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 poured 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 in 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 H 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 w^ide, 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 
