## 4 Results and Discussions
### Pattern Recognition of AE Data
The higher the  
Figure 10: Normalized signal features. (**a**) Confined specimen; (**b**) Unconfined specimen. frequency components are in a signal, the lower rise angle value the signal possesses. In the unconfined specimen, Cluster-5 and Cluster-6 exhibit higher hit rates at the higher concrete age compared to Cluster-4, and the average amplitude of signals for the clusters with higher frequency components is slightly higher than for signals in Cluster-4. However, the average duration for the signals in the cluster with the low-frequency components (Cluster-4) is much longer than the duration for Cluster-5 and Cluster-6. In Figure 11, the variation of cumulative signal strength in terms of the age of the concrete for each cluster is presented. The cumulative signal strengths were normalized by the maximum value for each specimen. In the confined specimen, the signals with the highest frequency components (Cluster-3) have dominant CSS from the early age. However, the CSS of Cluster-2 is very close to the CSS of Cluster-3 up to the concrete age of 150 days. After 150 days, the CSS rate for Cluster-3 increases, while the CSS rate of Cluster-2 continues with approximately the same rate. The signals in cluster Cluster-1 have negligible signal strength compared to Cluster-2 and Cluster-3 and initiate primarily after 120 days. In the unconfined specimen, the AE energy is primarily attributed to cluster Cluster-4 up to approximately 150 days. After 150 days, the clusters with the higher frequency components (Cluster-5 and Cluster-6) become prominent in terms of AE energy release. The distribution of total AE signal strength for the classified clusters and sensors at different ages of the concrete (66, 150, 195 days) are illustrated in Figure 12. These distributions are referred to as signal strength contribution factors (SSCF). The 66 th day and 195 th day were selected to illustrate the trend of data at the beginning and end of the evaluated time window. The 150 th day was selected because in both reactive specimens there was an obvious change in the rate of CSS of the clusters with high-frequency components in comparison to the lower frequency components. The figures  
Figure 11: Normalized cumulative signal strength. (**a**) Confined specimen; (**b**) Unconfined specimen. on the left show results of the confined specimen and the figures to the right show data from the unconfined specimen. In the confined specimen, most of the energy contribution is related to Cluster-2 (energy concentration in 50-150 kHz) and Cluster-3 (energy concentration in 150-300 kHz). The SSCF for Cluster-3 is increases with time, particularly after 150 days. Most of the AE energy for Cluster-3 is concentrated in sensor 3 (mid-thickness of the specimen) after 100 days. Cluster-2 and Cluster-3 both have prominent AE energy at sensor 2 before the 66 th day. Then the highest AE energy portion moves to sensor 3, whereas SSCF of Cluster-2 is much lower than Cluster-3, especially after 150 days. The SSCF for cluster Cluster-1 is negligible compared to other clusters. In the unconfined specimen, the highest SSCF is for cluster Cluster-4 (energy concentration between 0-150 kHz) at sensor 4 (attached to top reinforcement) at 66 days. However, this energy contribution decreases with time, and the SSCF of the clusters with high-frequency components (Cluster-5 and Cluster-6) increases with time. There is no obvious energy concentration in the sensor located at mid-thickness of the specimen (sensor 6), which  
Figure 12: Distribution of total AE signal strength in terms of clusters and sensors. (**a**) Confined specimen at age of 66 days; (**b**) Unconfined specimen at age of 66 days; (**c**) Confined specimen at age of 150 days; (**d**) Unconfined specimen at age of 150 days; (**e**) Confined specimen at age of 195 days; (**f**) Unconfined specimen at age of 195 days. is different from what is observed in the confined specimen. In both specimens, the SSCF declines in low-frequency signals and increases in high-frequency signals with time.