# Experimental Investigation on Effective Distances of Acoustic Emission in Concrete Structures
## 2 Experimental Program
### Test Procedure  
The experimental procedure is shown in Figure 5. The AE sensors were installed at the points shown in Figure 5a using high-vacuum grease with adhesive tape to enhance the coupling performance at the interface between the concrete and the sensor. The sensors were connected to the data acquisition equipment and a computer with the AE software installed. The balls were dropped through a hollow cylinder in a direction vertical to the surface (Figure 5b).  
Figure 4: Characteristics of AE.  
Table 2 shows the experimental cases. To investigate the effect of impact energy on the propagation characteristics, the experimental cases were established according to the four different weights of the ball and drop heights, respectively. Thus, a total of 16 cases were set, and 9 experiments were performed for all measurement points in each experimental case. The impact energy for generating the elastic waves in Table 2 was calculated as shown in Equation (1).  
\[\text{Impact energy (J) = ball mass (g) \times gravitational acceleration (m/s^{2}) \times drop height (m)} \tag{1}\]  
When the ball hits the structure surface, not all the impact energy is transmitted to the acoustic energy because some energy is lost, such as heat and vibration generated by friction between the surface and the ball. However, the intensity of the elastic wave is proportional to the impact energy of the steel ball. Thus, the impact energy indicated the energy of the elastic wave in the experiment. Moreover, the rebound effect of the ball on the recorded signal was filtered in the experimental results.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ball Number (\#)** & **Weight (g)** & **Height (m)** & **Impact Energy (J)** & **Measurement Distance (m)** \\ \hline \multirow{4}{*}{Ball 1} & \multirow{4}{*}{32.5} & 0.1 & 0.03 & \multirow{4}{*}{} \\  & & 0.5 & 0.16 & \\  & & 1 & 0.33 & \\  & & 2 & 0.65 & 0.1 \\ \hline \multirow{4}{*}{Ball 2} & \multirow{4}{*}{110.0} & 0.1 & 0.11 & 0.3 \\  & & 0.5 & 0.55 & 1.0 \\  & & 1 & 1.10 & 4.0 \\  & & 2 & 2.20 & 8.0 \\ \hline \multirow{4}{*}{Ball 3} & \multirow{4}{*}{260.6} & 0.1 & 0.26 & 11.0 \\  & & 0.5 & 1.30 & 14.0 \\ \cline{1-1}  & & 1 & 2.60 & 16.0 \\ \cline{1-1}  & & 2 & 5.20 & 17.0 \\ \hline \multirow{4}{*}{Ball 4} & \multirow{4}{*}{509.5} & 0.1 & 0.51 & \multirow{4}{*}{} \\  & & 0.5 & 2.55 & \\ \cline{1-1}  & & 1 & 5.09 & \\ \cline{1-1}  & & 2 & 10.18 & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Experimental cases by ball and dropping height.  
Figure 5: Experiment process.  
Figure 7: Duration according to impact energy.  
Figure 8: Amplitude according to impact energy.  
Figure 9: Signal strength according to impact energy.  
Figure 11: Average frequency according to impact energy.  
Figure 10: Absolute energy according to impact energy.  
In the experiments, the intensity AE parameters were more sensitive to impact energy than other AE characteristics. Therefore, the absolute energy and signal strength are highly recommended as significant factors for analyzing the external impact of concrete structures in AE monitoring systems.