Total Charge

Although the total charge passed Q_T increased as the specimen conductance increased, the values are not linearly proportional to one another. For samples which there are values of Q_T for both specimen lengths, the values are not inversely proportional to the specimen length; the value of Q_T for the 50 mm specimen was always significantly greater than twice the value for the 100 mm specimen. Figure 5 shows the measured current for the two specimens from Sample F. The current through the 100 mm specimen is nearly constant throughout the test. The current through the 50 mm specimen appears to increase linearly after approximately 100 minutes. Although the initial currents differed by a factor of two, the final currents did not. The difference is due to ohmic heating.

Figure 5: RCT current for the specimens of different length taken from Sample F.

Since fluid conductivity increases by approximately 2 percent per degree Celsius [10,11], and assuming that specimen pore fluid conductivity behaves in a similar manner, one could estimate sample temperature from the relative change in current. Unfortunately, the specimen temperatures were not measured at the time of the tests and a duplicate experiment that included temperature measurements could not be conducted because the specimens had been discarded.

To demonstrate the effect of temperature, and additional test was performed on a completely different sample from the Federal Highway Administration. For this specimen, the temperature was monitored using an immersion temperature probe incorporating a precision thermistor. The probe was demonstrated to be accurate to within one degree Celsius. The temperature of the solutions at each end of the specimen was measured periodically throughout the RCT test. The data are shown as filled symbols in Figure 6. Along with the symbols is a curve that shows the estimated specimen temperature based upon the increase in current passing through the specimen. Although the measured and estimated values are not equal, they agree to within ten degrees Celsius throughout the test. This is reasonable since the heat generated within the specimen is lost to the solutions and to the room. Also, the results are in general agreement with laboratory experience and with results reported elsewhere [20,21].

Figure 6: Calculated specimen temperature based upon relative changes in conductivity (solid line), and measured values (filled circles and squares).