# A Crack Characterization Method for Reinforced Concrete Beams Using an Acoustic Emission Technique
## 4 Methodology
### Experimental Test Setup and Data Acquisition  
We developed the testbed at our Ulsan industrial artificial intelligence laboratory (UIAI) located in Ulsan, Republic of Korea to acquire the AE data. Flexural tests are popular for evaluating the tensile strength of RC beams. Further, flexural tests can be used to determine the ability of the RC beams to endure failure resulting because of the bending. These tests are performed using center-point bending, three-point bending, or four-point bending. The three-point bending test is carried out in a way where half of the total load is projected at every one-third of the RC beam length. On the other hand, the full load is applied at the center of the beam in center-point bending. Based on the standard set by the American society for testing and materials (ASTM), the length of the RC beam has to be at least three times greater than the depth [43]. The depth of the RC beam we considered was 300 mm. The span length fixed for the experiment was 2400 mm. Since the length was quite large, a three-point bending test was conducted for a uniform distribution of the cracks in the beam. Figure 3: A pictorial illustration of the k-nearest neighbor (k-NN) algorithm. Figures 4 and 5 present the testbed for gathering AE signals from the test specimen. A diagram showing where the sensors were placed in the RC beam and the conducted three-point bending test is presented in Figure 4. Figure 5 displays the real-time setup developed at the UIAI laboratory. The length of the beam was 2400 mm, and the depth was 300 mm. For a particular amount of time, an external load was applied to initiate the cracks. The loads were projected at two points on the beam that were 800 mm apart from each other. We used a load velocity of 2 mm/s. Various types of sensors were used for acquiring the AE data. R15I [44] and WD [45] sensors were used in the experimental setup. For every type of sensor, the flexural test was conducted three times, resulting in six different bending tests. The experiments were conducted in the same condition for the reproducibility test. To choose a highly sensitive sensor, a verification of the sensor mounting was undertaken. We used the pencil lead fracture (PLF) test to mimic the acoustic wave on the surface of the concrete structure. For this purpose, a magnetic pencil with a Teflon shoe was used. Readers can refer to [46] for the details of the process. Figure 6 depicts the method to handle the magnetic pencil on the specimen surface. An angle of 30\({}^{0}\) from the plane of the specimen surface was used (illustrated in Figure 6). If the PLF produces a high amplitude of 99 dB or sensitivity within a \(\pm\)3-dB margin, we can consider the sensors to be significantly coupled. Through this process, we chose our sensor to be R15I. A total of eight sensors were placed on the concrete beam for data acquisition in each experiment. The RC beam had six surfaces, and our goal was to cover all of the surfaces using a total of eight sensors. We fixed the sensors on the concrete specimen using mounting tape and glue gel. We put the sensors as far as possible from the loading points, mostly near the ends of the beam, so that the crack formation did not harm the placement of the sensors. AE signals were collected for around 15 min at a 10-MHz sampling rate. The R15I sensors had an integrated amplifier and a preamplifier having a  
Figure 4: Schematic diagrams of the testbed: (**a**) the sensor placement, and (**b**) the three-point bending test for the beam. gain of 40 dB. There was no AE threshold set prior to performing the experiments, but an adaptive threshold level was used. The adaptive threshold was dependent on the maximum amplitude of the collected AE signals. Figure 7 presents the AE signals for three different scenarios. The first one, Figure 7a, represents the normal condition in the beam. Figure 7b,c represents the AE signals for micro-cracks and macro-cracks. Macro-cracks are often referred to as fractures in our study.