48 BULLETIN 407^ U. S. DEPAETMENT OF AGEICULTTJRE. 



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 dehvered 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 

 dehvering 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 difficulty 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 hke amount. The reason for 

 this is explained later. 



At the time of the inspection the surface was practically iutact over 

 the area of the original experiment. But owing to the imequal 

 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 bitmnen, 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. 



