24 BULLETIN 407, U. S. DEPARTMENT OF AGRICULTURE. 
EXPERIMENTS ON FALLS ROAD, MONTGOMERY COUNTY, MD. 
SURFACE TREATMENT (OILS, TAR PREPARATIONS, REFINED TARS). 
In the summer of 1913 a series of bituminous surface treatments 
was applied to Rockville Pike in Montgomery County, Md. The 
original report of these experiments is given in U. S. Department of 
Agriculture Bulletin No. 105 and a subsequent report of behavior 
was made in Department Bulletin 257. In order to make it possible 
to check the data secured from several of these experiments, another 
series of comparative test sections was constructed on the Falls 
Road, leading from Rockville to Potomac, Md. In the Falls Road 
series, sections 1, 2, 4, and 5 were treated with bituminous materials 
similar to those used on Rockville Pike sections 3, 5, 2, and 1, respec- 
tively. The coal-tar preparation selected for Experiment No. 3 on 
Falls Road had not been used in any previous service tests in this 
vicinity. Conditions surrounding the two experimental roads are 
much the same, except that traffic is lighter and the proportion of 
horse-drawn vehicles greater on the Falls Road. 
The specifications prescribed a uniform application of one-half 
gallon of bituminous material per square yard, but the amount was 
varied according to the surface condition of the several sections. The 
road surface was dry when the applications were made, and fine 
gravel was used to cover the bituminous material. The mechanical 
analysis of the gravel used is given in Table 24. 
The bitumen was transported to the road and spread by means of 
two 1,200-gallon automobile distributors equipped with speed and 
pressure gauges. The length of the nozzle-line was so adjusted that 
one-half of the width of the road was sprayed at one time. The 
nozzles were of the right-line type, from which the bitumen was ap- 
plied in a fan-shaped spray under a uniform pressure of approximately 
25 pounds. 
The Falls Road was reconstructed in 1913, and surfaced with 
water-bound macadam 15 feet wide. Early in the summer of 1915, 
when the experimental test sections were planned, the macadam 
surface had become thoroughly compacted, and, with a few excep- 
tions, was smooth, free from ruts or depressions, and in good condi- 
tion to receive the bituminous material. In August the contractor 
distributed all the gravel required for the work with a two-ton motor 
truck before he began the application of the bituminous materials. 
The action of this truck was so severe that when the gravel had been 
distributed, the water bond in sections 1, 2, and part of 3 was com- 
pletely broken, and several weak places in the foundation in section 
2 had formed. The latter were repaired, and an effort was made to 
reestablish the water bond. Owing to the protracted drought and 
the disturbing action of traffic, this work was only partially sue- 
