EOAD MODELS. 15 



The usual methods of construction are known as the mixing method 

 and the penetration method. In the mixing method a more or less 

 closely graded aggregate is mixed either by hand or machinery with 

 the proper amount of bituminous material before placing it on the 

 road. In the penetration method the wearing or second course of 

 stone is laid and rolled as in ordinary macadam; but instead of 

 applying the screenings, hot bituminous material is j:>oured or sprayed 

 over the stone in sufficient quantity to coat the fragments and pene- 

 trate the top course to a depth of 2 or 3 inches. In each of these forms 

 of construction it is customary to spread a second and light application 

 of hot bituminous material over the surface in order to fill the exposed 

 voids completely and make the surface waterproof. A sufficient 

 quantity of pea gravel or stone chips is then added to take up any 

 excess of bitumen, the surface is rolled thoroughly, and the road 

 thrown open to traffic. 



. PENETRATION METHOD. 



Plate VII, figure 2, illustrates the construction of a bituminous 

 macadam road according to the penetration method. 



Section A represents the prepared subgrade 16 feet wide, with the 

 crown one-half inch to the foot; section B, the first course of No. 1 stone, 

 4 inches hi thickness when compacted after rolling. This course is 

 partially filled with sand or stone chips when necessary to insure a 

 solid foundation. Section C shows the second course of No. 2 stone, 

 2 inches in thickness when compacted after rolling; section D, the 

 application of bituminous material at the rate of about 1J gallons to 

 the square yard; section E, a coating of stone chips which has been 

 rolled; section F, seal coat of bituminous material applied at the rate 

 of about one-half gallon per square yard ; and section G, the completed 

 surface, with clean stone chips lightly rolled. 



The construction of this road, as far as the completion of the No. 

 1 and No. 2 courses, is the same as an ordinary macadam road. 

 From that point it varies from the method of construction of a 

 macadam road in that hot bituminous material is flushed into the 

 No. 2. course before the screenings are applied. 



MIXING METHOD. 



Plate VIII, figure 1, illustrates the construction of a bituminous 

 macadam road according to the mixing method. 



Section A shows the prepared subgrade 16 feet wide, with the 

 crown three-eighths inch to the foot; section B, the foundation course 

 of No. 1 stone compacted to a depth of about 4 inches. This course 

 is partially filled with sand or stone chips, as shown in model, when 

 necessary to insure a solid foundation. Section C shows the wearing 

 course before and after rolling, consisting of a properly proportioned 



