## 3 Results and Discussion  
### Defect System Characterization  
The internal structure of concrete samples is depicted in Figure 6. The extracted concrete components such as coarse aggregates, voids and cracks can be evaluated tentatively by comparison of the density and orientation of internal structure particles. In accordance with color maps, each sample represents a heterogeneous structure with a unique allocation and orientation of coarse aggregates and voids within the concrete matrix. Samples No. 5 and No. 8 illustrate uneven coarse aggregate distribution density with big-size particles domination and gaps between them, whereas sample No. 9 shows the reverse tendency. To precisely assess the internal concrete structure, the corresponding components' geometric properties were calculated (Table 4). The average area rate of coarse aggregate in the entire sample set is 44.15% with average area and circularity of 111.10 mm\({}^{2}\) and 0.72, respectively. Void content consists of 0.78% on average, and the average area of the particle is 1.53 mm\({}^{2}\) with a circularity parameter of 0.97. Figure 6: Internal defect system of compression test samples. The pre-existing defect system exists in the form of mortar cracks in all samples tested. Regarding the entire sample set, the average area rate of cracks in mortar is 1.55%, and the average crack area is 17.94 mm\({}^{2}\). Sample No. 5 demonstrates a slight damage degree by only one continuous crack near the lateral surface (area rate is 0.31%). Other samples show a complicated damage system with randomly oriented cracks. Samples No. 9 and No. 12 can be characterized as concrete with a heavy damage degree with a crack area rate of 2.42% and 1.58%, respectively. Comparing Figures 1 and 5, there are some correlations between internal and surface defects regarding location. Therefore, by analyzing strain and displacement localization on the sample surface, the corresponding development of internal defects can be assumed. Concrete Physical Properties: Comparison of Accumulated Cracking Damage and the Dynamic Modulus of Elasticity  
The physical properties of the samples are shown in Table 5. According to the NDT results, all samples can be characterized as damaged with an average pulse velocity of 1951 m/s which is twice lower than the pulse velocity in good-quality concrete (4000 m/s) [43]. The result of resonant frequency tests also shows a low value of the elastic parameter: the average frequency and corresponding \(E_{D}\) value are 7587 Hz and 15.9 GPa, respectively. The obtained results are depicted in Figure 7. This figure demonstrates the correlation between accumulated cracking damage and \(E_{D}\) (R\({}^{2}\) = 0.51). All samples show various damage degrees under different physical properties: sample No. 5 has the highest \(E_{D}\) (28.2 GPa) under low crack density (0.31%), whereas the internal structure of sample No. 9 demonstrates a high degree of cracking damage (2.42%) and poor value of dynamic modulus of elasticity (\(E_{D}\) is 12.7 GPa), consequently. Both can be regarded as slightly and heavily damaged specimens. As for the rest of the samples, the dispersion of values is high, and they are placed in the middle part of the graph with a little shift to the left that demonstrates their damaged condition. Based on this result, we decided to conduct an evaluation of pre-existing defects' influence on the strain distribution for five following samples: No. 5, No. 8, No. 9, No. 11 and No. 12. As can be seen, samples No. 8, No. 11 and No. 12 have about the same value of accumulated cracking damage (1.49%, 1.48% and 1.58%, respectively) but a different value of dynamic modulus of elasticity \(E_{D}\) (14.9 GPa, 19.8 GPa and 11.6 GPa, respectively). To clearly understand this phenomenon, the compression test with AE measurement and strain localization will be conducted for these selected samples.