48 BULLETIN 407, U. S. DEPARTMENT OF AGRICULTURE. 
SUPPLEMENTARY REPORT OF EXPERIMENTS ON ROCKVILLE PIKE, 
MONTGOMERY COUNTY, MD., 1913. 
The original report of these experiments was given in U. S. Depart- 
ment of Agriculture Bulletin No. 105, and report of the first inspec- 
tion appeared in Bulletin No. 257. The following information covers 
the history of the experiments up to February 28, 1916: 
Experiment No. 1. — Refined Coal Tar, Hot Application. 
As expected at the time of the last report, this experiment required 
re-treatment during the summer of 1915. The work was done 
between July 12 and 23, using the same bituminous material as in 
the original treatment. The tar was delivered directly from the stills 
of the manufacturer in tank wagons, from which it was distributed 
under steam pressure at a temperature of 165° to 200° F. A nozzle 
delivering a conical spray was used and steam was furnished by a 
road roller which also drew the tank wagon. Since unskilled labor 
only was available, considerable difriculty was experienced in getting 
uniform distribution. Clean, washed gravel passing a f-inch mesh 
was used as covering material over the entire experiment, as it is 
considered no longer warrantable to continue the use of trap rock on 
those sections where it was originally used. The characteristics of 
a typical sample of the bituminous material are' here given, Table 48, 
and the cost data and quantities will be found in Table 55. 
At the time of this re- treatment experiment No. 1 was extended 
northward from station 61+20 to the forks of the road at station 
90 + 75, thus adding 2,955 feet to experiment No. 1 and reducing 
the length of experiment No. 2 by a like amount. The reason for 
this is explained later. 
At the time of the inspection the surface was practically intact over 
the area of the original experiment. But owing to the unequal 
distribution of tar by the conical hand nozzle used, and the conse- 
quent rather large variation in the amount of covering material held 
by the bitumen, the road surface plainly shows a continuous series of 
arc-like curves. Some of these are little more than visible and produce 
no marked vibration as traffic passes, but much the greater part are 
developing as flat ridges, which are decidedly perceptible to the 
occupants of a vehicle passing rapidly along the road. From present 
indications the lower areas, where the mat is thinner, are going to 
wear rapidly, and the amount of repairing necessary will correspond- 
ingly increase. 
