10 BULLETIlir 407, U. S. DEPARTMENT OF AGEICTJLTURE. 



on top of wliicli was placed a 10-inch course of cobble stones. The 

 drain was under the gutter line and was completed with a layer of 

 earth. 



ExPEKiMENT No. 3. — ^BiTtTMiNOtTS Macadam (Mixing Method); Sandstone with 

 Fluxed Native Asphalt A. 



Location: Station 18+00 to station 28+10. 



Length: 1,010 feet. 



Total area: 1,867.8 square yards. 



The compacted gravel base for this experiment had been under 

 traffic about 4 months before the surface was laid. A thin layer of 

 dust had developed and in a few instances the surface of the base 

 was not true to section. The section was revised by hand picking, 

 the addition of gravel to the hollows, and rolling. In general the 

 base presented a smooth, hard appearance and was thoroughly swept 

 with ordinary house brooms directly in advance of the placing of the 

 bituminous macadam. The stone was |-inch sandstone with physi- 

 cal properties and mechanical analysis as shown in Tables 4 and 6, 

 respectively. It was mixed with the hot bituminous material in the 

 proportions of 38 pounds of asphalt to 600 pounds of stone in a 

 semiportable mixing plant near station 95 + 00. The resulting per- 

 centage of asphalt to total weight of the mix was 5.95 per cent. The 

 mix was hauled in dump-bottom wagons to the point of placing and 

 dumped on spreading boards. It was spread at an average tempera- 

 ture of about 300° F. by shovels and rakes on the cleaned gravel base 

 to produce a compacted depth of 2 inches. 



The mixing plant raised the aggregate by means of a bucket-belt 

 from a feeding pit into a heating cylinder, where it was heated. The 

 heated aggregate was then elevated to a storage bin, from which it 

 was run into a weighing hopper and thence into the mixing box. 

 The asphalt was heated in two kettles, each of 1,000 gallons capacity, 

 and dipped by ladles into a weighing kettle from which the hot 

 material could be poured into the mixing box. The pouring began 

 as the chute from the aggregate weighing hopper opened, while the 

 blades of the mixer revolved continuously. A full minute was the 

 minimum time of mixing, and the mixed material was then allowed 

 to drop into the dump wagon. 



The resulting mix was very satisfactory, as all the particles of 

 aggregate were thoroughly coated. The temperature of the stone in 

 the weighing hopper was constantly tested by the hand, and the tem- 

 perature was checked at intervals by a thermometer, as was also the 

 temperature of the asphalt. No separation of the mix occurred 

 during the haul, and it arrived on the work almost without exception 

 at a temperature above 220° F. 



