ROADS AND PAVEMENTS 407 



In 'order to effect a more complete bond, an intermediate 

 layer of bituminous concrete known as the binder course, is 

 commonly placed between the concrete foundation and the 

 asphalt wearing surface. It is composed of clean broken stone 

 of small size mixed with bituminous paving cement. The 

 stones should vary in size from $ in. in smallest to 1 in. in great- 

 est dimension, and should be thoroughly screened. The stones, 

 which are heated to a temperature of from 230 to 300 P., 

 should be mixed with the paving cement in the proportion of 

 from | to 1 gal. of cement to 1 cu. ft. of stone. This mixture 

 should be spread, while hot, on the base course to such a depth 

 as will consolidate to a lihickness of about 1$ in.; it should then 

 be rammed and rolled, before it loses its plastic condition, until 

 thoroughly compacted. The binder course is substantially 

 the same for both a hydraulic and a bituminous base. 



The material for the wearing surface is laid on the foundation 

 or binder course, sometimes in one coat and sometimes in two 

 coats. When one coat is laid, the ingredients are made up by 

 either one of the following two formulas: 



Proportions 

 Indents pgf ^ 



f Asphaltic cement 12 to 15 



I < Sand 83 to 70 



[ Pulverized carbonate of lime 5 to 15 



{Asphaltic cement 13 to 16 

 Sand 63 to 58 

 Stone dust 28 to 23 

 Pulverized carbonate of lime 3 to 5 



When two coats are laid, the first coat should contain from 2 

 to 4% more asphaltic cement. The asphaltic cement and 

 the sand should be heated separately to a temperature of about 

 400 F. The proper amount of pulverized carbonate of lime, 

 while cool, should be mixed with the hot sand. This compound 

 should then be mixed with the asphaltic cement at the required 

 temperature and in the right proportions. In order that the 

 materials may be properly mixed, a special apparatus suited 

 to the purpose should be used. 



Laying Asphalt. Two Coats. The first coat of asphalt is 

 called the cushion coat, and the second the surface coat. The 



