Calculated Quantities

The results from the experiment are expressed in calculated values. For clarity, these quantities are defined explicitly. The total charge passed Q_T is calculated from the measurements of the currents I_i conducted at times t_i , that occurred at the i−th minute of the test:

$$Q_{\mbox{\tiny T}}= \frac{1}{2} \sum_{i=1}^{360} (I_i+I_{i-1}) (t_i - t_{i-1})$$ (3)

This method conforms to the procedure specified in ASTM C 1202. The initial RCT resistance R_o (ohms) was calculated from the initial current I_o (amps):

$$R_o = \frac{60}{I_o}$$ (4)

As indicated in Fig. 3, the bulk resistance R_B measured using IS was determined from the value of Z ' at the minimum value of -Z". Specimen conductivities σ_IS and σ_RCT were calculated using the measured resistance and the specimen length L and area A:

$$\sigma_{\mbox{\tiny IS}}= \frac{L}{A\,R_{\mbox{\tiny B}}} \hspace{0.5in} \sigma_{\mbox{\tiny RCT}}= \frac{L}{A\,R_o}$$ (5)

Since a number of studies reported in the literature have used a fixed frequency, it will be instructive to determine the validity of this approach. The quantity R₂₀ is the real component of Z(ω) at a frequency of 20 kHz, a frequency that has been used in similar experiments with a fixed frequency [16, 18, 19].