DUST PREVENTION- AND ROAD PRESERVATION, 1915. 33 
concrete. Sand and gravel of the mechanical analyses given in 
Tables 33 and 34 were used in the concrete, which was mixed in a 
mechanical mixer and allowed to stand 10 days before the wearing 
surface was applied. The proportion of the mix, by volume, was 1 
part cement, 3 parts sand, 7 parts gravel. 
Two types of bituminous concrete were chosen for the wearing 
course, which was spread so as to compact to a thickness of 2 inches. 
The northern half of each drive was surfaced with a mixture prepared 
according to the Topeka specification, while the southern half of 
each was paved with bituminous concrete prepared according to the 
District of Columbia specification. The analyses of the bituminous 
aggregates of these preparations are given in Table 35 and Table 36, 
respectively. An analysis of the oil-asphalt is given in Table 37. 
The bituminous concrete was prepared in a local plant and was 
delivered to the site of the work in trucks at a temperature of 220° F. 
or more. After being spread and sufficiently cooled the mixture was 
given a preliminary rolling with a 3-ton tandem roller and was fin- 
ished with a 12-ton roller. A seal coat of oil-asphalt represented by 
the analysis given in Table 37 was spread with a squeegee on the 
areas surfaced with the District of Columbia mixture, and CQV#i»#d 
with hot stone chips immediately. In the case of the Topeka mixture 
the seal coat was omitted and the surface was dusted lightly with 
Portland cement. 
Table 33. — Mechanical analysis of sand used in concrete base. 
Per cent. 
Total retained on 10-mesh sieve 10. 2 
Total retained on 20-mesh sieve 33. 9 
Total retained on 30-mesh sieve 53. 9 
Total retained on 40-mesh sieve 66. 4 
Total retained on 50-mesh sieve 77. 9 
Total retained on 80-mesh sieve. . 95. 9 
Total retained on 100-mesh sieve 96. 5 
Total retained on 200-mesh sieve 97. 9 
Passing 200-mesh sieve - 2.0 
Table 34. — Mechanical analysis of gravel used in concrete base. 
Per cent. 
Total retained on 1^-inch screen 2.1 
Total retained on 1-inch screen 5. 5 
Total retained on f-inch screen 18. 1 
Total retained on ^-inch screen 34. 7 
Total retained on |-inch screen „ 70. 7 
Total retained on 10-mesh sieve 94. 
Passing 10-mesh sieve 6. 
52086°— Bull. 407—16 5 
