DUST PREVENTION AND EOAD PRESERVATION. 
19 
The specification required that the bituminous material should be 
allowed to lie on the road surface for from one to three hours before 
being covered. It was soon noted, however, that on warm days a 
part of the material was lost by drainage over the shoulders and the 
practice of immediately covering the application with screenings or 
gravel was therefore adopted. Approximately the south half of each 
experiment was covered with trap rock screenings, while the north 
half was covered with fine river gravel. The latter material, used 
in the first three experiments, tended to run somewhat coarser than 
that obtained later, but no appreciable difference in results was noted 
after the road had been subjected to traffic for a short time. 
The mechanical analyses of the average run of trap rock and gravel 
are given in Table 20. 
Table 20. — Mechanical analyses of covering materials used in Rockville 
experiments. 
Material. 
Trap 
rock. 
Gravel. 
Passing f-inch, retained on £-inch screen 
Passing J-inch, retained on ^-inch screen 
Passing i-inch, retained on -J-inch screen 
Passing §-inch screen 
Total 
Per cent. 
30.5 
65.3 
3.7 
Per cent. 
5.2 
62.0 
29.0 
3.8 
100.0 
100.0 
Experiment No. 1. — Refined coal tar — Hot application. 
Location: Station 0+15 to station 61+20. 
Length: 6,105 feet. 
Bradley Lane was also treated for a width of 10 feet and a length 
of 665 feet east from Rockville Pike. 
Date of application: From September 9 to September 22. 
Temperature of tar: From 165° to 200° F. 
Table 21. — Analysis of refined coal tar used in experiment No. 1. 
Specific gravity 25°/25° C 1. 204 
Float test at 32° C. (time) 1/32" 
Per cent of free carbon 14. 93 
Distillation: By volume. By weight. 
Water percent.. 1 1.6 1.3 
First light oils to 110° C Trace. Trace. 
Second light oils 110°-170° C per cent. . .5 .4 
Heavy oils 170°-270° C do. . . . 2 13. 7 n _ 9 
Heavy oils 270°-315° C do s 7. 2 6. 3 
Pitch residue '. do ' * 77. 79. 8 
Total do 100. 99 . 7 
1 From leaky coils in tank car. 2 Four-fifths solid. 3 One-fourth solid. * Fairly glossy; brittle. 
