' A MODERN STREET. 221 



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 tem- 

 perature that will render the bitumen perfectly fluid, but not suffi- 

 ciently 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, 

 ii 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, 

 tne quality of a street appears to depend quite as much upon the techni- 

 cal 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.* 



* I wish herewith to express my obligations to the Warren-Scharf Asphalt 

 Paving Company for the illustrations accompanying this article. 



