## 3 Experimental Test Program and Results
### Corrosion Damage Localization in RC Prisms
PLBs were performed on all sides of the sample (including the bottom side), once when the sample was dry and once when the sample was partially saturated by the sodium chloride solution. It was found that the AE post-processing protocol was able to localize the PLBs executed at the bottom side of the sample. However, the localization error was found to be larger than AE signals originating from PLBs executed within the sensor array. Hence, less reliable source locations at the bottom of the sample will be filtered out during post-processing. A time lapse of the localization result is shown in Figure 9. In the beginning of the test, most events are localized close to the rebar meaning that these events are caused by the corrosion process itself and micro-cracking caused by the expansion of corrosion products. Figure 8: Localization results of sample AE-1 with indication of the error ellipsoids and cracks that were visible on the surface; (**top left**) side view, (**top right**) front view, (**bottom**) 3D views. Figure 7: Unfiltered localized AE events of sample AE-1; (**left**) side view, and (**right**) front view. Afterwards, cracks start to grow from the inside of the sample towards the outside. It can be noticed that many AE events were localized before the cracks could be observed on the surface meaning that the AE technique is able to assess the internal damage earlier than a visual inspection. The time lapse shows why crack 1 was less accurately localized. The cracks start to grow internally, near the rebar. Therefore, as this crack grows, this internal crack eventually prevents the AE signals originating from the bottom part to reach at least 4 sensors. Another sensor layout with sensors at the bottom would overcome this problem. However, also during on-site monitoring, sensors cannot always be positioned optimally, and hence dedicated post-processing is a necessity to avoid false detections and large localization errors. It can be concluded that the initial internal crack was localized correctly. This is validated by comparison with the zone of the rebar that was most heavily damaged, i.e., most mass loss. As damage progresses, crack growth is more difficult to localize correctly due to increased heterogeneity of the concrete. The aim of sample AE-2 was to investigate whether corrosion of a stirrup could be localized correctly. Figure 10 shows the AE source localization results without applying the post-processing protocol. The corroding parts of the rebar and stirrup are again colored in dark grey. The side of the stirrup where most mass loss was observed after cleaning, is indicated in red. The localization result is scattered and fails to provide accurate information on the most damaged part of the sample. A small shrinkage crack could be observed before the start of the accelerated corrosion process. Figure 11 shows the localization result after post-processing. Most AE events are localized around the stirrup at the side where the crack was visible on the surface. The initial shrinkage crack was found to locally promote the chloride ingress, as also observed in the literature [34]. Therefore, the corrosion process likely initiated at this point. This was validated by removing and cleaning of the stirrup as shown in Figure 5:0 A pit can clearly be seen at the spot where the crack and AE events are located. A few AE events were localized around the main rebar. The mass loss of the main rebar was 1.54%, whereas the mass loss of the stirrup was 2.95%. The stirrup was clearly more heavily corroded than the main rebar which was correctly represented by the AE measurements after application of the developed post-processing tool. Figure 5:1 Time lapse of the localization results of sample AE-1.