## 3 Results and Discussions  
### Two-Probe Configuration Results  
For the cement composites of two probe configuration, the change in resistivity as a function of the curing age are presented in Figure 3, and their values are given in Table 2. The cement composite test samples were prepared based on different sets of values of the CF content, CF length, and electrode spacing. The resistivity of all the cement composites with 0.1% CF increased, clearly, until 28 days of curing. This is attributed to the decrease in the pore solution. Generally, at 0.1% CF content, the resistivity of the cement composite is affected by the electrolyte because the CF content is not sufficient to predominate the electrical conductivity of the cement composite. Beyond 0.5% CF content, there was no significant increase in the resistivity with respect to the increase in the curing age. Therefore, it was concluded that the influence of CF was greater than that of the electrolyte on the electrical conductivity of the cement composite. Figure 3: Variation in resistivity of cement composites as a function of the curing age for the two-probe configuration; (**a–c**) electrode spacing of 10 mm, (**d–f**) electrode spacing of 25 mm, and (**g–i**) electrode spacing of 40 mm. L04, L06, and L10 correspond to the carbon fiber (CF) lengths of 4, 6, and 10 mm, respectively. 2E10, 2E25, and 2E40 indicate the electrode spacings of 10, 25, and 40 mm, respectively. CF0.1–2.0 indicate the CF content (0.1–2%) incorporated into the cement composites. The resistivity of the cement composites after 28 days of curing is plotted in Figure 4. Regardless of the CF length, the cement composites with 0.1% CF (L04-CF0.1, L06-CF0.1, and L10-CF0.1) showed insignificant changes in resistivity with increasing electrode spacing. This is because 0.1% CF did not affect the electrical conductivity of a cement composite, as mentioned in the previous paragraph. For a relatively high CF content, the cement composite containing 4 mm CF of 2% (L04-CF2.0) showed a marginal change in resistivity with increasing electrode spacing, while the resistivity of the cement composite with 10 mm CF of 2% (L10-CF2.0) decreased at an electrode spacing of 40 mm (Figure 4c). This is attributed to the fact that when the CF length is the same as the electrode spacing value (10 mm in this case), the distribution of CF is difficult; however, when the electrode spacing is four times the  
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**7 Days**} & \multicolumn{2}{c}{**14 Days**} & \multicolumn{2}{c}{**21 Days**} & \multicolumn{2}{c}{**28 Days**} \\ \cline{2-9}
**Mixture ID** & **Resistivity** & **S.D. ** & **Resistivity** & **S.D. ** & **Resistivity** & **S.D. ** & **Resistivity** & **S.D. ** \\ \hline L04-2E10-CF0.1 & 482.62 & 1.63 & 1010.59 & 2.65 & 1461.56 & 0.55 & 2949.77 & 20.21 \\ L  
CF length, dispersion of CF is improved because of increased space (CF length: 10 mm and electrode spacing: 40 mm). The effects of the CF lengths, electrode spacing and CF contents on the resistivity of cement composites are shown in Figure 5. The resistivity beyond the 2% CF content was determined by extrapolation. Interestingly, the resistivity of the cement composite increased with increasing CF length at a relatively narrow electrode spacing, as shown in Figure 5a,b. This result contrasts the CF length effect, wherein the longer the CF, the lower the resistivity of the cement composite [21; 42; 43]. At the electrode spacings of 10 and 25 mm, the cement composite with 4 mm CF (L04-2E10, L04-2E25) showed the lowest resistivity beyond 2% CF content. A relatively narrow electrode spacing with respect to the CF length might hinder the dispersion of CF. This is because a long CF tends to entangle with one another [38]. Additionally, [PERSON] [46] reported that the cube size can influenced affect the percolation threshold content of CF, particularly if it is smaller than CF length.