320 



Journal of Agricultural Research 



Vol.X. No. 7 



in diameter to dust. The particles were separated into definite sizes 

 by screening and were then recombined in fixed proportions with the 

 object of producing an aggregate that, on a small scale, fairly well 

 represented the one-size stone aggregate. The accurately proportioned 

 aggregate was heated and mixed with a predetermined proportion of 

 bituminous material. In order that the relative amount of bitumen 

 to aggregate would be the same in all mixtures, the precentage was 

 figured as a rational proportion, taking into account the specific gravity 

 of both the rock and bitumen in accordance with the suggestion of 

 Hubbard.^ The aggregate and bitumen were heated separately to 150° 

 C. and then thoroughly mixed together, after which the mix was 

 allowed to cool to 105° and maintained at that temperature in a 105° 

 oven until the test specimens were molded. The specimens were 25 by 

 25 mm. cylinders, compressed with a die and plunger under a pressure 

 of 132 kgm. per square centimeter on the machine commonly used for 

 preparing specimens in rock testing to show their cementing values.* 

 At the end of 24 hours and of 7 days the cylinders were tested for 

 toughness on the Page impact machine ^ with a 500-gm. hammer. The 

 specimens were stored under cover in the laboratory until half an hour 

 before testing, and during the 7-day period were immersed in water at 

 25° in order to bring them to a uniform temperature for testing. An 

 average was taken of the results on three cylinders, and it may be 

 stated that in all cases the three cylinders gave results in very close 

 agreement. 



Rocks of which the physical tests are given in Table I were used m 

 the first series of experiments. After crushing, the particles were re- 

 combined in the following proportion : 



Passing 8-mesh, retained on lo-mesh sieve 25 per cent. 



Passing lo-mesh, retained on 20-mesh sieve 25 per cent. 



Passing 20-mesh, retained on 50-mesh sieve 50 per cent. 



Table I. — Physical tests of rock used in preliminary toughness work 



No. 



Rock. 



Locality. 



6113 



5589 

 1817 

 7316 

 7682 



7445 

 4813 

 7332 

 6710 

 4444 

 1820 

 871 



Biotite gneiss 



Quartzite 



Metamorphic sandstone 



Diabase 



Biotite gneiss 



Sandstone 



Open-hearth slag 



Limestone 



Blast-furnace slag 



Basalt 



Chlorite gneiss 



do 



Fairfield. Conn 



Providence, R. I 



Newport, R. I 



Montgomery County, Md. 



Delaware County, Pa 



Picture Rocks, Pa 



Cape Breton, Canada 



Jack County, Tex 



Canal Dover, Ohio 



Cowlitz County, Wash 



Providence, R.I 



Louisa County, Va . - 



Lbs. 

 163 

 162 



170 



P.ct. 

 3-0 

 4-4 

 4.8 

 1.9 

 2.8 

 2.6 

 3-1 

 3-8 



4.6 



9.0 

 8.3 

 20. 6 

 14-3 

 15-5 

 12.9 

 10. 6 

 16.5 

 16. 1 

 8.6 

 4.9 



16 

 5 

 17 



102 

 14 

 43 



500-1- 

 118 

 300 

 7 



1 Hubbard, Prevost. the bitumen content of coarse bituminous aggregates. In Proc. Intern. 

 Assoc. Testing Materials, v. 2, no. 11, art. 25, pt. 2, 7 p. 1912. 



2 Jackson, F. H., Jr. methods for the determination of the physical properties of road- 

 building ROCK. U. S. Dept. Agr. Bui. 347, 27 p., 12 fig. 1916. 



