# Experimental Investigation on Effective Distances of Acoustic Emission in Concrete Structures
For practical application of AE techniques in such monitoring systems, the effects of elastic wave energy and wave propagation distance on the AE parameters should be investigated. This is because in monitoring systems, the degree of damage can be estimated using the received AE signals based on the AE energy parameters. In addition, the AE propagation distance determines the sensor arrangement in a monitoring system. These issues are meaningful for increased applicability of AE monitoring to concrete structures. Once the elastic wave is generated, it then travels throughout the material and can be detected at considerable distances from its point of origin [24]. Propagating from its source to the point of detection, the wave is subjected to all of the characteristics and variations of its acoustic path. During propagation, the wave energy is attenuated by spreading and scattering by both microscopic and macroscopic variations in the structure media [24; 25]. Considering the aforementioned attenuation characteristics of the AE, the wave energy has a significant effect on the propagation characteristics of the AE. If the AE characteristics according to the impact energy are analyzed in advance, the impact energy applied to the structure can be reversely estimated by measuring the AE parameters. Nevertheless, previous studies lack investigations of the propagation characteristics of AE according to the energy induced from external impact in concrete structures. Therefore, it is necessary to investigate the effect of acoustic energy on the wave propagation distance. Moreover, AE techniques should be tested in actual-sized structures to overcome limitations caused by scale problems. Because the acoustic waves in laboratory specimens do not spread widely in all directions owing to the small scale, the wave attenuation effect is not properly considered in lab tests. Thus, research on AE techniques in actual-sized structures is needed to evaluate the applicability of AE monitoring systems to the field. In field applications of AE monitoring systems, sensor arrangement is determined on the basis of the wave propagation distance considering the attenuation effects (Figure 2). Figure 1: Schematic diagram of acoustic emission (AE) monitoring system for foundation structures. Figure 2: Wave propagation distance of an AE monitoring system in concrete structures. Therefore, this study focuses on an AE monitoring system for concrete structures and investigates the AE characteristics in actual-sized structures to evaluate the field applicability of this system. To achieve this objective, experiments estimating the AE parameters in a concrete structure are conducted with regard to the wave energy and its propagation distance. In addition, the maximum propagation distance of the AE wave is suggested for practical application of the AE monitoring system. The results of this study can serve as guidelines for suitable sensor arrays covering all areas of target structure monitoring and cost-effective AE sensor spacing. Therefore, these results can improve the applicability of AE monitoring systems, contributing to promote the practice of technology and engineering.