## 4 Upscaling to RC Beams  
From the previous section, it can be concluded that the AE technique is able to detect and localize damage due to reinforcement corrosion in 3D. As a next step, the protocol was scaled up towards RC beams. However, for this sample scale, a larger monitoring area was required. Therefore, a 2D setup was proposed. Two RC beams with dimensions 150 \(\times\) 200 \(\times\) 1800 mm\({}^{3}\) were monitored with the 2D setup. They were reinforced with a ribbed rebar with a nominal diameter of 14 mm. A length of 1500 mm was exposed to corrosion. The mold was designed in such a way that a concrete cover of 30 mm was obtained. Figure 11: Localization results of sample AE-2 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 12: Picture of the stirrup of sample AE-2 after removal and cleaning at the end of the accelerated corrosion test. Figure 10: Unfiltered localized AE events of sample AE-2; **(left)** side view, and **(right)** front view. The corrosion process was accelerated using the same principle as for the mortar sample and RC prisms. Instead of immersing the beams partially in the sodium chloride solution, it was more convenient to place the beams upside-down. A bottomless wooden tank with a length of 1250 mm was placed on top of the samples and sealed with silicone. In this tank, the 5% sodium chloride solution was added. The current density was lowered to 50 \(\upmu\)A/cm\({}^{2}\) to slow down cracking of the concrete cover. A visual inspection was carried out weekly during which crack width measurements were performed with a crack meter. The accuracy of the crack meter was 0.05 mm. A schematic representation of the setup is shown in Figure 13. More details on the corrosion setup can be found in [PERSON] et al. [10] and [PERSON] et al. [22]. The same AMSY-6 Vallen acquisition system with six channels was used. Broadband piezoelectric sensors with a flat frequency response between 100 and 400 kHz were attached with hot melt glue. A 2D sensor layout was used to monitor the beams continuously. Four sensors were attached to the concrete, two at each side of the beam (Figure 13). The distance between the sensors along the length of the beam was 750 and 500 mm for Beam-1 and Beam-2 respectively. The amplitude threshold, pre-trigger time, duration discrimination time, and rearm time were set to 40 dB, 200 \(\upmu\)s, 450 \(\upmu\)s, and 5 ms, respectively. As a longer pre-trigger time was used for the RC beams, namely 200 \(\upmu\)s instead of 20 \(\upmu\)s for scales 1 and 2, also the shape of the AIC curve changed. Therefore, the threshold for the slope to check whether the TOA was accurately picked was adapted from \(-25\ \frac{1}{\upmu\text{s}}\) to \(-5\ \frac{1}{\upmu\text{s}}\). The sampling rate was set to 5 MHz. With 4096 samples being stored, the total length of the signal was 819.2 \(\upmu\)s. The digital frequency filter was set between 95 and 850 kHz. Figure 14 shows the initial localization results of Beam-1. The result is shown on pictures of the top surface of the beam and the crack that was observed on the surface is indicated. Please note that the crack is exaggerated to make its position clearer. The localization result is scattered, and many events are localized outside the sensor array. Figure 15 shows the filtered results obtained with the post-processing protocol. AE events are mainly localized around the crack; however, some events are also localized at the sides where no cracking on the surface was observed. Many events were recorded by three sensors, therefore, the localization error could not be calculated and used as a filter. This may be overcome by placing the sensors closer to each other. For events recorded by four sensors, the error was set to be maximum 25 mm (radius of the error ellipse). Figure 13: Schematic representation of the accelerated corrosion setup and 2D AE sensor configuration (S1–S4) of the RC beams.