## 6 Conclusions  
Concrete is one of the oldest building materials and it will continue to be widely used as the main structural material in the construction of buildings and facilities. The most important parameter of concrete is its compressive strength, and its determination using different methods continues to be a topical issue. The author's experimental research to determine the compressive strength of concrete was carried out over 6 years. The publication of the results in this paper was dictated by the fact that, regardless of the many methods developed and conducted over the years and the ongoing studies by many researchers, there is a lack of a universal algorithm for an effective and reliable assessment of the compressive strength of concrete. The determination of this information is still a goal in scientific circles. Based on this study to determine the compressive strength of concrete, the following conclusions can be drawn. * The results of the destructive tests were used as a reference and compared with those obtained using the elastic rebound method, UPVM (using two approaches) and the SonReb method. The compressive strength value observed in the destructive testing of test specimens ranged between 33.3 and 52.5 N/mm\({}^{2}\). In determining the number of tests and the number of methods used, the aim was to achieve the lowest price. * The relative error when using the elastic rebound method was large and reached 14.1%. The accuracy was low compared to the other non-destructive methods used in the experiments. The results regarding the rebound number were used in obtaining correlation curves for the tested specimens. This method is suitable for the rapid investigation of a large number of concrete or reinforced concrete structural elements in laboratory and field tests. To reduce the influence of different factors, the test should be repeated at least 10 times within each test region. * The relative error when using UPVM according to approach 2 was smaller (from 0.3% to 9.6%) compared to that observed using UPVM according to approach 1 (from 1.2% to 16.4%) and the elastic rebound method. Accuracy was greater when using approach 2 compared to approach 1 and the elastic rebound method. The results obtained regarding the ultrasonic signal velocity were used to develop correlation curves for the tested specimens. The method provided reliable results, the measurements were fast, the required equipment was relatively cheap. * When using a combination of methods, as with the SonReb method, more reliable results were obtained, with values close to the reference values. The relative error in this method was the smallest and reached 4.6%. The accuracy of this method was the highest (from 96% to 100%) compared to that of the other non-destructive methods used. The regression analysis performed and the correlation curves obtained in this study can be used to determine the compressive strength of concrete at any point of the entire structure solely on the basis of non-destructive testing, provided that the concrete used is the same. * The created nomograms could help to optimize the process of evaluating and tracking the compressive strength and behavior of concrete in the future monitoring and quality control of the investigated reinforced concrete elements. Only measurements of the rebound number with the elastic rebound method and the ultrasonic pulse velocity with UPVM are required to be made. * The use of these nomograms for other reinforced concrete structures with similar or different concrete compressive strength values may lead to large errors and inaccuracies in the assessments. The interpretation of the obtained results should be carried out by experienced engineers capable of analyzing deviations and inaccuracies that appear during the testing. If the measurements are carried out by persons with a lack of qualifications, the results may be interpreted incorrectly, leading to serious errors. The results are affected by a large number of factors that must be taken into account.