## 4 Conclusions  
The investigation of the influence of pre-existing defects on strain distribution in compression stress field is conducted. The internal concrete structure taken from the in-service irrigation structure was investigated by the X-ray CT method. Concrete damage was quantified by image analysis and the segmentation of concrete particles. The physical properties of concrete samples were investigated by such NDT methods as pulse velocity and the resonant frequency technique, and the dynamic modulus of elasticity was calculated. A compression test with measurement of AE behavior and deformation phenomena was carried out. Strain localization and a displacement contour map were obtained by the DICM technique. Moreover, the AE source location analysis was employed to acquire the AE event allocation. From the results, the relationship between concrete basic properties\(E_{D}\) and characteristics of AE and DICM in the compression field can be observed and the main conclusions of this study are summarized as follows:  
1. The X-ray CT technique is suitable for the investigation of internal concrete structures and can be used to quantify the damage degree by calculating the geometric properties of concrete components. The prevalent damage type in concrete is detected in the form of mortar cracks in all samples case: crack density varies from 0.31 % (sample No. 5) to 2.42 % (sample No. 9). 2. The deteriorated condition of concrete samples is confirmed by the low physical property values calculated by NDT: average pulse velocity and resonant frequency values are 1951 m/s and 7587 Hz for all samples, respectively. Dynamic modulus of elasticity, \(E_{D}\), calculated from the resonant frequency, shows a good correlation with accumulated concrete cracking damage: the stronger the cracking system, the lower modulus of elasticity. In slightly damaged sample No. 5, the high value of dynamic modulus \(E_{D}\) is observed (\(E_{D}=28.2\) GPa). In contrast, in sample No. 9 with a large number of cracks, a low \(E_{D}\) is detected (\(E_{D}=12.7\) GPa). Based on these results, the five samples with different damage degrees are selected for future investigation by AE and DICM tests. 3. The fracture process behavior of concrete under compression is monitored by the AE technique. According to the results, all samples show different AE energy release trends which are affected by the uniqueness and severity of the internal defect system. Sample No. 5 with low crack density has a step-up tendency similar to non-damaged concrete, where with stress increasing, the AE energy release events appear and accumulate until final failure where sudden emission of high intensity occurs. In contrast, the AE energy release trend observed in sample No. 9 which has a complex crackling system demonstrates the continuous emission process starting from the early loading stage, and the cumulative AE curve has a steeper slope corresponding high rate of fracture. This result confirms that the damaged condition of the concrete structure can be detected by the AE measurement results. 4. Circumferential strain and radial displacement analyzed by the DICM show the correlation between strain localization points and the location of pre-existing defects. Because the defect is a weak point of the material structure, thus, it leads to the fracture processes evolving near this area. In samples with a high degree of cracking damage, the strain allocation occurs in multiple locations at the same time and causes an unstable deformation process. 5. Intensity of the AE signal in terms of amplitude and peak frequency bands and its spatial distribution within the concrete matrix was received by the AE source location analysis. Calculated values demonstrate that the high-amplitude low-frequency AE events are prevalent in all samples corresponding to the different fracture types. Location analysis shows the correlation between AE event occurrence and area of pre-existing defects: AE event localization density is higher in the defect location and matches its main pattern.