8 BULLETIN 257, XJ. S. DEPARTMENT OP AGRICULTURE. 



The sand used, from which roots and grass were removed by the use 

 of a f -inch screen, was a clean, white silica obtained at the roadside. 



Dustless screenings, so-called, were obtained by rescreening on a 

 J-inch screen all rock that had passed the §-inch screen. All material 

 passing the f-inch mesh was classed as "extra fines." About 33 per 

 cent of the rock passed the J-inch screen, of which one-half was 

 recovered on the J-inch screen. There was no appreciable change 

 in volume between the pit run and the screened rock above one-half 

 inch. 



It was found necessary to extend the width of the subgrade from 

 12 feet to 15 feet on account of the thin depth of the original rock 

 outside the former width. This was done by trenching the sides and 

 backfilling with sufficient rock to give the required grade. About 15 

 per cent of the total cost of the work was expended in this manner. 



Experiment No. 1 (Residual Asphaltic Petroleum — Penetration Method). 



Location: Station 0+00 to station 1+02=102 feet. 

 Area: 170 square yards. 



Table 4. — Analysis of residual asphaltic petroleum. 



Specific gravity 25°/25° C 0. 989 



Flashpoint °C. 130 



Burning point °C . . 155 



Viscosity, Engler, 100 c. c. at 100° C, specific 9. 8 



Float test at 32° C. (time) 44" 



Float test at 50° C. (time) 27" 



Loss, 5 hours at 163° C, 20 grams per cent. . 13. 74 



Float test on residue at 32° C. (time) 7' 49" 



Float test on residue at 50° C. (time).. 2 / 0" 



Percentage of total bitumen insoluble in 86° B. naphtha 10. 05 



Fixed carbon per cent. . 4. 68 



Soluble in CS 2 (total bitumen) do 99. 92 



Organic matter insoluble do 05 



Inorganic matter insoluble do 03 



Total... 100.00 



Method. — The screened-rock course was lightly rolled to bring the 

 surface to a smooth cross section and leave an open surface which 

 would allow deep penetration of the bitumen. The rate of applica- 

 tion for the penetration course was 1.76 gallons per square yard, at 

 an average temperature of 95° C. No excess of bitumen remained 

 on the surface, which was then covered with sand and thoroughly 

 rolled. Before applying the seal coat all free sand was swept from 

 the surface with rattan brooms. The rate of application of the seal 

 coat was 0.56 gallon per square yard. A top dressing of sand was 

 applied and the surface thoroughly rolled. 



A. From stations, + 00 to + 50, the seal-coat application was 

 allowed to stand two hours before sanding. 



