404 JOSEPH STRUTHERS 



tice pavement, composed of coal or gas tar, sand, etc., was laid 

 in several cities during the period from 1870 to 1873, and in 

 1871 to 1873 successful experiments were carried out with an 

 artifical asphaltic sandstone pavement in New York city, and 

 in Newark, N. J. In 1878 a pavement composed of Trinidad 

 asphaltum, sand, and powdered limestone was successfully 

 introduced in Washington, D. C, and has since been the stand- 

 ard pavement of that city. At the present time more than 

 a himdred of the principal cities of the United States have 

 adopted some form of asphaltum pavement. 



In making asphaltum pavement the general procedure 

 is to grade the street to be paved, roll it with a steam roller, 

 and then cover it with a layer of cement concrete 5 or 6 inches 

 thick; or in case the street is macadamized or paved with stone 

 blocks, the concrete is unnecessary. In the latter cases a thin 

 layer of asphaltum concrete from 1 to 1.5 inches thick is laid 

 directly upon the old surface. The foundation ha v '^ ^ been thus 

 prepared, the asphaltum paving mixture, called the wearing 

 surface, is then spread over it with heated rakes to the desir- 

 ed thickness, which varies from 1.5 to 2.5 inches, depending 

 upon the traffic to pass over it. 



The ordinary Trinidad asphaltum paving mixture is made 

 by adding and thoroughly mixing with hot refined asphalt 

 about 15 per cent of its weight of residuum oil, the mixture 

 serving to cement together the sand and powdered limestone 

 which enter into the paving mixture. The proportions of the 

 ingredients vary according to their physical and chemical qual- 

 ity, as well as to the climate in which the pavement is to be 

 used, hot climates requiring less cement than cold. A typ- 

 ical pavement is composed of from 15 to 18 per cent of asphal- 

 tic cement, from 70 to 83 per cent of sand, and from 5 to 15 per 

 cent of limestone. 



For use in making pavements, the mined asphaltic lime- 

 stone is crushed into pieces not exceeding 2 inches in diameter, 

 and then reduced to about 10-mesh size in a ball or centri- 

 fugal pulverizer. The fine material is heated to 275° F. in 

 heaters, spread over the prepared roadbed, and compressed 

 by heated rammers, or otherwise, until it is of a thickness of 

 from 2 to 2.5 inches. This form of pavement is not popular 



