## 3 Experimental Results
### Heat Test and Thermal Imaging Analysis Results
The cement paste exhibited the best heating property because sand or fine aggregates did not interfere with the current flow in the composite and the environment was favorable for forming a CNT network. The temperature difference owing to the presence of damage was as high as 16.6 \({}^{\circ}\)C for cement paste, 13.4 \({}^{\circ}\)C for mortar, and 2.8 \({}^{\circ}\)C for concrete. Damage to cement paste and mortar can be detected based on heat properties. However, damage detection is challenging for concrete because the heat characteristic is the lowest, and the difference in the heating value owing to the damage occurrence is insignificant. The heat properties of each composite improved with increasing MWCNT concentration. Furthermore, as the curing period increased, the heat properties deteriorated because the effect of moisture inside the composites was reduced. Figure 7 shows the thermal images of the composites with 0.5 wt% MWCNT after 7 d of curing. The thermal images are images of the composite surface temperatures captured with a thermal imaging camera, based on which the heat property can be examined visually. The surface temperature was measured by adding the heating value generated by the applied voltage to the temperature of the composite before the voltage was applied. The temperature of the composites before the voltage was applied was approximately 22 \({}^{\circ}\)C to 25 \({}^{\circ}\)C. When a 60 V voltage was applied, among the composites with damage  
\begin{table}
\begin{tabular}{c c c c} \hline
**Specimen Name** & **Composite Type** & **Curing Days** & **Concentration (wt\%)** & **Temperature (\({}^{\circ}\)C)** \\ \hline CP-0.125-7D-X & & & 31.5 \\ CP-0.125-7D-O & & & 0.125 & 25.7 \\ CP-0.25-7D-X & & & 47.4 \\ CP-0.25-7D-O & & 7 & 0.25 & 40.6 \\ CP-0.5-7D-X  
(Figure 7a), the surface temperature of CP-0.5-7D-X was the highest at 90.3 \({}^{\circ}\)C, whereas that of CP-0.5-7D-O was the highest at 74.2 \({}^{\circ}\)C. The temperature variation of the composites without damage occurred at the midpoint between the electrodes because electric currents flow along the shortest distance. In contrast, the temperature variation of the composites with damage occurred around the position where the electrode was inserted, instead of the midpoint, because an empty space was present at the center. Damage detection using thermal image analysis is feasible for cement paste because it shows a distinct difference in the thermal image according to the presence of damage. A thermal image of mortar with damage is shown in Figure 7b. When a voltage of 60 V was applied, the highest surface temperature of the mortar was 46.5 \({}^{\circ}\)C for M-0.5-7D-X without damage and 34.1 \({}^{\circ}\)C for M-0.5-7D-O with damage. The surface temperature of the mortar was 43.8 \({}^{\circ}\)C lower than that of the cement paste with the same MWCNT concentration, and the temperature difference owing to the presence of damage was verified through the thermal images. Figure 7c shows the results of analyzing the concrete surface temperature according to the presence of damage using a thermal imaging camera. The surface temperature of the concrete was 34.4 \({}^{\circ}\)C for Co-0.5-7D-X and 31.5 \({}^{\circ}\)C for Co-0.5-7D-O The change in the thermal image of the concrete owing to the presence of the damage was not noticeable, compared to that of cement paste and mortar. Figure 8 shows the thermal images of composites with 0.5 wt% MWCNT after 28 d of curing. Figure 8a shows the thermal image corresponding to the highest surface temperature of the cement paste, generated by an applied voltage of 60 V. The surface temperature of the cement paste without damage was the highest at 65.5 \({}^{\circ}\)C for CP-0.5-28D-X, which is similar to that with 7 d of curing. The surface temperature of the cement paste with damage was 54.3 \({}^{\circ}\)C for CP-0.5-28D-O, which was 19.9 \({}^{\circ}\)C lower than that after 7 d of curing. Damage to the cement paste could be detected visually using thermal images.