2 BULLETIN 257, U. S. DEPARTMENT OE AGRICULTURE. 



the 1913 experiments at Miami and two water-gas tar products 

 donated by the manufacturers for this purpose. The oils had been 

 stored without protection from the weather, and were in poor con- 

 dition, principally through the presence of water. 



The excellent results obtained in experiment No. 6 of the original 

 series of experiments at Miami could be attributed beyond question 

 to the use of screened rock instead of the "run of the bank" material 

 used in the other experiments. Consequently only the screened-rock 

 type of construction was followed in these experiments. The rock 

 was quarried from the Lemon City pit, which was the nearest source 

 of supply. It was comparatively soft and decidedly unsuitable for 

 bituminous work of this nature, due to its tendency to crush to pow- 

 der when the screened-rock course was even lightly rolled to make it 

 conform to cross section. In some portions this created a surface 

 condition similar to that existing in experiments 3,4, and 5 at Miami, 

 described in the previous report. This soft quality and the presence 

 of a considerable proportion of fine material, which it was found 

 impracticable to screen out, made the surface very susceptible to the 

 binding effects of rain. 



The work began at station + 00 at the north end of the project 

 on January 6 and continued intermittently until February 7. 



The general method consisted in scarifying and regrading the old 

 surface, after which a course of new screened rock was spread to a 

 loose depth of 2\ inches. This course was shaped by a very light 

 rolling before spreading the bituminous material over it. The excess 

 of bitumen was absorbed by a covering of sufficient fine white sand 

 from near-by pits, and the road was finished by thorough rolling. 



Frequent rains and showers during the work considerably delayed 

 its progress and produced surface conditions which prevented the 

 best results being obtained. It is clearly demonstrated that the 

 greatest difficulty to be overcome in any future large-scale construc- 

 tion 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 the rock causes it to bind strongly when 

 wet, unless the dust is entirely removed in the screening. The sur- 

 face is thus rendered more or less impervious, and the treatment 

 becomes in effect only a surface treatment. This difficulty 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. 



The equipment was the same as that used at Miami, consisting of 

 a 10-ton gasoline roller, scarifier, grader, two 1 -barrel heating ket- 

 tles, and hand-pouring pots. Analyses of the bituminous materials 

 used are given in the tables following. 



