Page:Popular Science Monthly Volume 61.djvu/227

Rh not only attach the surface to the concrete, but will bold the surface in place and keep it from sliding from the center towards the gutters. The binder consists of small broken stone, which should be sound and clean. Each piece should be completely covered with a soft asphaltic cement containing more residuum oil than the surface mixture. The stone and cement should be thoroughly mixed by machinery at a temperature that will render the bitumen perfectly fluid, but not sufficiently high to burn or otherwise injure it. It is then dumped into carts and carried to the streets, where it is spread with hot rakes, and rolled to the proper contour.

The binder is an important part of an asphalt street surface, and should be carefully compounded and laid. If it is deficient in bitumen, it will absorb bitumen from the surface, causing the surface to crack and disintegrate. If it is well supplied with bitumen that is not too soft, the surface is preserved from becoming too dry by absorbing bitumen from the binder, or, in the technical phrase of the art, 'is nourished from the binder.'

The binder being laid and rolled presents a surface that should be exactly parallel to the surface of the finished street. Upon this surface the surface mixture of asphaltic cement and sand is brought in carts, while still hot, and is spread also with hot rakes and rolled, first with hand rollers and finally with heavy steam rollers, until cold. The rollers are prevented from sticking by strewing the surface with hydraulic cement or fine sand. The rolling is an.important element of a good asphalt-surfaced street, as upon that depends the complete solidity of the surface mixture.

As both good and bad streets have been made of about every variety of natural bitumen in the form of asphalt or asphaltum that can be had, the quality of a street appears to depend quite as much upon the technical skill of those who lay the street as upon the kind of material of which it is constructed. Of the various substitutes now being offered for natural bitumens too little has been demonstrated by use to warrant any conclusions concerning them.