BUREAU OF PUBLIC ROADS. 



421 



be classified as follows: Rock, 262; concrete, 252; sand, 205; gravel, 

 151; slag. 54; cement, 49; miscellaneous, 111. Samples were re- 

 ceived from 41 States, and also from Canada. The geographical 

 distribution of samples examined is as follows : 



MICROSCOPIC EXAMINATION AND CLASSIFICATION OF ROAD-BUILDING ROCK. 



The Petrographic Laboratory examined 686 samles of road-build- 

 ing material during the year, an increase of almost 9 per cent over 

 the preceding year and well above the average of routine work. Of 

 these samples, 249 were rock, 41 slag, 151 gravel, 189 sand, 45 clay, 

 and 11 miscellaneous. 



Of the samples examined in the physical, chemical, and petro- 

 graphic laboratories, 355 were materials for use in Federal-aid con- 

 struction. 



RESEARCH ON DUST PREVENTIVES AND ROAD BINDERS. 



The following investigations have been under way on bituminous 

 materials: I 



1. Comparison of old and new volatilization tests in gas and electric ovens. 



2. Exi)osure tests on bituminous materials of known chemical composition. 



3. Study of the distillation of California and Mexican peteroleum with refer* 

 ence to the manufacture of road oils. 



4. Bituminous joint fillers. 



5. Chemical composition of asphalts and tars. 



6. Methods of making float tests. 



7. Investigation of the action of copper salts on bituminous materials. 



8. Investigation of the toughness of bituminous aggregates. 



One investigation on the thickness of bituminous films on dilTer- 

 cnt types of aggregates has been completed and the data is ready to 

 be worked up in form for publication. A study of the asphalt con- 

 tent of oils has also been completed and is ready to be put in shape 

 for publication. A paper is now being published on the micro- 

 scopic examination of bituminous materials. A paper on the Effi- 

 ciency of Bituminous Surfaces and Bituminous Pavements under 



