## 3 Results and Discussion
### Fracture Behavior  
The mechanical properties and AE measurement results of the testing concrete samples are represented in Tables 6 and 7, respectively. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Compressive** & **Maximum** & **Initial Tangent** & **Secant** & **Strain Energy,** \\
**Name** & **Strength, \(\sigma\)** & **Strain, \(\epsilon\)** & **Modulus,** & **Modulus,** & **U** \\  & & **(\(\times 10^{-6}\))** & **\(E_{0}\)** & **(\(E_{c}\))** & **(\(J\))** \\ \hline  & **[N/mm\({}^{2}\)]** & **[-]** & **[GPa]** & **[GPa]** & **[J]** \\ \hline No. 5 & 18.3 & 1749 & 11.7 & 10.4 & 19.4 \\ No. 8 & 12.7 & 2309 & 2.8 & 5.5 & 14.4 \\ No. 9 & 12.0 & 1873 & 5.3 & 6.4 & 13.7 \\ No. 11 & 14.9 & 737 & 5.2 & 20.2 & 5.1 \\ No. 12 & 14.9 & 1797 & 5.2 & 8.3 & 20.1 \\ \hline Average & 14.5 & 1693 & 6.0 & 10.2 & 14.5 \\ Max & 18.3 & 2309 & 11.7 & 20.2 & 20.1 \\ Min & 12.0 & 737 & 2.8 & 5.5 & 5.1 \\ SD & 2.2 & 518 & 3.0 & 5.3 & 5.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Mechanical properties. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample Group** & \multicolumn{2}{c}{**Pulse Velocity, \(V_{p}\)**} & **Resonant Frequency** & \(E_{D}\) \\ \cline{3-5}  & & **[m/s]** & **[Hz]** & **[GPa]** \\ \hline  & Average & 1951 & 7587 & 15.9 \\ (No. 1–No. 12) & Max & 3730 & 13,561 & 36.6 \\  & Min & 977 & 4553 & 5.0 \\  & SD & 730 & 2630 & 8.6 \\ \hline  & No. 5 & 3730 & 11,529 & 28.2 \\  & No. 8 & 2035 & 7155 & 14.9 \\ Selected & No. 9 & 1226 & 6234 & 12.7 \\  & No. 11 & 2650 & 9470 & 19.8 \\  & No. 12 & 977 & 5567 & 11.6 \\ \hline  & Average & 2124 & 7991 & 17.4 \\ Selected & Max & 3730 & 11,529 & 28.2 \\ Min & 977 & 5567 & 11.6 \\ SD & 999 & 2208 & 6.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Physical properties. Figure 7: Relationship between accumulated cracking damage and \(E_{D}\). All samples had an initial damage degree confirmed before the compression test was conducted. Thereby, low mechanical properties were expected. The compressive strength of concrete samples did not exceed 20 N/mm\({}^{2}\) and ranged from 12.0 N/mm\({}^{2}\) (sample No. 9) to 18.3 N/mm\({}^{2}\) (sample No. 5). The strain value also was observed in all samples: maximum and minimum values are in samples No. 8 (2309 \(\times\) 10\({}^{-6}\)) and No. 11 (737 \(\times\) 10\({}^{-6}\)), respectively. Strain energy is stored in the material during work of external load and is needed for the crack formation and propagation processes. Sample No. 12 has the highest strain energy value (20.1 J), whereas sample No. 11 illustrates the lowest value (5.1 J). The number and distribution of pre-existing defects in each concrete sample are unique and vary in different degrees, and therefore, the AE hits and AE energy present obvious distinctions in obtained values. The AE hit numbers in samples No. 9 (119,056 hits) and No. 12 (74,888 hits) show a high degree of crack activity under load which corresponds to the high internal damage degree in these samples. Slightly crack-damaged sample No. 5 demonstrates low cracking activity with a total number of AE hits and AE energy of 33,133 hits and 13,319.6 V\({}^{2}\), respectively. The AE energy response and cumulative energy curve versus strain energy level during the loading process are shown in Figure 8.  
As can be seen, the AE energy release generated by concrete deformation is represented as a process of constant energy accumulation and intermittent energy discharge. Each specimen illustrates the diverse AE energy release trend which relates primarily to the distribution of internal defects and their severity. Generally, the AE energy release in non-damaged concrete has the following stages: the sparse activity period is characterized by the absence of any significant AE signals and the closure of existing micro-cracks within the internal structure. After this stage, the slow growth period occurs of the AE activity where internal damage generation starts.