## 4 Experimental Results
### Compressive Strength of Concrete Determined via UPVM  
Measurements were made with a Proceq Tico portable device, of which the transmitter and receiver frequency was 54 kHz. Ultrasonic waves cannot move through air (the speed of sound in air is about 340 m/s). Therefore, to ensure good contact between the transmitter, the receiver and the concrete surface, a special paste was used. The author's experimental research to determine the probabilistic concrete compressive strength was carried out over almost 6 years--at the age of 28, 244, 280, 293, 342, 1126 and 1926 days. These measurements were performed before the destructive tests. Ultrasonic pulse velocity measurements were carried out for:  
Three cubes with dimensions of 150/150/150 mm for each day at 28, 244, 280, 293 and 342 days;  
Twelve cubes on the 1126 th day;  
Eleven cored specimens with a height and diameter of 100 mm, taken from the reinforced concrete beams on the 1926 th day. \begin{table}
\begin{tabular}{c c c c c} \hline
**Age (Days)** & **Test Specimen, \(\mathbf{N^{0}}\)** & **Median R** & \(\mathbf{R_{s}}\cdot\mathbf{(N/mm^{2})}\) & \(\mathbf{f_{c,Schmidt}}\) (N/mm\({}^{2}\))** \\ \hline \multirow{4}{*}{28 th} & Cube 1 & 37 & 33.5 & 33.5 \\  & Cube 2 & 37 & 33.5 & 33.5 \\  & Cube 3 & 37.5 & 34.5 & 34.5 \\ \hline \multirow{4}{*}{244 th} & Cube 4 & 40.5 & 40.5 & 33.0 \\  & Cube 5 & 41 & 41.5 & 33.8 \\  & Cube 6 & 42 & 43.5 & 35.4 \\ \hline \multirow{4}{*}{280 th} & Cube 7 & 42 & 43.5 & 34.6 \\  & Cube 8 & 42 & 43.5 & 34.6 \\  & Cube 9 & 42.5 & 44.6 & 35.4 \\ \hline \multirow{4}{*}{293 rd} & Cube 10 & 42 & 43.5 & 34.2 \\  & Cube 11 & 43 & 45.7 & 36.0 \\  & Cube 12 & 43 & 45.7 & 36.0 \\ \hline \multirow{4}{*}{342 nd} & Cube 13 & 43 & 45.7 & 34.7 \\  & Cube 14 & 43.5 & 46.9 & 35.6 \\  & Cube 15 & 44 & 48.0 & 36.5 \\ \hline \multirow{4}{*}{1126 th} & Cube 16 & 49.5 & 60.4 & 45.3 \\  & Cube 17 & 47 & 54.7 & 41.0 \\  & Cube 18 & 46.5 & 53.6 & 40.2 \\  & Cube 19 & 46.5 & 53.6 & 40.2 \\  & Cube 20 & 46 & 52.5 & 39.4 \\  & Cube 21 & 48 & 57.0 & 42.8 \\  & Cube 22 & 44 & 48.0 & 36.0 \\  & Cube 23 & 48 & 57.0 & 42.8 \\  & Cube 24 & 47.5 & 55.9 & 41.9 \\  & Cube 25 & 45 & 50.0 & 37.5 \\  & Cube 26 & 46.5 & 53.6 & 40.2 \\  & Cube 27 & 45 & 50.0 & 37.5 \\ \hline \multirow{4}{*}{1926 th} & Core 1 & 46 & 52.5 & 45.5 \\  & Core 2 & 46 & 52.5 & 45.5 \\  & Core 3 & 48 & 56.6 & 49.0 \\ \cline{1-1}  & Core 4 & 52 & 67.6 & 59.1 \\ \cline{1-1}  & Core 5 & 47 & 54.7 & 47.8 \\ \cline{1-1}  & Core 6 & 47 & 54.7 & 47.8 \\ \cline{1-1}  & Core 7 & 45 & 50.7 & 44.3 \\ \cline{1-1}  & Core 8 & 46.5 & 53.6 & 46.7 \\ \cline{1-1}  & Core 9 & 50 & 61.8 & 54.0 \\ \cline{1-1}  & Core 10 & 48.5 & 58.0 & 50.2 \\ \cline{1-1}  & Core 11 & 50 & 61.8 & 54.0 \\ \hline \end{tabular}
\end{table} 
Table 1: Concrete compressive strength, determined based on its surface hardness. Ten measurements were performed for each specimen according to EN 12504-4 [49], as the transducers were placed symmetrically and opposite to each other (Figure 5). The device was calibrated with a reference cylindrical body. For each test specimen (cube or core, the arithmetic mean value of the ultrasonic velocity V\({}_{\text{UPV,mean}}\) was determined and then f\({}_{\text{c,UPV}}\) was calculated. The results regarding the probabilistic compressive strength of concrete, determined with UPVM at different concrete ages, were calculated using two approaches: approach 1--f\({}_{\text{c,UPV,app1}}\) (Equation (1)) and approach 2--f\({}_{\text{c,UPV,app2}}\) (Equation (2)). The value of the factor c in Equation (1) was assumed to be equal to 0.158 [3]. When determining f\({}_{\text{c,UPV,app2}}\), the value of the air-dry density of concrete \(\gamma_{\text{c}}\) determined by formula (4) was used. The difference between this value and the one determined experimentally (based on the measurement and weighing of specimens) varies from 0.1% to 4.2%. The results are presented in Table 2.