164 



AGRICULTURAL ENGINEERING 



protect the macadam roads which are now in use, but 

 another form of construction must be adopted for new 

 roads. At the present time a rather large number of mate- 

 rials are being used as binders experimentally. One class 

 of these binders is known as bitumen, which includes 

 not only the natural asphalt products but also similar 

 material obtained from gas plants in the nature of tars. 

 In addition to bitumen, various grades of oils are sometimes 

 used to protect roads. Some of these are known as dust 

 preventives. 



Method of Constructing Bituminous Macadam Roads. 

 There are two general methods of constructing bituminous 

 __^^^^^^^^ macadam roads. One is 

 known as the penetration 

 method, and the other 

 the mixing method. In 

 the penetration method, 

 the foundation or sub- 

 grade is prepared sub- 

 stantially as for the 

 water-bound macadam 

 road, and the first or 

 second layer of stone 

 also applied in the same 

 way. On the second, 

 or upper, course or layer, 

 bitumen is applied at 

 various rates, averaging 

 perhaps !}/ gallons per 

 square yard. Following 

 this a layer of stone 

 chips is applied, and then another layer of bitumen at the 

 rate of perhaps % gallon per square yard. 



Fig. 86. Model of a bituminous macadam 

 road made by the penetration method. A 

 represents the prepared sub-grade. B rep- 

 resents the first course of stone, and C the 

 second course. D shows the first applica- 

 tion of bitumin. E shows the application 

 of a course of stone chips. F shows sec- 

 ond application of bitumen. G shows the 

 completed road with a layer of clean stone 

 chips, lightly rolled. (Bui. 36, Office of 

 Public Roads, U. S. Dept. of Agr.) 



