METHODS OF SAMPLING AND TESTING HIGHWAY MATERIALS 63 
mouth, carefully ground to receive an accurately fitting solid-glass stopper with 
a hole of about 1-millimeter bore in place of the usual capillary opening. The 
lower surface of the stopper is mad* 1 concave in order to allow all air bubbles 
to escape through the bore. The depth of the cup-shaped depression shall be 
about 4.8 millimeters at the center. The stoppered flask has a capacity of about 
25 cubic centimeters and when empty weighs about 25 grams. 
5. Before making a determination, the pycnometer with stopper shall first be 
calibrated by weighing it clean and dry upon an analytical balance. This 
weight is called a. It shall then be filled with freshly boiled distilled water at 
a temperature of 25° C. (77° F.), the stopper firmly inserted, all surplus mois- 
ture wiped from the surface with a clean, dry cloth, and again weighed. This 
weight is called &. 
6. When determining the specific gravity of road oils or road tars which flow 
readily, the material shall be brought to a temperature of 25° C. (77° F.) and 
poured into the pycnometer until it is full, with care to prevent the inclusion of 
air bubbles. The stopper is then firmly inserted and all excess of material 
forced through the opening is carefully removed with a clean, dry cloth. The 
pycnometer and contents are then weighed and this weight is called c. The 
specific gravity of the material shall be calculated from the formula: 
Specific gravity = _ _ 
7. When determining the specific gravity of tar and asphalt products which 
are too viscous for the method described in section 4. a small amount of the 
material shall be brought to a fluid condition by the gentle application of heat, 
care being exercised to prevent loss by evaporation. When sufficiently fluid, 
enough is poured into the clean, dry pycnometer to about half fill it. Precau- 
tions shall be taken to keep the material from touching the sides of the tube 
above the final level and to prevent the inclusion of air bubbles. The tube 
should be slightly warmed before filling. The pycnometer and contents are 
then cooled to room temperature and weighed with the stopper. This weight is 
called c. The pycnometer is next removed from the balance, filled with freshly 
boiled distilled water, and th» j stopper firmly inserted. It is then completely 
immersed for not less than 30 minutes in a beaker of distilled water maintained 
at 25° C. (77° F.). after which it is removed and all surplus water is wiped off 
with a clean cloth. It is immediately weighed. This weight is called d. The 
specific gravity of the material shall be calculated from the formula : 
Specific gravity =— — — - 
(6— a) — (d— c) 
8. When making the specific-gravity determination it is important that: 
(a) Only freshly boiled distilled water shall be used. 
(6) When weighing the pycnometer completely filled the temperature of its 
contents shall be within 1° C. (1.8° F.) of 25° C. (77° F.). 
(c) Precautions shall be taken to prevent expansion and overflow of the 
contents from the heat of the hand when wiping the surface of the pycnometer. 
(d) The presence of all air bubbles shall be eliminated in filling the pyc- 
nometer and inserting the stopper. 
(e) Weighings shall be made quickly after filling the pycnometer and shall 
be accurate to 1 milligram. A number of trial fillings and catchweights may 
be necessary to obtain the desired degree of accuracy. 
if) To prevent breakage of the pycnometer when cleaning it out after a de- 
termination has been made upon a very viscous or semisolid material, it will be 
found advisable to warm it in an oven at not over 100° C. until most of the 
material may be poured out and then to swab it with a piece of soft cloth or 
cotton waste. When cool, it may be finally rinsed with carbon disulphide. 
benzol, or other solvent and wiped clean. 
9. The limit of accuracy of the test is ±0.005 specific gravity. 
DISPLACEMENT METHOD 
(Used for hard, solid bitumens) 
10. For materials which are hard enough to be broken and handled in frag- 
ments at room temperature the following method will prove convenient : A small 
fragment of the bitumen (about 1 cubic centimeter) is suspended by means of 
a silk thread from the hook on one of the pan supports, about 1V< inches above 
