"The determination of the organic matter not bitumen, or, as it is often called, the foreign organic matter, is made by burning, in a tared platinum crucible, the residue left in the tube after ex tracting the bitumen." t Standard Consistency. The softness or the consistency of an asphaltic cement is determined by measuring the distance a standard needle will penetrate the mass in a specified time. There are two forms of apparatus in use for this purpose, Bowen's and Dow's.
1. Bowen's Penetration Apparatus. This apparatus consists of a needle of standard weight and size, whose vertical motion is registered by an index moving over a graduated disk. This is ac complished by inserting a large sewing needle in the free end of a lever arm which is supported by a thread wound around a spindle. The spindle carries an index which moves over a graduated disk. On the spindle is a drum round which winds a thread supporting a weight which acts as a counterbalance to the weight of the lever arm. This counter-weight keeps the thread taut; and when the lever arm is raised, it turns the pointer on the dial. The penetra tion of a sample is taken by lowering the needle until it is just in contact with the surface, and then releasing a clamp which allows the needle to penetrate into the sample for a specified time—usually 1 second. At the end of the time the clamp is closed, and the de gree of penetration is noted from the dial.
The sample must be kept at a standard temperature, usually 77° F. (25° C.) for at least half an hour before making the test. This is most accurately done by keeping the machine and samples in a small room maintained at the standard temperature; but is usually done, particularly at the mixing plant, by immersing the samples in a tank of water.
Preceding 1899, this apparatus was the standard in Washington, 1). C., and the penetration of the Trinidad asphaltic cement used during the year ending June 30, 1897, by the two paving companies doing work there was respectively: average 77 and 85, maximum 83 and 100, minimum 74 and 76.
2. Dow's Penetration Apparatus. This consists of an ordinary No. 2 sewing needle fastened into the end of a small brass rod which in turn is fastened into the end of a metal tube about 4 centimeters (1.6 inches) long and 1 centimeter (0.4) inch in diameter. Mercury is poured into the tube to give it any desired weight from 30 to 300 grams. The brass rod and the tube with needle end down, slides freely up or down through a frame, and can be held in any position by a clamp. The motion of the sliding part is communicated by a
thread to an index arm moving over a graduated disk. The test samples are kept in a water-jacketed copper box, which rests in a tank supplied with inlet and outlet pipes whereby a constant tem perature may be maintained. The apparatus has several minor devices to facilitate its use and to insure accuracy.* The unit is the distance in hundredths of a centimeter that a No. 2 needle will sink into an asphalt paving cement in five seconds when weighted with 100 grams, the cement and the apparatus being at a temperature of 77° F. (25° C.). The penetration can be meas ured with accuracy to one fiftieth of a millimeter (0.000,8 inch). This instrument is a later invention and is claimed to be more ac curate than the Bowen apparatus. Since Jan. 1, 1899, this instru ment has been the standard in Washington, D. C., and the average penetration of the Trinidad asphalt cement used by the two com panies doing the work in that city during the year ending June 30, 1899, was 36 and 45. In a rough way the unit of this apparatus is about twice that of the Bowen machine.
The proper softness or viscosity of the asphaltic cement depends upon the kind and the amount of the traffic, the range of atmospheric temperature, the character of the sand used (§ 621), and the susceptibility of the bitumen to changes of temperature (§ 615) and to changes with age (§ 616). No general standards have been established, and but little accurate information is known to the public concerning the experience in any city except Washing ton, D. C. The degree of penetration employed in that city with both the Bowen and the Dow apparatus is stated in the preced ing section.
Variation of Consistency with Change of Temperature. The susceptibility of the paving cement to changes in temperature is determined by taking the penetration of the substance at several different temperatures and noting the variation caused by the rise or fall. The chief cause of the susceptibility to changes in tem perature is the presence of the paraffin added in the fluxing agent. Paraffin at ordinary temperatures is an inert solid; but as soon as heated to the melting point it quickly liquefies, and then acts as so much additional flux, thus suddenly changing the consistency of the asphaltic cement.