372 ANNUAL BEPORTS OF DEPARTMENT OF AGRICULTURE. 



MICROSCOPIC EXAMINATION AND CLASSIFICATION OF ROAD-BUILDING ROCK. 



The microscopic laboratory examined and classified G81 samples 

 of road-building material, a decrease of about 23 per cent as com- 

 pared with the last fiscal year, but well above the number examined 

 during 1915. Of these samples 415 were rock and slag, 249 gravel, 

 sand, clay, etc., and 17 miscellaneous. 



RESEARCH ON DUST PREVENTIVES AND ROAD BINDERS. 



A paper upon "The Toughness of Bituminous Aggregates" was 

 prepared for publication in the Journal of Agricultural Research. 

 Another paper, "The Effect of Exposure on Fluid Bitumens," was 

 presented before the American Chemical Society and will be pub- 

 lished in the Journal of Industrial and Engineering Chemistry. 



The following subjects are under investigation: The eifect of 

 variations in refining methods upon the characteristics of road oils 

 and asphalts produced from crude petroleum; the thickness of 

 bituminous films upon different t^q^es of rock aggregates; the effect 

 of water upon the viscosity of road oils; relation between exposure 

 and volatilization tests on tar products; the physical properties of 

 coarse bituminous aggregates; the effect of colloids on bituminous 

 materials; bituminous materials suitable for the surface treatment 

 of waterbound gravel roads. 



EXPERIMENTAL BITUMINOUS ROAD CONSTRUCTION AND MAINTENANCE. 



Supervision and inspection of experimental bituminous roads has 

 been confined largely to the vicinity of Washington, although further 

 work was done in connection with the use of Florida coralline rock 

 in bituminous macadam construction, and the experimental surface 

 treatment of a gravel road in Alabama was undertaken. Continuous 

 plant and laboratory inspection was furnished for the experimental 

 construction of a bituminous concrete road in Virginia in which 

 partly crushed and graded pit-run gravel was used as the mineral 

 aggregate. The surface treatment of Virginia sand gravel and clay 

 gravel roads with two grades of tar and a road oil was made the 

 basis of three maintenance experiments. Plans have been made to 

 investigate the use of screened bank gravel in the construction of 

 bituminous gravel roads according to the penetration method. It 

 is also hoped to continue experiments in the construction of natural 

 soil and asphalt mixed roads in some of the Southern States. 



In all of the experimental work of the year, which included the 

 maintenance of roads constructed in previous years, this division 

 cooperated with the Division of Construction and Maintenance. It 

 prepared a number of road-material specifications and made 52 indi- 

 vidual inspections. A circular summarizing the material and cost 

 data for the construction and maintenance of all experimental roads 

 in the vicinity of Washington was prepared for publication. 



NONBITUMINOUS ROAD-MATERIAL INVESTIGATIONS. 



A paper upon " Laboratory Tests of Brick Pavements " was pre- 

 sented before the xVmerican Society of Civil Engineers, and a new 

 impact tester to be used in field tests of various types of roads was 



