## 4 Upscaling to RC Beams
### Results of the RC Beams  
Figure 9 shows the clustering results of Beam-1 (left) and Beam-2 (right). Both peak (top) and center (bottom) frequency of each AE event over time are shown. Moreover, the moving average (200 datapoints) is visualized. The grey shaded area indicates the time period in which the surface crack must have occurred. This is the period between the last crack measurement where no crack was observed and the first crack measurement where the surface crack was visually observed in the zone of interest between the AE sensors. Note that a technical issue with the AE equipment was noticed for Beam-2 during which no AE signals were saved. This time period is indicated by the dashed area.  
For Beam-1, the moving average of the peak and center frequency starts to decrease around day 5. At this moment, also an increase of the events belonging to the red cluster can  
Figure 8: Schematic representation of the accelerated corrosion setup and 2D AE sensor configuration (S1–S6) of the second test series of RC beams (Beam-3 and Beam-4).  
be noticed. Therefore, as this lies within the period that the crack occurred, this indicates the onset of macro-cracking. For Beam-2, an increase of the red cluster was observed as well as a sudden drop of the moving average of the peak frequency around day 6. The average peak and center frequency start to decrease earlier, namely around day 3. To indicate the grey area representing the results of the crack measurements, only the crack measurement points in between the AE sensors are taken into account which is in this case a zone with a length of 500 mm. However, a crack was observed outside the zone covered with AE monitoring on day 6. This crack must have occurred between day 1 (first crack measurement, no crack) and day 6 (second crack measurement, small crack outside AE zone) explaining the group of AE events of the second cluster (red) between days 3 and 5.  
Figure 10 shows the clustering results of Beam-3 (left) and Beam-4 (right). For Beam-3, a small decrease in average peak and center frequency and increase of the number of events of the red cluster is observed around day 4. Indeed, at the first crack measurement, two longitudinal cracks with an average crack width of 0.05 mm were observed. A second and more clear decrease of the peak and center frequencies can be observed around day 8 meaning that the longitudinal cracks grew further. For Beam-4, the moving average of the peak and center frequency starts to decrease from day 2. Unfortunately, no crack measurement was performed at this moment. Based on the observations of the other samples, macro-cracking may have started early during the corrosion test. Besides longitudinal cracks on the top surface, this beam also showed severe cracks at the sides.  
It can also be observed from Figures 9 and 10 that the typical accelerated corrosion setup aims at obtaining very high corrosion levels in a short time interval, resulting in early concrete cracking and high damage rates that might not be representative for on-site corrosion damage processes. Therefore, a further validation of the clustering algorithm on samples with slower or natural corrosion rates will be done in future work as a next step towards on-site AE monitoring. Nevertheless, the results of the corroded beams  
Figure 9: Clustering results for Beam-1 (**left**) and Beam-2 (**right**) represented as peak frequency (**top**) and center frequency (**bottom**) versus time. The period in which the surface crack occurred is indicated by the grey area.  
indicate that the developed AE clustering algorithm is able to distinguish the initiation of corrosion-induced concrete macro-cracking.