## 3 Results and Discussions
### Four-Probe Configuration Results  
For the cement composites of four-probe configuration, the change in resistivity as a function of curing age is presented in Figure 6, and their values are given in Table 3. Regardless of the CF length, the cement composites incorporating more than 0.5% CF showed constant resistivity despite the increase in curing age. In Figure 6, the resistivity of the cement composites with 0.5% CF can be clearly distinguished from the other cement composites with different CF contents; no further decrease in the resistivity was observed beyond 1.0% CF content, indicating that the percolation threshold was 1.0% CF content [11; 22; 24]. This result is more clearly seen in Figure 7.  
For the cement composites of four-probe configuration with 10 mm electrode spacing, the change in the resistivity as a function of CF content is shown in Figure 7. Similar trend in the decreasing resistivity with increasing CF lengths was also observed in the four-probe configuration, as in the two-probe configurations (Figure 5). This is because the relatively short 4 mm CF (L04) is easily dispersed, the resistivity of the cement composite decreased continuously with the increase in CF contents, while relatively long 6 or 10 mm CFs (L06 and L10) were difficult to be dispersed in the electrode spacing of 10 mm. Poor dispersion may cause an increase in the resistivity of the cement composite. This phenomenon became worse with increasing CF content. Figure 5: Change in resistivity of cement composites as a function of the carbon fiber (CF) content measured by the two-probe configuration for different CF lengths and electrode spacings of (**a**) 10 mm (2E10), (**b**) 25 mm (2E25), and (**c**) 40 mm (2E40). L04, L06, and L10 correspond to the CF filament lengths of 4, 6, and 10 mm, respectively. Figure 6: Resistivity of cement composites as a function of the curing age measured via four-probe configuration for different carbon fiber (CF) contents and electrode spacing of 10 mm (4E10) for CF lengths of (**a**) 4 mm, (**b**) 6 mm, and (**c**) 10 mm. L04, L06, and L10 correspond to the CF lengths of 4, 6, and 10 mm, respectively. CF0.1–2.0 indicate the CF content (0.1–2%) incorporated into the cement composites. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Mixture ID**} & \multicolumn{2}{c}{**7 Days**} & \multicolumn{2}{c}{**14 Days**} & \multicolumn{2}{c}{**21 Days**} & \multicolumn{2}{c}{**28 Days**} \\ \cline{2-9}  & **Resistivity** & **S.D.** & **Resistivity** & **S.D.** & **Resistivity** & **S.D.** & **Resistivity** & **S.D.** \\ \hline L04-4E10-CF0.5 & 225.21 & 184.30 & 249.31 & 182.93 & 290.19 & 195.22 & 286.05 & 205.15 \\ L06-4E10-CF0.5 & 111.28 & 71.07 & 178.34 & 78.80 & 183.01 & 89.74 & 177.04 & 89.75 \\ L10-4E10-CF0.5 & 242.40 & 96.21 & 267.67 & 93.00 & 226.16 & 69.07 & 220.29 & 69.20 \\ L04-4E10-CF1.0 & 38.23 & 0.98 & 34.66 & 2.70 & 32.31 & 0.82 & 26.31 & 1.30 \\ L06-4E10-CF1.0 & 68.20 & 10.27 & 67.55 & 12.02 & 62.89 & 9.89 & 56.76 & 9.80 \\ L10-4E10-CF1.0 & 70.42 & 15.84 & 68.14 & 16.54 & 66.71 & 17.22 & 60.11 & 17.86 \\ L04-4E10-CF1.5 & 29.52 & 1.28 & 26.35 & 2.91 & 25.36 & 2.13 & 19.15 & 1.71 \\ L06-4E10-CF1.5 & 44.92 & 0.44 & 52.35 & 2.88 & 54.22 & 2.41 & 48.46 & 2.37 \\ L10-4E10-CF1.5 & 57.84 & 13.34 & 57.79 & 15.95 & 61.42 & 21.25 & 54.18 & 20.48 \\ L04-4E10-CF2.0 & 21.09 & 2.83 & 20.12 & 1.92 & 19.48 & 2.28 & 13.53 & 2.32 \\ L06-4E10-CF2.0 & 36.58 & 4.38 & 40.54 & 5.59 & 36.64 & 4.24 & 30.55 & 4.25 \\ L10-4E10-CF2.0 & 52.21 & 9.01 & 47.48 & 7.82 & 44.82 & 8.64 & 38.91 & 8.09 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Resistivity [Ohm-cm] of cement composites as a function of the curing age for the four-probe configuration; each resistivity is the average of 3 test samples, and S.D. stands for standard deviation.