## 5 Processing of the Results and Discussion
### Relative Error in Determining Compressive Strength using Non-Destructive Methods
Evaluation the Accuracy of the Compressive Strength Determined Using Non-Destructive Methods Compared to Reference Values  
Accuracy (%) was calculated as the ratio of the reference concrete compressive strength for a given specimen to the probabilistic strength determined using a non-destructive method. Probabilistic compressive strength for given specimen was determined from median value of the rebound when using Schmidt hammer, and from mean value of the ultrasonic pulse velocity when using UPVM for calculation by both approaches. The same values were used in the SonReb method. Figure 13 presents the accuracy of the elastic rebound method, the ultrasonic method using both approaches and the SonReb method in the experimental determination of concrete compressive strength compared to reference values based on the testing of 27 cubes at ages of 28, 244, 280, 293, 342 and 1126 days. Figure 14 shows the accuracy obtained in the testing of 11 cores at a concrete age of 1926 days. Figure 12: Correlation between the reference compressive strength and the compressive strength determined via different non-destructive methods for concrete samples aged 1926 days (source: author [PERSON]). The highest accuracy was observed when using the SonReb method--from 96% to 100%, followed by UPVM--from 91% to 100% when using approach 2, whereas the lowest accuracy was obtained when using approach 1--from 86% to 99%--and with the elastic rebound method--from 86% to 100%. Typical Multiple-Correlation Relationship in the Form of a Nomogram for the Determination of Concrete Compressive Strength  
In experimental studies of reinforced concrete structures of buildings and facilities, it is very convenient to plot nomograms in order to determine the compressive strength of the \(\mathrm{f_{c,SonReb}}\) values of concrete at an arbitrary point of the structure. Multiple-correlation  
Figure 14: Accuracy of different non-destructive methods in determining the compressive strength of concrete against the reference value based on the testing of 11 cores at the age of 1926 days (source: author [PERSON]). Figure 13: Accuracy of different non-destructive methods in determining the compressive strength of concrete against the reference value based on the testing of 27 cubes at the ages of 28, 244, 280, 293, 342 and 1126 days (source: author [PERSON]). relationships were obtained based on the concrete strengths determined in the testing of cored specimens or cubes, the results concerning the rebound number obtained using a Schmidt hammer, and the ultrasonic pulse velocity at a sufficient number of test points in the structure. Using these curves for arbitrary locations of the structure, the value of the compressive strength \(\mathrm{f_{c,Son\text{Reb}}}\) can be determined solvly on the basis of the results of non-destructive methods. This reduces the number of cores that need to be taken and leads to a reduction in the weakening of existing structures. Based on the power dependencies obtained from Equation (8), with the corresponding coefficients a, b and c, nomograms were drawn in Microsoft Excel. For concrete ages ranging from 28 to 1126 days, the nomogram shown in Figure 15 can be used, and for the age of 1926 days, the nomogram shown in Figure 16 can be used. Figure 16: Nomogram for concrete aged 1926 days. (source: author [PERSON]). Figure 15: Nomogram for concrete ages ranging from 28 to 1126 days (source: author [PERSON]).