Heat Test. This test is employed to determine the amount of the lighter and more volatile hydrocarbons in the bitumen. It separates the petrolenes from the heavier hydrocarbons (malthenes and asphaltenes). The test is made by determining the loss in weight suffered by the material upon being heated for a definite time at a constant temperature. It is designed to show whether the asphalt cement will be materially changed by heat in forming the surface mixture. In making the test, cylindrical dishes about 2i inches in diameter and i to 2 inches high are commonly em ployed. Mr. Richardson * recommends the use of 20 grams of the asphalt cement, heated to a temperature of 325° F. or 400° F. for 7 hours. For this purpose it is necessary to have an oven which can be maintained at a uniform temperature. Mr. Dow has suggestedt that the temperature to be used in the test should be varied according to the heat required in working the material.
" It is to be hoped in the near future that an improved modification of this test can be adopted on the following lines: As there is considerable variation in the degree of temperature at which different asphaltic cements become sufficiently liquid to mix with the mineral ingredients, this test would be more practical if the cements were tested at the temperatures at which it is practical to work them. As for example: one cement may be so fluid at 25o° as to be capable of mixing just as well as another one at 300°. In the testing of these two cements it would only be necessary to test the one at 25o° and the other at 300°. This modification in the test will be adopted as soon as an apparatus has been perfected for determining the viscosity of molten asphalt cements at high temperatures." Penetration Test. The consistency of asphalt cement is determined by measuring the penetration of a No. 2 needle under a standard weight (usually zoo grams), in a given interval of time (commonly 5 seconds). The tests must be made at a standard temperature (usually 77° F.). Machines for making this test have been devised by Mr. A. W. Dowt and by Clifford Richardson. $ The Dow apparatus consists of a No. 2 needle in serted in a short brass rod which is held in an aluminum rod by a binding screw. The aluminum rod is secured in a framework so balanced that when it is supported on the point of the needle the framework and rod will stand in an upright position, allowing the needle to penetrate perpendicularly without the aid of support. The frame, aluminum rod, and needle weigh 50 grams; additional weight, when desired', is placed on the bot tom of the frame. The motion of the sliding part is communicated by a thread to an index arm moving over a graduated disk.
"To make the penetration test, the samples of asphalt cement contained in circular tins, along with the glass dish, are placed in a receptacle containing at least 5 inches of water, which should have been pre viously brought to the temperature at which it is desired to make the test. While the samples are under the water it should be stirred every few minutes, best with a thermometer, and the temperature kept con stant when necessary by the addition of hot or cold water as the case may require. The samples should remain under water at least 15 minutes, and in cases where their temperature is not near that at which the test is made they should be left in possibly half an hour. After the samples have remained in the water a sufficient time to have attained its temperature they are ready to be penetrated." For the purpose of determining the effect upon con sistency of changes of temperature, tests are made of the penetration at different temperatures. Mr. Dow recommends the following standards: "The needle which I have adopted as a standard for penetration is a No. 2, manufactured by R. J. Roberts, Redditch, England. All the needles, however, obtained in a package cannot be used for penetrating, as they vary somewhat in shape, and only those are selected which give a penetration corresponding to the standard needle. The standards that I have adopted for this machine are : At 32° F. or lower, the distance in one
hundredths of a centimeter that a No. 2 needle will penetrate into the sample in one minute of time when weighted with 200 grams. For tests made at a tem perature of 77° F., the distance in one-hundredths of a centimeter that a No. 2 needle will penetrate into the sample in 5 seconds of time when weighted with zoo grams. For tests made at a temperature of 100° F., or above, the distance in one-hundredths of a centi meter that a No. 2 needle will penetrate in 5 seconds of time weighted with 5o grams.
"The following is a table giving the penetration and ductility of three classes of asphalt cement, which I have designated as A, B, and C: " It has been found from practical experience that it is not safe to use an asphalt that is more susceptible to changes in temperature than sample A, given in the table, for if it were more susceptible than this, and made to a softness to give sufficient ductility at low temperatures, it would be too soft for use at high temperatures. The average paving cement gives penetrations such as represented by B in the table. Sample C in the table represents the least susceptible cement which I have found on the market. This non susceptibility to change in temperature would be of great advantage if it were not for the fact that the cement is lacking in ductility. There is a law which I have found that invariably applies to the proper ties of asphalt cements, that is, that the less susceptible cement is to change in temperature, the less ductile it is at normal temperatures, and inversely, the more susceptible the more ductile is the cement." Test for Ductility. Mr. A. W. Dow * has proposed a test of the ductility of asphalt cement by determining the distance in centimeters that a prism of cement can be drawn out before breaking. The prism he uses is 5 centimeters in length with a square cross-section of centimeter. The test piece is molded with the ends in clips, which may be attached to apparatus for applying the pull. The clips are pulled apart' at a speed of i centimeter per minute, while immersed in water at the required temperature. " Sufficient work has not been done on the ductility test at low temper atures to be able to state any standard at the present time, but it has been found that it is not safe for an asphalt having a consistency of 4o penetration at 77° F. to pull less than 20 centimeters at this temperature in the above ductility test." Im˘act Test. An impact test for the purpose of determining the toughness of asphalt surface mixtures has been proposed by Messrs. Richardson and Forrest.t "The test pieces were made as follows: The surface mixture was brought to such a temperature as would be found necessary in handling it upon the street, a weighed amount, such as has been found by experi ence would yield a cylinder after compression of inch in height, is placed in a cylindrical mold, closely resembling the ordinary diamond mortar of the labo ratory, of a diameter of if inch. The mold is supported on the rigid block of timber in by 9f inches square by 321 inches high. The warm steel plunges is placed upon the top of the hot mixture, above which is a cylinder of steel weighing io pounds, running in grooved guides, which can be allowed to fall upon it from a height of 3 feet. After a few gentle taps to seat the plunger, the weight is raised and allowed to fall freely io times. The cylindrical mold is then inverted and the plunger introduced at the other end in a space left for this purpose by a boss on the base supporting the mold. Ten additional blows are then given on this end of the cylinder. In this way it has been found that satisfactory and uniform compression is obtained. The cylinders are then weighted to determine if the density is satisfactory, and measured to see that they are of uniform height, i inch or nearly so. On cooling they are ready to be tested, in the same manner employed by Mr. Page for rock cylinders, at whatever temperature may be selected" (see Art. 37).