## 3 Experimental Results
### Heat Test and Thermal Imaging Analysis Results
The difference in the thermal images of the mortar owing to the presence of damage is illustrated in Figure 8b. The surface temperature of the mortar was 40.3 \({}^{\circ}\)C for M-0.5-28D-X  
Figure 7: Thermal images at 7 curing days: (**a**) cement paste, (**b**) mortar, and (**c**) concrete (all at 60 V). and 28.7 \({}^{\circ}\)C for M-0.5-28D-O, which was up to 6.2 \({}^{\circ}\)C lower than that obtained after 7 d of curing, and the difference in surface temperature owing to the occurrence of damage was 11.6 \({}^{\circ}\)C. As with the 7 d of curing, the deteriorated heat property of mortar cured for 28 d was verified visually using thermal images. Thus, damage to the mortar could be detected visually using such images. The difference in the thermal images of concrete owing to the presence of damage is illustrated in Figure 8c. The surface temperature of the concrete was 30.5 \({}^{\circ}\)C for Co-0.5-28D-X, which was 3.9 \({}^{\circ}\)C lower than that with 7 d of curing, and 29.8 \({}^{\circ}\)C for Co-0.5-28D-O, which was 1.7 \({}^{\circ}\)C lower than that with 7 curing days. Because the difference in surface temperature owing to the presence of damage was only 0.7 \({}^{\circ}\)C, it was difficult to verify the change in the thermal images. The damage to the concrete with MWCNTs could not be identified through a visual inspection based on thermal imaging analysis. Figure 9 shows the temperature variation of the composites over time. The heat properties of each specimen were measured for 60 min, and a voltage of 60 V was applied. The plots of cement paste according to the parameters are shown in Figure 9a,b. The temperature gradient of the cement paste with 0.125 wt% MWCNT remained constant after the highest temperature. In contrast, the temperature gradients of the cement pastes with 0.25 wt% and 0.5 wt% MWCNT increased over time and decreased after the highest temperature. As shown in Figure 10, the temperature decreased because cracks were generated in the composite and the mesh became corrupt owing to the high temperature. The highest temperature and temperature gradient of the cement paste with damage decreased compared to those of the cement paste without damage. The heat property deteriorated because the length of the shortest distance path of the current increased owing to the occurrence of damage and the cross-sectional area decreased, which led to an improvement in the electrical resistance property. Furthermore, the initial temperature gradient over time and the highest temperature both increased with increasing MWCNT concentration, but both decreased as the curing period increased. After 7 d of curing, the temperature gradient of the MWCNT cementitious composites increased because the current inside the MWCNT cementitious composite could flow efficiently owing to  
Figure 8: Thermal images at 28 curing days: (**a**) cement paste, (**b**) mortar, and (**c**) concrete (all at 60 V). the moisture in the composite. The temperature variations over time of the mortar with MWCNTs are shown in Figure 9c,d. The temperature gradient of the mortar decreased compared to that of the cement paste at the same concentration. However, the temperature variation of mortar over time remained constant after the highest temperature at every concentration, unlike the cement paste. The temperature reduction owing to the occurrence of damage was the greatest in M-0.5-7D-X and M-0.5-7D-O, and the temperature gradient over time also decreased. Figure 9e,f shows the temperature plot of the concrete over time, which did not decrease after the highest temperature, and the temperature gradient after 7 d of curing was higher than that after 28 curing days. Figure 9: Temperature–time history: (**a**) cement paste (curing for 7 d), (**b**) cement paste (curing for 28 d), (**c**) mortar (curing for 7 d), (**d**) mortar (curing for 28 d), (**e**) concrete (curing for 7 d), and (**f**) concrete (curing for 28 d).