Aug. 13, 191 7 



Toughness of Bituminous Aggregates 



323 



tions in the relative proportions of different size particles in one of the 

 fractions of crushed aggregate used, a 30-mesh screen was introduced in 

 separating the aggregate. The mineral aggregate for the second series 

 of cylinders was therefore proportioned as follows: 



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



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



Passing 30-mesh, retained on 50-mesh. screen 50 per cent. 



It is likely that this elimination of an intermediate size of particle is 

 partly responsible for the uniformly lower results obtained throughout 

 the second series. The physical tests on the rocks used are given in 

 Table V. The bituminous materials consisted of refined water-gas tars 

 practically identical with those used in the first series, three oil asphalts 

 of 145, 91, and 50 penetration, respectively, and a fluxed native asphalt. 

 The data regarding these materials will be found in Tables II and III. 



Table V. — Physical tests of rocks used in second series of toughness tests 



Locality. 



9445 

 9095 

 9094 



8136 

 8992 

 9321 

 8993 

 8989 

 9281 



Diabase 



Biotite schist 



Quartzite 



Biotite granite 



Biotite gneiss 



Altered diabase por- 

 phyry. 

 Feldspathic sandstone . 



Open-hearth slag 



Feldspathic quartzite . . 



Blast-furnace slag 



Chlorite gneiss 



Limestone 



Montgomery County, Md. . . 



Cumberland, Me 



Minnehaha, S- Dak 



Knox County, Me 



Cumberland County, Me . . . 

 Cumberland, Me 



Jlontgomery County, Md. . . 

 Mahoning County, Ohio. . . . 



Fulton County, Ga 



IMahoning County, Ohio . . . 



Jamestown, R. I 



Seneca County, Ohio 



0-35 

 .41 

 .68 

 .40 

 .26 

 •27 



3- IS 



. 13 



■33 

 1. 18 



. 21 

 2. 16 



Before deciding upon the proportions of bitumen to be used in this 

 series of tests, a number of cylinders were made up with concrete sand 

 which had been screened and reproportioned in the same manner as 

 adopted for the rock-test pieces. The oil asphalt 8950 was used, and the 

 results in three cylinders for each proportion are given in Table VI. Since 

 the proportion of 88 aggregate to 1 2 bitumen, which was used in the first 

 series of tests, gave within one point of the highest toughness obtained, 

 it was decided to continue its use through the second series. This series 

 of results with the sand and bitumen mixtures shows very clearly within 

 what narrow limits the percentage of bitumen should be controlled in 

 order to obtain its maximum efficiency as a binder. By taking the 

 highest toughness obtained, which is 1 1 , it will be seen that this was pro- 

 duced within a range of bitumen content of 0.43 per cent by weight and 

 that well-defined decreases continue from this maximum by either re- 

 ducing or increasing the bitumen content. 



