## 8 Conclusions  
The reliable study of engineering problems and the identification of further areas for improvement require thorough research on background and existing investigations. In this study, a detailed analysis of theoretical and experimental findings of digital image correlation was provided. The main trends in computational approaches were identified On the basis of the conducted literature review, it is obvious that the digital image correlation technique has come through multi-stage evolution and transformation. Starting from the first theoretical investigations on image-based measuring methods, much attention was paid to image recognition and matching procedures. A great number of scientific studies were also dedicated to calibration methods and the development of analytical concepts aiming to increase the accuracy of the results. In general, after various improvements and modifications, the DIC-technique has become a unique methodology for investigation of the stress-strain state of constructions. In addition, the work includes a thorough comparative analysis of alternative techniques, which, in addition to the DIC, are effective for diagnostics of RC structures' stress-strain state. Specific advantages of image-based technologies in comparison with other methods, determining their wide and effective application were identified. It provides the possibility to study cracking and fracture processes in structural elements, obtaining the full field of deformation and most extended picture of structure performance. The method is applicable to use for a wide range of engineering issues and for different structural materials and provides reliable data on a micro- and macroscale. Further development of the DIC technique would surely enable more precise measuring of stress-strain parameters and reliable assessment of structural behavior at different loading stages. In recent research, specific attention was paid to the development and improvement of the matching process, synchronization and coordination of parameters and correlation functions in time and calibration of image systems. These questions are being currently discussed in engineering. However, there is still a lack of knowledge, and further research is required. Among the issues, which are not thoroughly covered in scientific works, are: the specifics of DIC use for different types of natural and artificial materials, the impact of external factors and environmental conditions. A specific feature of the DIC method is the possibility to apply it not only for laboratory testing, but also for site research. In such cases, accurate measurement and the reduction of errors should be of particular importance. The absence of detailed analysis of this problem is the reason for numerous challenges faced by scientists when using the DIC method for site investigations. It should also be noted that there is no exact unanimous decision on the specifics and parameters of the speckle pattern, which provides the highest accuracy. Although there is a clear understanding of the importance of parameters, such as size of dots, measurement bases' sizes and density of the pattern, there is no exact strict methodology to predefine these values during experiment planning. In particular, the necessity for further study is obvious for damaged reinforced concrete structures, which are composite elements with non-homogeneous micro-and macrostructure. The detailed analysis has shown that the specific advantage of DIC technology is the possibility to obtain the stress-strain data along the whole studied area. This unique feature makes the image-based method almost irreplaceable for engineering tasks. Therefore, this method requires more attention in further research, specifically for damaged structures. In the presence of different damages and defects, the stresses and strains in localized areas need to be taken into account. In these areas, the stress-strain map in some cases is non-continuous, which causes additional inaccuracy of obtained data.