## 5 Processing of the Results and Discussion
### Relative Error in Determining Compressive Strength using Non-Destructive Methods  
For concrete samples aged 28, 244, 280, 293, 342 and 1126 days, for each specimen, the relative error in determining the compressive strength of concrete with the elastic rebound method, the ultrasonic non-destructive method using two approaches and the SonReb method against f\({}_{\text{c,cube}}\) was calculated (Figure 9). The same was carried out for a concrete age of 1926 days (Figure 10). The reference value of the compressive strength of the concrete was the one obtained via destructive testing of cubes prepared on the day of laying the concrete mix or via the testing of cored specimens according to EN 12390-3 [34]. Figure 10: Relative error in the determination of the probabilistic compressive strength via non-destructive methods at the concrete age of 1926 days (source: author [PERSON]). Figure 9: Relative error in the determination of the probabilistic compressive strength of concrete via non-destructive methods versus f\({}_{\text{c,cube}}\) (source: author [PERSON]). The relative error was determined using the formula:  
\[\varepsilon_{\gamma_{\mathrm{o}}}=\frac{\mathrm{f_{c,NDM}-f_{c,cube}}}{\mathrm{f_ {c,cube}}}.100\;[\%], \tag{9}\]  
where \(\mathrm{f_{c,NDM}}\) is the probabilistic compressive strength of concrete determined via a non-destructive method. The relative error when using the Schmidt hammer ranged from 0 to 14.1%, with that of UPVM--approach 1 ranging from 1.2% to 16.4%, UPVM--approach 2 ranging from 0.3% to 9.6% and the SonReb method ranging from 0 to 4.6%. The smallest relative error was obtained using the SonReb method. The relative error calculated with the ultrasonic velocity approach using approach 2 was also acceptable. Correlation between the Reference Compressive Strength of Concrete and That Determined via Different Non-Destructive Methods  
Figure 11 shows the dependence of the reference compressive strength \(\mathrm{f_{c,cube}}\) on the probabilistic strength determined by the elastic rebound method \(\mathrm{f_{c,Schmidt}}\), with both approaches using the ultrasonic velocity approach, \(\mathrm{f_{c,UPV,app1}}\), \(\mathrm{f_{c,UPV,app2}}\) and with the SonReb method, \(\mathrm{f_{c,SonReb}}\), for concrete samples aged 28, 244, 280, 293, 342 and 1126 days, and the results for those aged 1926 days are shown in Figure 12.  
Figure 11: Correlation between the reference compressive strength and the compressive strength determined via different non-destructive methods for concrete samples aged 28, 244, 280, 293, 342 and 1126 days (source: author [PERSON]). We preferred to construct the function graph in Microsoft Excel because the resulting graphs are easy to understand and provide an opportunity to analyze the numerical data. The accuracy of the experimental results of the non-destructive tests correlation with the reference compressive strength values can be evaluated using the R\({}^{2}\) parameter. The closer the R\({}^{2}\) value is to 1, the more accurate the correlation between the data. In tests conducted on the 28 th, 244 th, 280 th, 293 rd, 342 nd and 1126 th days (Figure 11), the R\({}^{2}\) value for the SonReb method was the highest and was equal to 0.9796, whereas that obtained using UPVM approach 2 was 0.8725, for UPVM approach 1 it as 0.8628 and for the elastic rebound method it was 0.8182. In tests conducted on the 1196 th day (Figure 12), the R\({}^{2}\) value obtained for the SonReb method was the highest and was equal to 0.8824, whereas that obtained for UPVM approach 2 was 0.8732, that for UPVM approach 1 was 0.8742 and the value obtained using the elastic rebound method was 0.8011. The correlation was best when using the SonReb method and was the smallest when using the elastic rebound method.