SAMPLING AND TESTING HIGHWAY MATERIALS. 



55 



IV. ACCURACY. 



(9) The li mit of accuracy of the test is 0.5° 0. (0.9° F.). 



V. PRECAUTIONS. 



(10) The use of freshly boiled distilled water is essential, as otherwise air bubbles 

 may form on the specimen and affect the accuracy of the results. Rigid adherence 

 to the prescribed rate of heating is absolutely essential in order to secure accuracy 

 of results. 



A sheet of paper placed on the bottom of the glass vessel and conveniently weighted 

 will prevent the bituminous material from sticking to the glass vessel, thereby saving 

 considerable time and trouble in cleaning. 



33. METHOD FOR EXAMINATION OF BITUMINOUS MIXTURES. 



A. CENTRIFUGAL METHOD. 



The extractor shown in figure 30 was designed upon lines suggested by an examina- 

 tion of machines in use by A. E. Schutte and C. N. Forrest. 10 It consists of a one-fifth 

 horsepower, 1,100 revolutions per minute ver- 

 tical-shaft electric motor, a, with the shaft 

 projecting into the cylindrical copper box b, 

 the bottom of which is so inclined as to drain 

 the spout c. A three-sixteenths-inch circular 

 brass plate 9J inches in diameter is shown in <t- 

 d, and upon this rests the sheet-iron bowl e, k- 

 which is 8J inches in diameter by 2| inches 

 high, and has a 2-inch circular hole in the 

 top. Fastened to the inner side of the bowl 

 is the brass cup /, having a circle of one- 

 eighth-inch holes for the admission of the 

 solvent, and terminating in the hollow axle, 

 which fits snugly through a hole at the cen- 

 ter of the brass plate. The bowl may be 

 drawn firmly against a felt-paper ring g, three- 

 fourths inch wide, by means of the 2J-inch 

 milled nut h, for which the hollow axle is 

 threaded for a distance of three-fourths inch 

 directly below the upper surface of the plate. 

 The axle fits snugly over the shaft of the 

 motor, to which it is locked by a slot and 

 cross pin, i. 



The aggregate is prepared for analysis by 

 heating it in an enamel-ware pan on the hot 

 plate until it is sufficiently soft to 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 1 

 kilogram may be cut off with hammer and chisel. The disintegrated aggregate is then 

 allowed to cool, after which a sufficient amount is taken to yield on extraction from 50 

 to 60 grams of bitumen . It is placed in the iron bowl and a ring three-fourths of an inch 

 wide, cut from the felt paper, is fitted on the rim, after which the brass 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 previously treated in the empty 

 extractor with a couple of charges of carbon disulphide in order to remove any small 



Fig. 30.— Centrifuge extractor, 

 type.) 



(Reeve 



10 Any extractor of similar design may be used. 



