## 3 Results and Discussion
### Sensing Capability under Fatigue Test Conditions  
Figure 9 reports an example of a sensor electrical response (in Volts) recorded under a cyclic test. In the fatigue test setup, a small pre-load was applied to the specimens before the test started. In this condition, the upper side of the specimen experiences compression, and the opposite side experiences tension during the cyclic testing in the three-point-bending configuration. In the area subjected to compression, there is an increased chance for adjacent MWCNTs to connect each other or reduce the gap between them. As a result, the system conductivity increases, i.e., the potential difference between two probes of the sensor (Delta V) decreases. On the other hand, in the area subjected to tension, the tensile stress causes MWCNTs slipping, pulling off, and cutting along with cracking of the concrete matrix. These flaws disconnect the MWCNTs conductive network and cause a decrease in the system conductivity. [PERSON] et al. (2013); [PERSON] et al. (2015); [PERSON] et al. (2015); [PERSON] et al. (2015) mentioned that the sensing behavior under flexural loading is a composition of sensing properties under compression and tension. Furthermore, the sensing behavior heavily depends on the components of the self-sensing concrete and the layout of the electrodes. Therefore, different sensing behaviors of self-sensing concrete under flexural loading were observed in previous research. As shown in Figure 9, for the specific settings used in the present study, the electrical response of the concrete sensor undergoes a decrease during fatigue loading. In fact, dominant behavior in flexural is near compression and not tension.  
[PERSON] et al. reported a very similar finding in some of their research studies as detailed in ([PERSON] et al., 2016; [PERSON] et al., 2017). They mentioned that damage in the fatigue process was observed in the FRC composite and an irreversible decrease in the electrical resistivity response of the composite was observed. They assumed that the resistivity decrease could be attributed to damage in the composite matrix. They concluded that the greater the stress level, the greater the damage, and the greater the resistivity decrease ([PERSON] et al., 2016; [PERSON] et al., 2017).  
By investigating the sensor behavior, it appears that a relation between the fractional change in electrical resistance and stress can be found in each cycle. Indeed, the electrical response of the sensor decreased with loading and increased during unloading. However, when a large number of loading cycles are applied, another superimposed trend can be observed, as shown in Figure 9. According to this trend, the baseline of the electrical response of the sensor decreases irreversibly and gradually cycle by cycle and it is approximately linear up to failure, apart from the initial rapid drop. Investigation of previous literature studies ([PERSON] et al., 2016; [PERSON] et al., 2015; [PERSON] et al., 2015) showed that this trend (descending or ascending trend of the electrical response of the sensor over time) is also visible in their results.  
Figure 9: Electrical response of self-sensing concrete during the fatigue test.