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Asphaltic Cement

residuum, asphalt, fluxing, agent, flux, bitumen, petroleum and oil

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ASPHALTIC CEMENT. The cementitious element of the wearing coat consists of commercially refined asphalt mixed with a softening or fluxing agent. The asphalt has already been dis cussed (see Art. 1).

Softening Agent.

Many of the asphalts are so hard and brittle that before being used for paving purposes, it is necessary to soften them by the admixture of oil or liquid bitumen. The selec tion of the proper fluxing agent for the harder asphalts is a very im portant matter. For example, refined Trinidad asphalt, which consists of 52 to 56 per cent pure bitumen, is usually mixed with about 18 pounds of fluxing material (petroleum residuum) per hundred pounds of asphalt; and hence the bitumen added in the fluxing material is equal to about one third of that originally in the asphalt, or the bitumen added in the softening agent is equal to about one fourth of that in the finished pavement.

There is a considerable difference of opinion as to the value of different fluxing agents, and the selection of a proper flux involves several unsettled chemical problems.

The properties required of an asphaltic flux are: 1. It should contain a material volatile under 300° F., as otherwise the volatile matter will be given off while the paving cement is being heated preparatory to its being mixed with the sand of the wearing coat. and consequently the asphalt will lose its cementing power. 2. The flux should be as fluid as possible in order that the greatest softening effect may be produced by the least quantity, as ordinarily the flux ing agent is comparatively expensive. 3. The softening agent should be chemically stable and not lose its fluidity by molecular change. 4. The fluxing agent should dissolve the asphalt and not simply form a mechanical mixture with it. The asphalt consists of asphaltine and petroline (§ 586), the former being entirely devoid of cementing power and the latter highly cementitious; and therefore the fluxing agent should dissolve the asphaltine.

There are two general classes of asphaltic flux in common use: (1) petroleum residuums or artificial bituminous fluxes, and (2) malthas or natural bituminous fluxes. The first is composed of liquid paraffins, and the second of fluid natural bitumens of the same nature as asphalt. There are two forms of each in more or less general use. There are therefore four fluxing agents; viz.: (1) residuum from the paraffin petroleums of Pennsylvania; (2) a spe cially prepared paraffin,—petroleum residuum known as Pittsburg flux; (3) residuum from the asphaltic petroleums of California; and (4) maltha. Until recent years the first was the only fluxing mate

rial in use, but at present all four are in more or less common use.

Residuum. " The best [paraffin] petro leum residuum is a heavy thick oil at 70° F., which begins to solidify at 58° F., and becomes solid at 48° F., the solidification being due to a crystallization of a portion of its constituents. At a temperature of 90° F. it becomes very limpid. It is very non adhesive in character; and, when in a solid condition from cold or other causes, it is very waxy in consistency and entirely lacking in cementing properties. It gradually loses its fluidity with age, ap parently by the separation from the residuum of a light brown amorphous solid." * Whether or not petroleum residuum com pletely dissolves the bitumen of asphalt has been an open question for several years, but some recent experiments t seem to show that it does not. " Judging from the physical properties of this residuum and its chemical relations to asphalt bitumen, it is not a desirable flux." "The best petroleum residuums comply with the following tests: 1. The specific gravity ranges between 20° and 23° Baum6. 2. The flash point. (as taken in a New York Board of Health oil tester) is between 300° and 425° F. 3. On keeping the residuum at a temperature of 400° F. for thirty hours, it must lose between 2 and 6 per cent of oil. The residue in the retort should be fluid at 75° F., and on cooling should not show a coarse crystallization. The quantity of residuum necessary to soften [Trinidad] asphalt. into a cement containing bitumen whose penetration is 80° (District of Columbia standard), should not be over 33 per cent of the total quantity of bitumen in the asphalt. 4. The residuum must show only the slightest signs of having been 'cracked' [i. e., that the hydro-carbon compounds have been broken up into new components in the course of manufacture]. An oil that has been `cracked' on being examined through the microscope reveals numerous black particles floating in it. The particles are insoluble in petroleum naphtha. but are soluble in carbon bisulphide, and resemble asphaltine." Pittsburg Flux. This is made by heating paraffin-petro leum residuum with sulphur, which favorably changes the paraffin, and has been used to a limited extent.

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