# Experimental Investigation on Effective Distances of Acoustic Emission in Concrete Structures
The AE monitoring system consists of an AE sensor, an amplifying and filtering system, and data acquisition equipment. On the basis of the monitored AE signal, the system can detect signs for failure before the critical failure occurs in the materials. In monitoring systems, AE techniques have been applied to detect the crack location, quantify the degree of damage, and determine the crack classification of concrete structures [14; 15; 16]. Accordingly, many studies have been conducted on AE characteristics, including propagation distance, source location estimation, and crack classification in the concrete material [17; 18; 19; 20; 21; 22; 12; 23]. Table 1 summarizes the reports of AE characteristics in studies conducted after 2011 according to main research topics. The AE system can be usefully applied to monitor concrete structures in civil engineering. For example, this system can be particularly effective for foundation monitoring because damages in foundations are not visible using a typical monitoring system (Figure 1). Even though the internal crack \begin{table}
\begin{tabular}{c c c} \hline \hline
**References** & **Target Characteristic** & **Investigation Results** \\ \hline [PERSON] et al. (2011) & Propagation distance & The RA value (rise time/amplitude) tends to increase with propagation distance, whereas the frequency parameters do not vary significantly with distance. \\ \hline [PERSON] et al. (2011) & Propagation distance and crack classification & For reliable crack classification, the propagation distance between the source crack and the acquisition point should not exceed a threshold value. Shear cracks generate elastic waves with higher RA values than those in tensile cracks. \\ \hline [PERSON] and [PERSON] (2017) & Propagation distance & A new method based on AE is proposed. When the frequency is between 100 kHz and 150 kHz, the cumulative amplitude attenuation reaches its maximum value, and the frequency components decay exponentially. The value is lowest when the frequency is between 200 kHz and 250 kHz. \\ \hline [PERSON] (2018) & Source location & Damage monitoring using AE source location based on a computational geometry method is suggested; the convex hull volume correlates well with the damage until 60\% of the ultimate load is reached. \\ \hline [PERSON] and [PERSON] (2017) & Evaluation of damage degree & The four-point bending test of the reinforced concrete beams showed that as the level of damage increased in microcracks, visible cracks, steel yield, and concrete crushing, the values of AE parameters such as count, hits, rise time, acoustic emission energy, and duration increase; however, this does not occur for average frequency. \\ \hline [PERSON] et al. (2018) & Evaluation of damage degree & A relationship between AE activities and mechanical damage is established, and good correlation is found with empirical laws. \\ \hline [PERSON] and [PERSON] (2007) & Source location & The accuracy of the AE location is discussed; the source location capability decreases when concrete cracking occurs. \\ \hline [PERSON] et al. (2018) & Crack classification & High AE energy is released during the development of shear cracks, whereas low AE energy is released during that of tensile cracks. \\ \hline [PERSON] et al. (2017) & Propagation distance & In monitoring the cracking of textile reinforced cement, the distance between the location of the damage and the sensor had a significant effect on the propagating waveform under the bending test. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Literature review of investigations of acoustic emission (AE) characteristics in concrete materials. of foundations is not visible, it may have a critical effect on the foundation safety, which determines the overall safety of buildings or structures. If the foundation is damaged by external impact, the induced damage can be detected by AE passive techniques in the early failure stage.