## 4 Results  
### Results of Monitoring Corrosion Using UGWL  
The change in the recorded UGWL amplitudes during the exposure period are attributed to corrosion activity, given this was the only variable in the experiment. Further, according to previous findings by [PERSON] et al. [1,2], an increase in the measured amplitude values indicate the onset of corrosion in RC structures. This is because early corrosion improves the bond between concrete and rebar, thus increasing the energy leaked into the surrounding concrete. The first statistically significant increase in amplitudes (53.5%) is observed on day 9 at 30 cm away from the edge (Figure 5). Sample measured amplitudes using UGWL method is presented in Appendix A (Table A1). The expected exponential decay of amplitude measurements of the leaked waves along the length of specimen is shown using two theoretical curves in Figure 5. [PERSON] [27] previously determined these two exponential curves, i.e., the upper and lower bounds, by assuming a high and a low attenuation coefficient. It was observed that the dimensions of the test specimen affected the attenuation. The smaller the dimensions of the test specimen, the more likely that the reflection within the concrete would be detected by sensors, resulting in more variable and higher amplitude readings at the starting end of the guided wave, as well as quicker attenuation. Thus, use of a higher attenuation coefficient is more suitable for smaller specimens. In a study conducted by [PERSON] [27], the smaller specimen measured 45.7 cm \(\times\) 45.7 cm \(\times\) 12.7 cm (18 in \(\times\) 18 in \(\times\) 5 in). The attenuation coefficient associated with this smaller size (determined as 0.052) represents the lower bound and it is demonstrated as solid lines in all of the results plots is the paper. The larger specimen used by [PERSON] [24] measured 168 cm \(\times\) 45.7 cm \(\times\) 12.7 cm (66 in \(\times\) 18 in \(\times\) 5 in) and showed lower attenuation compared to the smaller specimen. This upper limit exponential curve is represented by a dashed line in the plots. The attenuation coeffici  
Figure 5: Specim A UGWL data: days 0, 3, 6 and 9.  
from the leaked waves in larger specimens is identical to the attenuation of the guided wave itself, as expected. In this paper, with the specimens having the dimensions of 91.4 cm \(\times\) 45.7 cm \(\times\) 12.7 cm (36 \(\times\) 18 \(\times\) 5 in), it is more likely that the attenuation coefficient in this case is similar to that of the smaller specimen (higher attenuation) obtained by [PERSON] [27]. However, both attenuation curves are used to develop an envelope to better understand this set of specimens' attenuation behavior. Figure 6 illustrates select days of measurements from day 0 up to 175 days for the full array of sensors. When the entire period of 175 days is considered, the most significant changes are observed at 38 cm (15 in) and 53 cm (21 in) from the transmitter end. The percent change between \"Day 0\" and \"Day 30\" of corrosion for the point at 38 cm (15 in) reaches 95%. Figure 7 further illustrates the change in amplitudes for these two points over the entire period of the experiment. Given that the specimen was continuously submerged in NaCl solution during this period, without any other interventions or change in circumstances, the changes in the UGWL amplitude are attributed to corrosion progression. As can be seen in Figure 7, there is some fluctuation in the UGWL amplitude measurements. This fluctuation is attributed to testing or user errors (such as the sensor location being slightly different from the previous test, different amounts of couplant used, etc.) as well as the effects of ambient noise. However, when the change in the condition is significant enough (i.e., when a significant amount of corrosion product is built up), there is a significant (or \"permanent\") change in the amplitude values. Table 3 quantifies the change in amplitudes for these two points over the entire period of the experiment.