SAMPLING AND TESTING HIGHWAY MATERIALS. 65 
METHOD FOR EXAMINATION OF BITUMINOUS MIXTURES. 
A. Centrifugal Method. 
The aggregate is prepared for analysis by heating it in an Centrifugal 
enamel-ware pan on the hot plate until it is sufficiently soft to met hod 
be thoroughly disintegrated by means of a large spoon. Care 
must be taken, however, that the individual particles are not 
crushed. If a section of pavement is under examination, a piece 
weighing somewhat over i kilogram may be cut off with hammer 
and chisel. The disintegrated aggregate is then allowed to cool. 
Not less than 500 grams of aggregate containing particles larger 
than three-quarters inch in diameter or 200 grams of aggregate 
with all particles smaller than three-quarters inch are placed 
in the bowl of the centrifuge extractor, and a ring three-quarters 
of an inch wide, cut from felt paper, is fitted on the rim, after 
which the cover plate is placed in position and drawn down 
tightly by means of the milled nut. If the bitumen is to be 
recovered and examined, the felt ring should be treated pre- 
viously in the empty extractor with a couple of charges of 
carbon disulphide in order to remove any small amount of 
grease or resin that may be present, although a proper grade of 
felt Should be practically free from such products. The bowl 
is then placed on the motor shaft of the extractor and the slot 
and pin are carefully locked. An empty bottle is placed under 
the spout and 150 cubic centimeters of carbon disulphide (carbon 
tetrachloride, benzol, or chloroform may also be used as solvents) 
is poured into the bowl through the small holes. After allow- 
ing the material to digest for a few minutes, the motor is 
started, slowly at flr.«t in order to permit the aggregate to dis- 
tribute uniformly. The speed should then be increased sufficiently 
by means of the regulator to cause the dissolved bitumen to flow 
from the spout in a thin stream. When the first charge has 
drained, the motor is stopped and a fresh portion of disulphide 
is added. This operation is repeated from four to six times 
with 150 cubic centimeters of disulphide. With a little expe- 
rience the operator can soon gauge exactly what treatment is 
necessary for any given material. When the last addition of 
solvent has drained off. the bowl is removed and placed with 
the cover plate uppermost on a sheet of manila paper. The 
cover plate and felt ring are carefully laid aside on the paper 
and, when the aggregate is thoroughly dry, it can be brushed 
on a pan of the rough balance and weighed. The difference 
between this weight and the original weight taken shows the 
amount of bitumen extracted. The aggregate may then be 
tested as occasion requires. 
When it is desired to examine the bitumen, or recover the 
solvent, a suitable distillation apparatus may be used, taking 
necessary precautions when handling inflammable solvents. 
The solution of bitumen should be allowed to stand overnight 
in order to permit the settling of any fine mineral matter that 
is sometimes carried through the felt ring in the extractor. The 
solution is then decanted into the flask, and the solvent is driven 
off by means of heat from an incandescent lamp until the residue 
is of a thick sirupy consistency. Meanwhile the solvent is con- 
densed and recovered in the flask. The residue is poured into 
an 11-centimeter porcelain evaporating dish and evaporated on 
a steam bath. The most scrupulous care must be taken at all 
times that no flames are in its immediate vicinity. Evaporation 
is carried on at a gentle heat, with continual stirring, until 
foaming practically ceases. It is advisable to have a large watch 
glass at hand to smother the flames quickly should the material 
ignite. As the foaming subsides, the heat of the steam bath 
may be gradually raised, and evaporation continued until the 
bubbles beaten or stirred to the surface of the bitumen fail to 
give a blue flame or odor of sulphur dioxide when ignited by 
67096°— 24 5 
