TESTS FOR BITUMINOUS ROAD MATERIALS. 



19. DETERMINATION OF SPECIFIC GRAVITY OF BITUMINOUS MATERIALS. 



A. HYDROMETER METHOD (USED FOR THIN FLUID BITUMENS). 



The specific gravity of thin fluid bituminous road materials is determined at 25° C. 

 as compared with water at that temperature. This may be done with the above- 

 mentioned apparatus by first pouring a sufficient quantity of the material into the tin 

 cup, which is then placed in the large dish containing cold or warm water as occasion 

 may require. The material in the cup should be stirred with the thermometer until 



a 



Fig. 15. — Hydrometer method of determining specific gravity. 



it is brought to a temperature of 25° C, after which it should be immediately poured 

 into the hydrometer jar and its gravity determined by means of the proper hydrom- 

 eter. In case the hydrometer sinks slowly, owing to the viscosity of the material, 

 it should be given sufficient time to come to a definite resting point, and this point 

 should be checked by raising the hydrometer and allowing it to sink a second time. 

 The hydrometer should never be pushed below the point at which it naturally comes 

 to rest until the last reading has been made. It may then be pushed below the read- 

 ing for a distance of three or four of the small divisions on the scale, whereupon it 

 should immediately begin to rise. If it fails to do so, the material is too viscous for 

 the hydrometer method, and the pycnometer method should be employed. 



The direct specific gravity reading obtained by the foregoing method is based upon 

 water at 15.5° C. taken as unity. For all practical purposes this reading may be 

 corrected to water at 25° C, considered as unity, by multiplying it by 1.002. Thus: 

 Specific gravity 25° C./25° 0.=specific gravity 25° C./15.5 C.X 1-002. 



B. PYCNOMETER METHOD (USED FOR VISCOUS FLUID AND SEMISOLID BITUMENS AND 



EMULSIONS). 



The inconvenience and difficulty of employing the ordinary narrow neck pyc- 

 nometer when determining the specific gravity of viscous fluid and semisolid 

 bitumens has led to the use of a special form shown in figure Hi. 

 36 



