## 2 Impact-Echo Data  
A PSC bridge provides compressive stress to the concrete in order to minimize the generation of tensile stress; it uses a method in which tensile stress is applied later using a duct. In this paper, we used the PSC specimens manufactured by the Korea Institute of Civil engineering and building Technology (KCIT, Goyang-si, Republic of Korea) ([[https://www.kict.re.kr](https://www.kict.re.kr)]([https://www.kict.re.kr](https://www.kict.re.kr)); accessed on 2 March 2022) to verify our proposed method, which was manufactured based on the same structure and material as close as possible to the actual PSC bridges. One is a specimen manufactured with various thicknesses (Specimen\(-1\)), and the other is a specimen manufactured with the same thickness and vertical shape (Specimen\(-2\)). These specimens have both a void structure and a normal structure. Figure 1 is an illustration of a cross-sectional view of a PSC bridge's internal structure.  
The IE method is a method developed for the non-destructive testing (NDT) of concrete structures based on the principle of measuring the impact-generated stress waves that transmit an impact to the concrete surface and are reflected from the surface of different materials. In this paper, we consider that the principle of reflection from different materials along with the economic advantages of the IE method can include information on internal defects. The IE equipment which is manufactured by Olson Instruments Inc. (Wheat Ridge, CO, USA) ([[https://www.olsoninstruments.com](https://www.olsoninstruments.com)]([https://www.olsoninstruments.com](https://www.olsoninstruments.com)); accessed on 22 February 2022) was used in this paper and the principles of the IE method are shown in Figures 2 and 3. Additionally,  
Figure 1: Structure of PSC girder bridge.  
Figure 4 shows the measured IE data samples in Specim\(-1\). In Figure 4, the IE data have a considerable amplitude during specific areas of the measurement time.  
However, there is one thing we need to consider when using the IE method: the pattern of IE data appears irregular. This phenomenon is affected not only when measuring different specimens or bridges, but also on the compressive strength and modulus of concrete. In concrete structures, the compressive strength and modulus of concrete change continuously over time [31]. The causes are climate changes, temperature variations, or  
Figure 4: Example of IE data in Specim\(-1\) (top: normal, bottom: defect).  
Figure 3: (**a**) Basic principle of IE; (**b**) examples of IEs that may contain information of internal defect.  
Figure 2: IE equipment used in this paper.  
other environmental factors [32; 33]. For example, the constant stresses and vibrations caused by vehicles crossing a bridge have an impact on it. Climate change is particularly dangerous for concrete since it expands and contracts in response to temperature variations. As a result, concrete ages more quickly over time and structures lose strength from corrosion [33]. For this reason, it can lead to a difference in the pattern of IE even in the same conditions.  
Therefore, we compared IE data measured in Specim\(-1\) and Specim\(-2\), as shown in Figure 5. Here, the IE data of Specim\(-1\) show a value measured at a different time from the IE data of Figure 4. Figure 5a,b are the normal data measured at Specim\(-1\) and Specim\(-2\), but it can be seen that the patterns of the IE data are different. Similarly, it can be seen that different patterns appear in Figure 5c,d, which are the void data. In particular, it can be confirmed that Figure 5a,c show different patterns compared to Figure 4. Finally, it is clear from Figures 4 and 5 that most of the IE data have an amplitude in a specific area. Therefore, we chose this specific area and used its amplitude value as a training feature.