and consequently the quantity of water used varies widely with the method of doing the work, but is usually about 4 to 6 cubic feet per cubic yard of stone. Sometimes men with heavy brooms are kept upon the road sweeping the binding material about to assist in working it in, and also to secure a more uniform distribution of it. While applying the screenings care should be taken to pick off any coarse stone—particularly flat ones,—as they do not bind well and their subsequent loosening causes the road to ravel 377).

As the rolling and sprinkling proceed, fine material should be added where needed, i. e., as open spaces appear. All the filler should not be put on in the beginning, since a thin layer can be worked in to better advantage than a thick one; and, besides, it is desirable to use only enough to fill the voids.

Occasionally the surface of the road becomes muddy and sticks to the roller. This can be remedied in either of two ways: viz., by sprinkling the roller and keeping it constantly wet, or by keep ing the sprinkling wagon immediately in front of the roller and having the binder always fully saturated. The rolling is con tinued until the water is forced as a wave in front of the roller and until the surface behind the roller is mottled or puddled and is covered with a thin paste. The binding, or the puddling of the surface, can not be done satisfactorily when the surface freezes nightly.

When finished, if the road is allowed to dry and is then swept clean, the surface will be seen to have the appearance of a rude mosaic, the flat faces of the fragments of stone being crowded against one another and the interspaces being filled with the bind ing mat eiial—the latter occupying about half of the area. Such a surface when dry will stand considerable sweeping with a steel broom or brush without the fragments of stone being loosened. The water used in construction not only aids in working the binder into the interstices, but also develops the cementing power of the rock dust.

Usually after the rolling has been completed a thin coat ing of binding material is sprinkled over the surface. Authorities differ as to the amount of fine material to be left on the finished surface, some specifying as little as * inch and some as much as 1 inch, the usual quantity being f to I inch. If only enough binding material to fill the interstices between the coarser fragments is left upon the road, the fine material will be blown and washed away, and soon there will not be enough to level up between the large bits and to hold the surface stones in place, when the wear will come directly upon the stones. On the other hand, if any con

siderable quantity of fine material is left upon the surface, it is speedily ground up, and becomes offensive dust if it is not sprinkled, and equally objectionable mud if it is sprinkled. It is probably best to put on a quantity just sufficient to give a thin layer, say, to 3/8inch, over the surface, and when this amount is blown or washed away renew it. By this Method, the wear on the body of the road will he prevented, a minimum amount of sprinkling will be required, and there will be as little dust as possible. The surface coat is also serviceable in decreasing the tendency of the binding material to dry out and to lose part at least of its cementing power; i. e., the surface coat is serviceable to prevent the raveling of the road (see § 377). Fine material over and above that required to fill the interstices is useful only to prevent raveling and to keep the wear from the surface of the stone; and therefore sand is as good for the top dressing as stone dust, and is usually much cheaper. Loam or clay does fairly well for a top dressing, but it readily grinds to dust and blows away when dry, and when wet makes mud. It is desirable that this coat of fine material shall be sprinkled and rolled before the traffic is admitted.

The road is now finished; and after it has dried out for a day or two, it may be thrown open to traffic.

Amount of Binder.

The amount of binder required de pends upon the hardness of the stone and the amount of rolling preceding the application of the binder (see § 347). The voids in the broken stone can be reduced by rolling to 20 or 25 per cent, say 22 per cent, of the compacted glass;* and the completed road will contain 4 to 7 per cent, 5 per cent, of voids; t and therefore enough binder must be added to fill about 17 ( =22 — 5) per cent of voids. The binder itself usually contains 40 to 50 per cent of voids. and therefore the volume of filler required is 40 to 50 per cent more than the voids to be filled, i. e., 40 to 50 per cent more than 17 per cent of the original volume of stone; or, in other words, the amount of filler required is 25 to 35 per cent of the thickness filled. This allows a little for waste and for the thin coating spread upon the finished surface. If the binder is applied before the rolling has progressed very far, more fine material will be required, since some of it will work in between the fragments of stone and prevent them from coming into as close contact as they otherwise would. In this case, part of the surplus binder will be flushed to the surface during the sprinkling and rolling, as mortar flushes to the surface in tamping concrete; but in no case does all the surplus thus work out, and consequently the road is not as durable as though only enough binder had been used to fill the voids; and, farther, the binder which flushes to the surface must be removed as mud. An excess of binder is often used to reduce the cost of construction by decreasing the amount of sprinkling and rolling required; but such a practice adds to the cost of maintenance, and the road is less durable and more dirty.