10 BULLETIN 257, U. S. DEPARTMENT OF AGRICULTURE. 



flowed readily and, after standing a few minutes, entirely pene- 

 trated into the rock course. Sand was then applied and swept into 

 the voids. The sand dressing, however, while filling the voids, did 

 not incorporate with the bitumen, but remained for the most part 

 dry and loose. The individual tar-coated rock particles were left 

 projecting and unprotected and would have worn away rapidly 

 under traffic. All free sand was therefore swept from the surface, 

 and a light coal tar was applied cold at the rate of 0.33 gallon per 

 square yard and allowed to stand for about four hours until com- 

 pletely absorbed. The surface was then covered with sand and 

 thoroughly rolled. The sample of the light tar was lost in shipment, 

 and consequently no analysis is given. 



Experiment No. 3 (Refined Coal Tar — Penetration Method). 



Location: Station 1+94 to station 3+68.8=174.8 feet. 



Area: 291.5 square yards. 



Material: Refined coal tars, analyses of which are given in Table 5. 



Method. — The general nature of this experiment is the same as that 

 of the preceding, except for the omission of a second application of 

 bituminous material and the use of dustless screenings instead of sand. 

 From station 3 + 25 to the end of the next experiment heavy rains 

 had so bonded the rock surface that it had to be harrowed, regraded, 

 and rolled in order to provide for suitable penetration. The smooth- 

 ness of the original cross section could not be restored, and the surface 

 of this portion of the road remains slightly irregular. The applica- 

 tion of bituminous material was made at the rate of 1.20 gallons per 

 square yard, and was immediately covered with dustless screenings, 

 filling the voids and thinly covering the surface, which was then 

 thoroughly rolled. The screenings crushed somewhat under the 

 roller, but seemed to absorb the bituminous material and form a 

 firm, rigid surface. 



Tar of lot No. 1 was used from station 1 + 94 to station 2 + 76. 

 From the latter point to the end of the section material from a second 

 shipment, designated in Table 5 as lot No. 2, was used. This mate- 

 rial did not flow and penetrate as satisfactorily as lot No. 1, even 

 when heated to 185° C. 



Experiment No. 4 (Residual Asphaltic Petroleum — Penetration Method). 



Location: Station 3+68.8 to station 5+84=215.2 feet. 

 Area: 358.6 square yards. 



Table 6. — Analysis of residual asphaltic petroleum. 



Specific gravity, 25°/25° C 0. 980 



Flash point °C. . 125 



Burning point °C . . 143 



Viscosity, Engler, 100 c. c. at 100° C, specific 5. 5 



