Variation of Consistency with Age. "All bitumens undergo a more or less rapid change with aging, a fact which appears to be due to two or possibly more causes. Two distinct changes manifest themselves. 1. A surface hardening takes place, probably due to indirect oxidation, and possibly to the volatilization of the light oils. It begins at the surface and gradually extends into the mass. 2. A hardening of the entire mass takes place, evidently due to poly merization, i. e., to new atomic arrangement. Both of these changes take place in all bitumens, but one or the other may predominate.
"The test is made as follows: The penetration of the sample is determined, after which it is put away for a week, when the penetra tion is again ascertained. If the sample is found to have appreciably hardened, a slanting cut is made into it with a keen, sharp knife, thus exposing a gradual descent from the surface into the interior of the cement. Penetrations are now taken down the side of this cut, beginning at the surface. In this way the increase in hardness of the surface, and also of the interior, over the original consistency is determined. It is well to continue this test for as long a period as possible, making examinations at intervals of every few weeks." * Stability at High Temperature. The asphalt must remain at a high temperature for a considerable time in mixing it with the softening agent as well as in mixing the cement and the sand; and it is important that the asphalt mixture shall in this process lose none of its valuable properties. Owing to the great area exposed to evaporation the effect of high temperature is especially severe after the cement has been mixed with the sand. The lack of stability resulting from the loss of the light oils is manifested in different ways in different bitumens. Although generally true, it does not of necessity follow that the bitumen losing the most oil undergoes the greatest change in consistency. There are two methods of making this test, both of which should be used: 1. "The asphalt cement is mixed with standard sand in such proportions that the mixture will contain 10 per cent of bitumen.
This is done by keeping the ingredients in an oven for 15 minutes at 300° F., and then incorporating them by stirring. One portion of this mixture is then put aside to cool, while the other is kept at the temperature of 300° F. for a half hour longer. The bitumen is then extracted from both; and after having arrived at the same temperature, their penetrations are compared.
2. "The second method consists in keeping a quantity of the substance, equivalent to 20 grams of bitumen, at the tempera ture of 400° F. for thirty hours. The method of procedure is as follows: The substance is weighed in a short-necked, tubulated, 2 ounce retort, the tare of which has been previously taken. The
retort is then hung in a copper cylinder so that the neck just pro trudes. The copper cylinder is then jacketed with asbestos and provided with a thermometer. After being heated up to 400° F. for thirty hours, the retort is allowed to cool, and is then weighed. The per cent of loss should not be more than 8 per cent. The retort is then broken open and the character of its contents compared with that of the original substance." * Effect of Water and Ammonia. " The action of water and dilute ammonia on an asphalt mixture is determined by molding an inch cube of the mixture under a pressure of 1,000 pounds. The cube is broken in two, one portion being placed in water or dilute ammonia, while the other portion is kept in the air. The two pieces are compared from time to time. If the piece has been acted upon by the liquid, the corners will be found to give away readily with a slight pressure of the finger. After soaking some time, it is well to evaporate the liquid to dryness and to note if any bitumi nous residue remains." * Adhesiveness. Formerly the adhesiveness of cement was supposed to be measured by the proportion of petroline (bitumen soluble in ether or naphtha) it contained; but the results differ so widely with different asphalts, with variations in the details of making the experiments, and with the purity of the solvents, that the test has been practically abandoned.
The adhesiveness has been tested in a few cases by cementing together with asphalt the ends of hydraulic cement briquettes and then pulling them apart with a cement-testing machine. Brass briquettes have been used instead of the cement ones, the surface to which the asphalt is applied being left rough. When using these briquettes, they are heated in water to about 140° F., taken out and dried, and the separate ends dipped into the molten asphalt. The two ends of the briquette are then pressed together and allowed to cool, after which they are tested in the same manner as are hydraulic cement briquettes.
At present the only test in general use is to compare different asphalts for adhesiveness by the sense of touch.
Cohesion. Attempts have been made to determine the tensile and the compressive strength of asphaltic cement, by much. the same methods as those employed in testing hydraulic cement, but owing to difficulties in making the tests the practice has not been generally adopted. The strength of asphaltic cement varies materially with the temperature, and flows perceptibly at ordinary atmospheric temperatures; and therefore the details of the experi ments materially affect the results.*