jan. 2 4 , i 9 i6 Effect of Variables on Asphalt Penetration Test 
813 
Although all of the samples examined hardened very materially upon 
setting for 28 days, it is of interest to note that when these samples were 
remelted, allowed to cool in air for 30 minutes, immersed in the water 
bath at 25 0 C. for hours, and again tested, the penetrations, to all 
intents and purposes, were the same as those originally obtained by 
the 30-minute air cooling and ij^-hour immersion in the bath. This fact 
does not, however, indicate that the materials do not permanently 
harden with age, as Hubbard and Reeve 1 have shown that all types of 
bitumen permanently harden upon prolonged exposure. 
As a result of the foregoing observations, the 30-minute air cooling 
and i>£-hour immersion in the bath prior to the test was adopted as the 
method of preparing samples prior to studying the effect of the vari¬ 
ables, temperature, load, and time. 
EFFECT OF VARIATIONS IN TEMPERATURE 
The penetration of an asphalt cement is frequently determined and 
sometimes specified at three temperatures. The temperature most 
commonly employed and at which the consistency of the material is 
rated is 25 0 C. This is known as normal temperature, and the customary 
load and time factors used are 100 gm. and 5 seconds. 
The penetration test is next frequently made at o° C. with a load of 
200 gm. applied for 1 minute. In some cases the test may be made 
with a load of 100 or 200 gm. applied for 5 seconds. For this test the 
sample is usually packed in finely crushed ice, which completely covers 
it, and the needle is brought in contact with its upper surface through a 
hole in the ice worked out with the finger. The needle itself, as well as 
the exposed surface, may, therefore, at the time of test be at a somewhat 
higher temperature than o°. For this reason 4 0 C. has been selected by 
some for a low-temperature test, as it is a temperature which may be 
accurately maintained in the water bath. 
Another temperature at which the penetration test is made is 46° C. 
Where possible, a load of 50 gm. is applied for 5 seconds, but in the case 
of materials which are very soft at this temperature the 50-gm. load is 
applied for 1 second. 
In order to study the effect of variations in temperature upon the 
penetration test, a number of samples of each of the 12 asphalt cements 
were prepared, and after cooling in air for 30 minutes were placed for 
1% hours in the bath maintained at the test temperature. The results 
of these tests are given in Table V. 
1 Hubbard, Provost, and Reeve, C. S. The effect of exposure on bitumens. In Jour. Indus, and Engin. 
Chem., v. 5, no. 1, p. 15-18. fig. i-a. 1913* 
