# Experimental Investigation on Effective Distances of Acoustic Emission in Concrete Structures
## 5 Conclusions  
The energy of acoustic waves has a significant effect on the wave propagation distance. Thus, the present study investigated the AE propagation characteristics in concrete structures according to the impact energy, and experiments were conducted to evaluate the AE parameters. According to the experimental results, the concept of effective distance for AE monitoring system was presented. This study contributes to improving the field applicability of AE monitoring systems by suggesting suitable spacing for AE sensors. The conclusions of this work are summarized in the following points:  
1. The AE measurement experiments were conducted in actual-sized reinforced concrete structures by dropping a steel ball. The amplitude, count, duration, absolute energy, signal strength, and average frequency of the AE signal were investigated according to the impact energy of the ball. The AE parameters tended to increase with an increase in impact energy. Among the AE parameters, absolute energy was determined to be the most sensitive and is thus recommended as a significant factor in AE monitoring systems to evaluate the external impact on a structure. However, the count frequency did not differ significantly with an increase in impact energy.  
2. The attenuation effects of the elastic wave were quantitatively evaluated with the AE parameters according to the propagation distance. The experimental results indicated that the signal strength and duration are the key factors for evaluating the attenuation effect of the AE signal. In addition, the signal reception levels were divided into four categories according to the amplitude: perfect, good, normal, and weak. At the propagation distance range at perfect and good levels, the measured AE signal can be regarded as a main AE signal generated from the damage of the material. The effective distance was suggested to be the maximum propagation distance of the main AE signal corresponding to the good level in the representative case. The effective distance increased as the impact energy increased, although the increased effective distance was limited because the damage signal is of high frequency and, therefore, it is effectively attenuated.  
Conceptualization, T.-M.O. and M.-J.K.; Data curation, T.-M.O. and M.-K.K.; Formal analysis, J.-W.L.; Funding acquisition, H.K.; Investigation, T.-M.O., M.-K.K. and J.-W.L.; Methodology, M.-K.K.; Project administration, H.K.; Resources, H.K.; Visualization, J.-W.L.; Writing--original draft, T.-M.O. and M.-J.K.; Writing--review & editing, M.-J.K. All authors have read and agreed to the published version of the manuscript  
This research received no external funding.  
This work was supported by a National Research Council of Science and Technology (NST) grant by the Ministry of Science, Information and Communications Technology (ICT) and Future Planning (MSIP) of the Korean government (Nos. CRC-16-02-KICT and NP2016-055) and Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science and ICT of Korea (GP2018-009).  
The authors declare no conflict of interest.