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Tests for Asphalt Cement

bitumen, tube, crucible, carbon, disulphide, total and materials

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For the purpose of controlling the character of the surface mixtures to be used upon asphalt pave ments, tests are commonly made of the asphalt cement, as well as of the surface mixture itself. In testing asphalt cement the total amount of bitumen is usually determined; the hydrocarbons composing the bitumen are separated into their various classes, and the consistency of the mixture as well as the effect of temperature upon the consistency is examined.

Total Bitumen. The total bitumen in asphalt, or asphalt cement, is determined by testing its solubility in carbon disulphide. The following method is recom mended as standard by the "Committee on Standard Test for Road Materials," of the American Society for Testing Materials.* "It was decided, owing to the great variety of ,con ditions met in asphalt and like bitumen, that it was impossible to specify any one method of drying that could be at all satisfactorily applied in every case; it is therefore supposed that the material for analysis has been previously dried either in the laboratory or in the process of refining or manufacture, and that mois ture if preserved, is only hydroscopic or in such a con dition as to be easily removed.

"The material to be analyzed, if sufficiently hard and brittle, is ground and then spread in a thin layer in a suitable dish (iron or nickel will do), and kept at a temperature of 125° C. for one hour. In the case of paving mixture, where it is not desirable to crush the sand grains, a lump may be placed in the drying oven until it is thoroughly heated through, when it can be crumbled down with a spatula. From 2 to 12 grams (depending on its richness in bitumen) of the substance after drying is weighed in a large-sized test-tube (8 inches long by i inch in diameter), the tare of which has been previously ascertained. The tube is then tightly corked with a good sound cork and shaken vigorously until no asphalt can be seen adhering to the bottom or sides, or if a paving mixture or like material, until the solvent has disintegrated all lumps. Care must be taken that the bottom of the cork should be free from any cracks or holes that might retain insoluble matter from the asphalt, or if not, the end of the cork inserted in the tube must be covered with tin foil. Care should be taken while shaking, to keep one finger on the cork to prevent it being blown out.

"The tube should then be put away, still corked loosely, in an upright position and not disturbed for two days, after which the carbon disulphide is decanted off into a second tared tube. As much of the solvent should be poured off as is possible without losing any of the residue. The first tube is again filled with fresh carbon disulphide and shaken as before and put away for two more days. The second tube is also corked and put away in an upright position. At the end of the two days the contents of these two tubes are decanted off on to a weighted Gooch crucible fitted with an asbestos plug filter, the contents of the second tube being passed through the filter first. The asbestos filter shall be made of ignited long fiber amphibole, packed in the bottom of a Gooch porcelain crucible to a depth of about one-eighth of an inch. The residue in the second tube is then treated with about 2 cubic centimeters of carbon disulphide, care being exercised to disturb it as little as possible, the treatment merely being to remove the small portion of solvent containing bitumen. The Gooch crucible is then washed with clean carbon disulphide until the filtrate is colorless. The crucible and the two tubes are then dried at F. and weighed. The filtrate containing the bitumen is evaporated and the bituminous residue burnt, and the weight of the ash added to that left in the two tubes and Gooch crucible. The sum of these weights deducted from the weight of the substance taken gives the weight of the bitumen extracted." Bitumen Soluble in Naphtha. This test is employed for the purpose of determining the relative amounts of asphaltenes and malthenes present in the bitumen. When used in specifications it is designed to insure a proper relation between these classes of hydrocar bons, the object being to avoid materials containing too great percentage of asphaltenes, and the use of light oils as fluxes. The Committee of the American Society for Testing Materials recommend that the same method be employed as for obtaining the total bitumen. They also recommend that the naphtha used be described by giving the temperatures between which it distills and its specific gravity. Naphtha of a density of 88° Baume at 60° F. is commonly employed for this purpose.

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