SAMPLING AND TESTING HIGHWAY MATERIALS. 



37 



This pycnometer consists of a conical or Erlenmeyer-shaped flask about 4.5 cm. 

 high, 4.0 cm. diameter at bottom, and 2.5 cm. diameter at the mouth. It is care- 

 fully ground to receive an accurately fitting solid glass stopper with a hole about 1 mm. 

 bore in place of the usual capillary opening. The lower surface of this stopper is made 

 concave to allow air bubbles to escape through the bore. The depth of the cup-shaped 

 depression is 4.8 mm. at the center. The flask has a capacity of about 25 c. c. and 

 weighs when empty about 25 grams. Its principal advantages are (1) that any de- 

 sired amount of bitumen may be poured in without touching the sides above the de- 

 sired level; (2) it is easily cleaned; (3) on account of the 1.0 mm. bore the stopper 

 can be easily inserted when the flask is filled with a viscous oil. 



When working with semisolid bitumens which 

 are too soft to be broken and handled in fragments, 

 the following method of determining their specific 

 gravity is employed : 



The clean, dry pycnometer is first weighed 

 empty, and this weight is called a. It is then 

 filled in the usual manner with freshly distilled 

 water at 25° C, and the weight is again taken and 

 called b. A small amount of the bitumen should 

 be placed in the spoon and brought to a fluid 

 condition by the gentle application of heat, with 

 care that no loss by evaporation occurs. When 

 sufficiently fluid, enough is poured into the dry 

 pycnometer, which may also be warmed, to fill it 

 about half full without allowing the material to 

 touch the sides of the tube above the desired 

 level. The flask and contents are then allowed to 

 cool to room temperature, after which the tube is 

 carefully weighed with the stopper. This weight 

 is called c. Distilled water, at 25° C, is then 

 poured in until the pycnometer is full. After 

 this the stopper is inserted, and the whole cooled 

 to 25° C. by a 30-minute immersion in a beaker of 

 distilled water maintained at this temperature. All f 

 surplus moisture is then removed with a soft cloth, OS***™ 

 and the pycnometer and contents are weighed. 

 This weight is called d. From the weights ob- 

 tained the specific gravity of the bitumen may be 

 readily calculated by the following formula: 



(b-a)-(d-c) 



/cm. 



Specific gravity 25° C./25° C.= 



Fig. 16. — Hubbard-Carmick pycnometer. 

 (Dimensions only approximate.) 



Both a and b axe constants and need be deter- 

 mined but once. It is therefore necessary to make 

 but two weighings for each determination after the first. Results obtained according 

 to the method given above are accurate to within 2 units in the third decimal place, 

 while the open-tube method is accurate to the second decimal place only. 



The specific gravity of fluid bitumens may be determined in the ordinary manner 

 with this pycnometer by completely filling it with the material and dividing the 

 weight of the bitumen thus obtained by that of the same volume of water. 



The pycnometer may be readily cleaned by placing it in a hot-air bath until the 

 bitumen is sufficiently fluid to pour. As much is drained out as possible and the 

 interior swabbed with a piece of cotton waste. It is then rinsed clean with a little 

 carbon disulphide, and after drying is again ready for use. 



