RURAL ENGINEERING. 687 



having been noted (E. S. R., 31, p. 686). The materials used and methods 

 employed in the experiments, the several kinds of roads and varying condi- 

 tions of traffic, and analyses of materials and cost data are also given. 



A second series of experimental bitimiinous treatments of coralline rock 

 roads in Dade County, F!a., using light and lieavy oils and two water-gas tar 

 products, " clearly di^nonstrated that the greatest difhculty to he overcome in 

 any future large-scale construction of bituminous coralline rock roads is the 

 harmful effect of rains on the prepared surface before the application of the 

 bitumen. The high cementing power of tlie rock causes it to bind strongly 

 when wet unless the dust is entirely removed in the screening. The surface 

 is thus rendered more or less impervious, and the treatment becomes in effect 

 only a surface treatment. This dillicuity may be met by performing such work 

 In the dry season of the year or by keeping at all times the prepared surface 

 work and the bituminous application well together." After a year of service 

 none of these experiments sliowed any decided evidence of failure. In con- 

 nection with tliese experiments a section of ordinary coralline rock surface was 

 treated with calcium chlorid at the rate of 1.49 lbs. per square yard, the pur- 

 pose being to maintain a damp surface on the coralline rock road. After sev- 

 eral weeks of wear the surface was damp at all times, but pitted to some extent, 

 showing bare rock in spots. 



Experiments were also conducted at West Palm Beach, Fla., with oil, tar, oil 

 asphalt, and coralline rock to develop a more economical and durable road 

 surface than is obtainable by the use of coralline rock under ordinary methods 

 of construction. Experience gained from the construction in the experiments 

 at Miami, Fla., previously noted, and from their inspection after long usage 

 by traffic " showed conclusively that the best method of treatment involved the 

 use of rock from wliich the finer particles had been removed to provide for 

 uniform penetration of the bitumen. . . . The general method of construction 

 preparatory to the application of the bitumen was, in brief, as follows : The 

 preparation of a thoroughly compacted subgrade parallel to the finished sur- 

 face and composed of the rock originally in the roadbed ; upon this, a course 

 of screened rock with a uniform loose depth of 3 in., lightly compacted to allow 

 a maximum penetration of bituminous material." An inspection of these ex- 

 periments the following year showed that the residual asphaltic petroleum was 

 used with uniformly good results in one case, while in a second case the sec- 

 tion, although in good condition and free from any wearing defects, presented 

 a varied appearance which is attributed to the varied weather conditions under 

 which the experiment was constructed. The refined coal-tar experiments were 

 in good condition in one case, but in a second case the tar had become hard 

 and friable, necessitating a surface treatment to prevent wear. One oil asphalt 

 section was in excellent condition, while a second section had a mottled appear- 

 ance, due to bleeding of the bitumen to the surface. A water-bound coralline 

 rock macadam section showed a smooth, unbroken surface true to cross section. 

 A comparison of this experiment with the bituminous-bound experiments indi- 

 cated that the water-bound macadam had worn down about J in. below the ad- 

 jacent bituminous-bound section. 



Supplementary reports of experiments at Miami, Fla., with oil and coralline 

 rock in 1913; on Rockville Pike, Md., with tar and asphaltic petroleum in 1913; 

 at Washington, D. C, with surface treatment with tar preparations and oils in 

 1912 ; at Chevy Chase, Md., with bituminous concrete, cement concrete, oil-cement 

 concrete, vitrified brick, and bituminous surface treatments on concrete in 1912 ; 

 at Chevy Chase, Md., with bituminous construction and surface treatment in 

 1911; at Jamaica, N. Y., with oil-cement concrete, oil asphalt, tar, and fluxed 



9635°— No. 7—15 1 



