12 BULLETIN 257, U. S. DEPARTMENT OP AGRICULTURE. 



Method. — The work on the experimental road was discontinued 

 from April 16 to May 18, owing to delay in the arrival of the oil 

 asphalt. During this time heavy rains had rendered the prepared 

 rock surface unfit for penetration work, and it was therefore har- 

 rowed, regraded, and rolled to proper condition from station 5 + 84 

 to station 8 + 60. The larger voids were partly filled with dustless 

 screenings. The rate of application of bituminous material was 1.75 

 gallons per square yard, and the average temperature was 175° C. 

 The penetration was not good, owing to the rapid congealing of the 

 oil asphalt on a dense surface of this type. The top dressing of dust- 

 less screenings was immediately applied in sufficient quantity to fill 

 the voids and thinly cover the surface, which was then thoroughly 



rolled. 



Experiment No. 6 (Oil Asphalt — Penetration Method). 



Location: Station 6+32 to station 7+17=85 feet. 



Area: 141.6 square yards. 



Material: Oil asphalt, analysis of which is shown in Table 7. 



Method. — The rock course was compacted with a rather heavy 

 rolling. An application of oil asphalt was made at the rate of 1.52 

 gallons per square yard and at an average temperature of 185° C. 

 This was immediately covered with sand. As in experiment No. 2, 

 the sand did not become incorporated with the bitumen, but remained 

 to a large extent dry and loose. All free sand was swept entirely 

 off the road, and a sand-asphalt mixture was spread upon the penetra- 

 tion course. This was composed of 1 cubic foot of roadside sand to 

 1 gallon of oil asphalt, both at a temperature of 175° C. The mix- 

 ture filled the interstices and formed a wearing surface about one- 

 quarter of an inch thick above the topmost particles of the rock. 

 It bonded with the lower course both by irregularities in the latter 

 and by reason of the melting of the first bituminous coat through 

 contact with the hot mixture. 



Experiment No. 7 (Water-Bound Macadam). 



Location: Station 7+17 to station 8+60=143 feet. 

 Area: 238.3 square yards. 



Method. — As in the previous experiments, the subgrade was pre- 

 pared and compacted parallel to the cross section of the finished road, 

 and a course of screened rock spread to a uniform depth of 3 inches. 

 This was heavily rolled until there was no visible yielding under the 

 roller. No material was available from the first screening through 

 a |-inch screen. Dustless screenings and dust were therefore mixed 

 in equal parts and spread over the surface, after which the roadway 

 was flushed with water and thoroughly rolled. 



