10 BULLETIN 407, U. S. DEPARTMENT OF AGRICULTURE. 
on top of which was placed a 10-inch course of cobble stones. The 
drain was under the gutter line and was completed with a layer of 
earth. 
Experiment No. 3. — Bituminous 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 f-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. 
