jan. 34. 1916. Effect of Variables on Asphalt Penetration Test 
807 
It is assumed that in all instances laboratories take representative sam¬ 
ples. The handling of the sample, once it is taken, however, is subject 
to a number of conditions which are not ordinarily strictly specified. 
In the first place, the sample must be melted by the application of heat 
and, to prevent any change during the melting process, it should be 
heated at as low a practicable working temperature as consistent with 
the time required to melt it. That is, all asphalts and asphalt cements 
tend to harden upon being heated, due either to loss by volatilization or 
to so-called oxidation or reaction with atmospheric air. This tendency 
is increased as both the temperature and time of melting are increased. 
The method followed in preparing all of the samples for this investiga¬ 
tion was as follows: 
About 6 ounces of each of the 12 materials were placed in pint tin cups. 
The 12 cups were then placed upon a }^-inch asbestos board resting 
directly upon a gas hot plate. The samples were stirred occasionally to 
expedite melting, and removed from the hot plate as soon as completely 
fluid. At no time were the samples heated sufficiently to produce fuming. 
Upon removal from the hot plate the samples were poured into 3-ounce 
cylindrical tin dishes, measuring 5.5 cm. in diameter, with vertical sides 
approximately 3.5 cm. in height. While still fluid, all air bubbles which 
rose to the surface were removed by means of a tiny gas flame, which was 
rapidly passed over the surface and which merely caused the bubbles to 
break without in any way injuring the sample. 
As the effect of the size of the container upon the results of tests had 
been investigated by Reeve, 1 it was felt that by the use of the dish above 
stated no danger of influencing results from this cause need be feared. 
In this connection it is of interest to note that Reeve's work demonstrated 
that a dish of 5 cm. or more in diameter could not influence the results 
of tests, although appreciable variations in results were in some cases 
caused by dishes smaller than 2.5 cm. in diameter. 
EFFECT OF VARIATIONS IN METHOD OF PREPARING MELTED 
SAMPLES FOR TESTING 
Undoubtedly the most common method of preparing a melted sample 
for the penetration test is to allow it to cool in air at room temperature 
for approximately an hour, then to immerse it for an hour in water main¬ 
tained at the temperature at which the test is to be made. The sample 
is then tested under water at this temperature. In certain cases, cooling 
the sample in ice water or crushed ice prior to immersing it in the constant- 
temperature bath has been resorted to, and the penetrations so obtained 
have frequently been somewhat lower than those obtained by the method 
first described. As great a difference as 15 points in one asphalt cement 
1 Reeve, C. S. Effect of diameter of bitumen holder on the penetration test. In Proc. Intemat. Assoc. 
Testing Materials (6th Cong. New York 1912), v. 2, no. ix, Paper 25^ 4 p. 191a. 
17211°—16-3 
