DUST PREVENTION" AND ROAD PRESERVATION, 1915. 13 
the mechanical analysis in Table 6. It was taken exclusively from 
the Garfield pit, where it was excavated by steam shovel and then 
hauled 1,000 feet to a storage pile near station 95-1-00. It was fed 
into the mixer through a 2-inch screen. 
When the gravel was heated the clay formed a fine powder or dust 
and much of it blew away in dropping the material from the storage 
bin into the weighing hopper and from there into the mixing box. 
The oil asphalt was mixed with the heated gravel in the proportions 
of 36.5 pounds to 500 pounds of gravel. The resulting percentage 
of bitumen by weight was approximately 6.8. Typical analyses of 
the mixture are given in Table 10. 
The method of placing the bituminous gravel concrete was exactly 
as described for the bituminous mixed macadam experiments with 
stone aggregate. The depth of spreading was approximately 3 
inches loose, and the compacted thickness was approximately 2 
inches. The mix was allowed to set from 1 to 2 hours before rolling, 
to avoid a wavy surface. 
The rolled surface before sealing presented a smooth, dense appear- 
ance, except for occasional small superficial pits. A seal coat of the 
same asphaltic material used in the mix was applied at a tempera- 
ture of about 300° F. at the rate of one-half gallon to the square yard. 
Clean pea gravel was at once spread uniformly and the surface rolled. 
The maximum grade for this section is 5.24 per cent. 
Experiment No. 10. — Bituminous Gravel Concrete. Pit-Run Gravel with 
Fluxed Native Asphalt B. 
Location: Station 89+50 to station 118+10. 
Total length: 2,880 feet. 
Total area: 5,445.2 square yards. 
This experiment was constructed exactly as experiment No. 9, 
except that fluxed native asphalt B was used instead of oil asphalt. 
Typical analyses of the mixture are given in Table 11. The maxi- 
mum grade is 4.08 per cent. 
Experiment No. 11. — Bituminous Gravel Concrete. Pit-Run Gravel with 
Repined Tar. 
Location: Station 118+10 to station 126+95. 
Total length: 885 feet. 
Total area: 1,573.3 square yards. 
This experiment was constructed exactly as experiments Nos. 9 
and 10, except that a refined tar was used throughout. The analysis 
of the tar is given in Table 9. The proportions of the mix were 43 
pounds of tar to 500 pounds of gravel, which resulted in a percentage 
of 7.92 of tar to total weight of mixture. Typical analyses of the 
mixture are presented in Table 12. 
