16 BULLETIN 586, U. S. DEPARTMENT OF AGRICULTURE. 
In addition to the surfacing and shoulders some extra work was 
done. Guardrail was built from station 57+50 to station 62+00, 
station 198+00 to station 204+00, and station 248+00 to station 
250+00. Several pipe culverts were lengthened and new headwalls 
built. A new 4 by 4 by 33 foot concrete box culvert was built at 
station 133 +55. 
BituMInovus GRAVEL CONCRETE. 
Location, station 0+-00 to station 217-++00. 
station 223-+00 to station 250+80. 
Total length, 24,500 feet. 
Total area, 38,442.6 square yards. 
The materials for this experiment were: 
Asphalt.—Oil asphalt; see typical analysis in Table 18. 
Aggregate.—Crushed and screened gravel from the Ayers pit. Fine 
ageregate passing }-inch circular openings, coarse aggregate passing 
14-inch and retamed on }4-inch circular openings. Fine aggregate 
contained less than 5 per cent of clay. 
Limestone dust.—Ordinary ground limestone used as filler. 
Pea gravel.—Washed Potomac River gravel, screened through 
34-inch and over }-inch screen. Used as top-dressing for seal coat. 
The mixing plant was of the stationary type, operated by steam 
power. The gravel was excavated by hand, run through a jaw 
crusher, and screened to remove tailings before it was heated. The 
heating was done in two rotary drums, wood being used as fuel. 
The fine and coarse aggregates then were separated by means of a 
1_-inch screen and stored in two bins immediately over the mixer. 
The asphalt was heated in two kettles supplied with steam coils, and 
the mixing was done in an ordinary twin pug mixer. The average 
time for mixing a batch was 1 minute. Table 19 shows analyses of 
several samples of mix taken at the plant. Table 20 shows similar 
analyses of road samples of mix. 
Most of the hauling was done with 1-ton autotrucks, which carried 
three batches of the hot mix ata load. On the longest haul distances 
several two-horse wagons carrying four batches to a load were used 
to supplement the trucks. 
The hot mix was dumped from the trucks or wagons onto a steel 
dumping board and spread with hand shovels and asphalt rakes. 
Then it was “‘set’”’ by rolling with a 23-ton tandem roller and com- 
pacted to its final shape with an 8-ton tandem roller. The loose 
depth of the hot mix averaged about 2? inches, and the compacted 
depth was not less than 2 inches. The temperature at which the 
material was spread ranged from 200° to 300° F. 
The seal coat was of the same asphalt used in the mix, covered 
with pea gravel. The asphalt was spread at a temperature of about 
350° F., about one-third gallon per square yard bemg used. The 
pea gravel was spread at the rate of about 1 cubic yard to 150 square 
