## 3 Results and Discussion
### Signal Location Analysis  
Applying the AE source location analysis, AE sources detected during the entire loading process were identified and the results are represented in Figure 10.  
The background of each graph depicts the corresponding internal sample structure where pre-existing defects are clearly detected. The signal parameters were sorted by amplitude and peak frequency. In this research, the amplitude analysis was conducted for three bands: 42 to 59 dB, 60 to 69 dB and 70 to 99 dB. In peak frequency analysis, the signals were sorted out by 10 to 59 kHz, 60 to 99 kHz and 100 kHz and higher. The location of each AE event and its intensity are clearly observed. During the concrete deformation and fracture process, the AE frequency spectrum range was very wide. The distribution characteristics of frequency-amplitude greatly differed under various systems of pre-existing defects in the internal structure of the material. It is investigated that the amplitude value of low-frequency signals (10-59 kHz and 60-100 kHz) was bigger than that of high-frequency (bigger than 100 kHz). Usually, high-frequency waves are generated from small defects relating to the damage process beginning, whereas low-frequency ones can be produced only from large cracks developed during the final failure [44]. The dispersion of the signals in samples No. 5 and No. 8 is higher and distributed through sample height (Figure 10a,b). In sample No. 5 (Figure 10a), the concentration point is observed: AE signals of high amplitude (70-99 dB) are allocated near the strain concentration point and where a defect exists that is evidence of the crack initiation point. The high number of AE events of low frequency (10-59 kHz and 60-99 kHz) is also located at this point corresponding to the main crack initiation and propagation. In sample No. 8 (Figure 10b), the AE events of high magnitude and low frequency can be noticed in the upper side of the specimen caused by the popping out of the concrete piece. The main fracture occurred near the lower sample side where the AE event concentration was denser. In other samples, the AE event accumulation illustrates compact behavior with a higher degree of event quantity. There is no obvious event localization point; events of different amplitude and frequency bands are presented in each part of the samples. The presented internal structure of samples also shows an uneven allocation of pre-existing defects that influence AE behavior. From Figure 10, the main deformation parts can be distinguished as follows: the majority of AE events in sample No. 9 are distributed in the central part of the cylinder (Figure 10c), the upper and left side of the specimen (Figure 10d) and on both sides of the central part (Figure 10e). The AE event distribution pattern in these samples replies to the internal defect system contour: events are located in the upper part of sample No. 11 and move to the lower one by the left side where a longitudinal crack exists (Figure 10d). In Figure 9e, two main concentration clouds of AE events can be observed which also correspond to the major crack allocation positions. It can be mentioned that these samples have some portion of low-frequency high-amplitude AE signals. It can be related to the fact that the coalescence of many cracks under relatively high stresses can involve cracks with diverse smaller scales. It can indicate the fracture instability stage of concrete [45]. Figure 10: _Cont._  
To get a more comprehensive understanding of the signal source locations, we decided to apply an image-gridding method for establishing a relationship between defects and AE event characteristics. The binary images of cracks extracted were divided into a grid of smaller cells, and relative crack density was counted in each grid cell. The grid scheme is presented in Figure 11. In this research, the number of cells in each sample was equal to nine, and their size was the same varying in each sample in accordance with its dimensions.