## 3 Experiment Setup
### Specimen A2_3 Made from SFRC  
The graph of differences in measured velocities [m\(\cdot\)s\({}^{-1}\)] of ultrasound waves for specimen A2_3 before and after blast load of 600 g TNT is shown in Figure 12. It can be seen from the graph that negative values were not measured at all (when the passage of the ultrasound waves accelerated through the specimen). The number and distribution of measured values is better seen in Figure 13. The expected deceleration of the ultrasound waves occurred in the majority of measuring points (41 out of 45 points). There was no change in measured velocities in eight points (points of green color). Figure 13 shows the reverse side of the specimen with visible transverse cracks oriented from the center to the corners of the specimen, which is characteristic for a fast dynamic load. Figure 9: Schema of differences in measured velocities [m\(\cdot\)s\({}^{-1}\)] of ultrasound wave for specimen A2_3 before and after blast load of 600 g TNT: acceleration (red color)–52.9% of points. Figure 11: Schema of differences in measured velocities [m\(\cdot\)s\({}^{-1}\)] of ultrasound waves for specimens A2_2 before and after blast load of 350 g TNT: acceleration (red color)–42.9% of points. Figure 10: Graph of differences in measured velocities [m\(\cdot\)s\({}^{-1}\)] of ultrasound wave for specimens A2_2 before and after blast load of 350 g TNT. Evaluation of Differences in the Results of Ultrasound Wave Velocity Measurement, Batch of Specimens A2_1 to A2_3  
A characteristic feature of porous materials, which includes concrete [15; 16], is the ability to compact themselves under compressive loading, i.e., reducing the porosity or increasing their density. The course of such compaction can be described by an idealized curve of porosity dependence, resp. density of material at applied pressure [17; 18]. The initial compaction of the material has an elastic character due to the elastic deflection of  
Figure 12: Graph of differences in measured velocities [m\(\cdot\)s\({}^{-1}\)] of ultrasound wave for specimen A2_3 before and after blast load of 600 g TNT. Figure 13: Schema of differences in measured velocities [m\(\cdot\)s\({}^{-1}\)] of ultrasound waves for specimen A2_3 before and after blast load of 600 g TNT: acceleration (red color)–no point; no change (green color)–8 points; deceleration (blue color)–41 points. the cell walls of the material. After exceeding this limit of compressive stress, a permanent plastic deformation occurs, resulting in a volume change of the material accompanied by an increase in the density of the material until complete compaction (Figure 14). When using the ultrasound method, the dependence of the velocity of ultrasound passage on the properties of the material can be described by the formula [19]:  
\[v_{u}=10^{3}\cdot k\cdot\sqrt{\frac{E}{\rho}} \tag{1}\]  
where \(v_{u}\) [m\(\cdot\)s\({}^{-1}\)] is the speed of signal propagation, \(E\) [MPa] is the modulus of elasticity, \(\rho\) [kg\(\cdot\)m\({}^{-3}\)] is the density of the material and \(k\) is the coefficient of environmental dimensionality [-]. As can be seen from Equation (1), the rate at which the ultrasound signal passes through the material depends on the modulus of elasticity and the density of the material. As shown in [20; 21; 22], the modulus of elasticity of porous materials is also dependent on the porosity itself, i.e., \(\mathrm{E=f}\left(\rho\right)\). With a decreasing value of porosity, as the density of the material increases, the value of the modulus of elasticity increases also. The increasing value of the ratio of the terms below the square root in Equation (1) therefore results in an overall increase of the value of the speed of propagation of the ultrasound signal in the evaluated specimens. This fact was found after evaluating the measured values on specimens A2_1 and A2_2 (Figure 8, Figure 9, Figure 10, Figure 11). The average percentage increase in the rate of propagation of the ultrasound signal in the specimen A2_1 was 0.97%. For specimen A2_2 it was 1.77%.