## 4 Results and Discussions
### Acoustic Emission Energy Release  
To enable comparisons between the confined and unconfined specimens only AE data recorded by the internal sensors is discussed. These sensors are less sensitive than the resonant sensors, which results in lower volumes of data. The main reason for choosing the broadband sensors is that their broad frequency response makes them suitable for frequency analysis. The resonant sensors attached on the bottom surface of the specimens are more representative of what may be used in practice for optimized detection and source location. Three dimensional source location of large-scale specimens requires a method to accurately calculate time of arrival for very weak signals, which is a future step of this research. This study involves frequency analysis and therefore focuses on broad band sensors. Analysis of the resonant sensor data is a future consideration. In Figure 1, the red marks and corresponding labels in red font denotes the schematic sensor locations for both confined and unconfined specimens. Sensor coordinates are presented in Table 2. The last column of the table represents the coordinates of normal vectors, which are perpendicular to the sensing surfaces of sensors and the directions of vectors are toward the outside of the sensors. The orientation of sensors is shown in Figure 1d. \begin{table}
\begin{tabular}{c c c c} \hline \multicolumn{2}{c}{**External Sensor**} & \multicolumn{2}{c}{**Internal Sensor**} \\ \hline
**Amplitude (dB)** & **Duration (\(\upmu\)s)** & **Amplitude (dB)** & **Duration (\(\upmu\)s)** \\ \hline
41–43 & 400\(<\) & 32–35 & 155\(<\) \\
44–45 & 500\(<\) & 36–42 & 260\(<\) \\
46–47 & 600\(<\) & 43–100 & 330\(<\) \\
48–49 & 650\(<\) & - & - \\
50–53 & 820\(<\) & - & - \\
54–56 & 940\(<\) & - & - \\
57–65 & 1080\(<\) & - & - \\
66–100 & 1400\(<\) & - & - \\ \hline \end{tabular}
\end{table} 
Table 1: Duration-amplitude rejection limits. Figure 5: Amplitude and cumulative signal strength of AE data in terms of concrete age. (**a**) Confined specimen; (**b**) Unconfined specimen; (**c**) Control specimen. The AE cumulative signal strengths of the internal sensors for the reactive specimens are presented in Figure 6a,b. The total CSS versus time for the confined specimen is much higher than the CSS for the unconfined specimen at 195 days (the CSS for the confined specimen is 2.35 times the value for the unconfined specimen). Moreover, in Figure 5a,c, it can be seen that a significant portion of released AE energy is attributed to sensor 3, which is located at mid-height of the confined specimen. The CSS rate for sensor 3 (5805 pVs/Day) is much larger than for sensors 1 and 2 (1545 and 1212 pVs/Day), thereby increasing the difference in the CSS between the sensors. On the other hand, this trend in the confined specimen is not observed in the unconfined specimen where AE energy release is not concentrated in one specific sensor. The distribution of energy was approximately uniform among the sensors. However, the CSS for the sensors at the top reinforcement mesh (sensors 4 and 5) is larger than in sensor 6. These observations illustrate that the confined specimen has larger AE energy release in the middle layers of the specimen, which is increasing with the progression of the ASR reaction, than the unconfined specimen. This may be due to a larger expansion strain through the thickness of the confined specimen. This large expansion is expected to cause more damage through the thickness of specimen and consequently more AE energy release at middle of the th \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sensor No.** & **X (m)** & **Y (m)** & **Z (m)** & \(\overset{\rightarrow}{\mathbf{n}}\) \\ \hline Sensor 1 & 1.08 & 1.91 & 0.86 & (0,0,\(-\)1) \\ Sensor 2 & 1.85 & 0.78 & 0.86 & (0,0,\(-\)1) \\ Sensor 3 & 2.34 & 1.46 & 0.5 & (\(-\)1,0,0) \\ Sensor 4 & 1.1 & 1.59 & 0.86 & (0,0,\(-\)1) \\ Sensor 5 & 1.83 & 0.88 & 0.86 & (0,0,\(-\)1) \\ Sensor 6 & 2.36 & 1.59 & 0.5 & (\(-\)1,0,0) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sensor coordinates.