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+00. It was fed 

 into the mixer through a 2-uich 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 mixtui'e 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 smaU superficial pits. A seal coat of the 

 same asphaltic material used in the mix was apphed 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. 



Totallength: 2,860 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 gi-ade 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. 



Totallength: 885 feet. 



Total area: 1,573.3 square yards. 



This experiment was constructed exactly as experiments Nos. 9 

 and ] 0, 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. 



