DUST PREVENTION AND ROAD PRESERVATION, 1915. 51 
during the coming summer. The surface has not become so muddy 
at any time during the winter of 1915-16 as during that of 1914-15. 
The bumps due to faulty distribution at the time of original treat- 
ment are still visible and cause considerable trouble. They have 
been repeatedly cut and trimmed, but owing to the excess of bitu- 
minous material the spots have required top dressing in the hottest 
weather, and this builds them up again. There has been considerable 
repair work necessary over all of the experiment, but no place shows 
marked weakness. Patches made on this experiment are unusually 
smooth and serviceable, and so far there appears no tendency for 
the original surface to break or give way around patches. 
Experiment No. 5. — Residual Asphaltic Petroleum, Hot Application. 
This experiment, like No. 4, shows less tendency to become muddy 
during thaws, although dampness of subgrade continues to be very 
noticeable. During the past summer a great deal of patching has 
been necessary and the breaks have been somewhat more numerous 
in the bottoms and poorly drained sections than elsewhere, although 
they have been general. It is now expected that the experiment 
will require re-treatment during the coming season. 
Experiment No. 6. — Water-Gas Tar Preparation, Hot Application. 
This experiment required but little attention during 1914, but 
early in the spring of 1915 it began to break up rather rapidly. 
The nature of the failure was very similar to that described for 
experiment No. 2 in 1914. Breaks in the mat at the sides of the 
road progressed rapidly toward the center and any raw edge of the 
mat had a tendency to crumble away rapidly. It was always neces- 
sary to give immediate attention to patching, and even with care 
in some instances patches had to be made that extended from the 
sides nearly to the center of the road and were from 4 to 6 feet long. 
It finally became necessary to re- treat November 8 to 12. The 
work was done by contract, using the same type of bituminous 
material as originally applied and covering with clean, washed 
gravel passing a f-inch screen. The contract price was $1,170 or 
11.1 cents per square yard. The characteristics of the material 
used are shown in Table 50 and additional data are given at the end 
of this report. 
Table No. 50. — Analysis of water-gas tar preparation used on experiment No. 6. 
Specific gravity 25°/25° C 1. 136 
Float test at 32° C, time 2' 11" 
Free carbon (insoluble in CS 2 ), per cent 1. 71 
