HIGHWAY CONSTRUCTION 



109 



quality of the refined asphaltum. Since agitation has. been adopted, 

 however, the greater part of the earthy and siliceous matters is retained 

 in suspension; and it has come to be considered just as desirable for 

 a part of the surface mixture as the sand which is subsequently added. 

 The refined asphaltum, if for local use, is generally converted into 

 cement in the same still in which it was refined. 



The average composition of both the land and lake varieties is 

 shown by the following analyses : 



Average Composition of Trinidad Asphaltum 



The characteristics of refined Trinidad asphaltum are as follows: 

 The color is black, with a homogeneous appearance. At a, tempera- 

 ture of about 70 F., it is very brittle, and breaks with a conchoidal 

 fracture. It burns with a yellowish-white flame, and in burning emits 

 an empyreumatic odor, and possesses little cementitious quality. To 

 give it the required plasticity and tenacity, it is mixed while liquid with 

 from 16 to 21 pounds of residuum oil to 100 pounds of asphaltum. 



The product resulting from the combination is called asphalt 

 paving-cement. Its consistency should be such that, at a temperature 

 of from 70 to 80 F., it can be easily indented with the fingej^, and on 

 slight warming be drawn out in strings or threads. 



Artificial Asphalt Pavements. The pavements made from Trini- 

 dad, Bermudez, California, and similar asphaltums, are composed of 

 mechanical mixtures of asphaltic cement, sand, and stone-dust. 



The sand should be equal in quality to that used for hydraulic 

 cement mortar ; it must be entirely free from clay, loam, and vegetable 



