## 6 Review of Alternative Methods for Diagnostics of RC Structures' Stress-Strain State
Authors in [110] provide a detailed theoretical description and experimental evaluation of the proposed ViSP. The specific feature of ViSP is that the positions of the laser rays provide the information on the rotation angles of the manipulators, and the projected laser rays can be inside the screen boundary; thus, the position of the lasers is controlled to make the projected laser rays be always on the screen. A comparatively small distance between the camera and the screen causes robustness to the external environmental changes. Among the main drawbacks of this system, the most significant is the limited measurable range due to the limited screen size [108,109]. Figure 7: Principle of ultrasonic testing [101]. Similar optical-based systems with a phototransistor receiver and a laser light source were described in the works of [PERSON] et al. [112] and [PERSON] et al. [113], where relatively low errors (0.1-0.5 mm) were admitted. Technologies with the use of sensor equipment enable identifying dynamic and static movements and specifics of the stress-strain state. The optical methods described above have the following specifics: rather small displacement measurement errors and the possibility to provide constant monitoring of the structure during a long period of time. On the other hand, authors in [108] noticed a relatively small distance between the receiver and the radiation source and small displacement measurement range. There are a number of specific advantages of technologies, similar to DIC, which determine their increasing spread in engineering practice. As was stated by [114], camera-based measuring devices such as digital image correlation, electronic speckle interferometry and laser speckle measurement systems correlations are effective for strain measurement, even in special measuring condition cases. Within these methods, the Laser Doppler Vibrometry (LDV) system, based on the Doppler effect principle, needs to be discussed in more detail. According to this approach, the vibration of the structure can be calculated based on the laser beam Doppler shift reflected off the vibrating surface [115]. The Doppler effect itself is defined as the certain change in the wave motion frequency, noted by the observer when there is relative motion between the source and the observer [116]. The abovementioned approach allows full noncontact excitation and measurements for modal analysis of various structures and elements. The contactless vibration measurement method utilizing a laser Doppler vibrometer system was thoroughly studied in the paper [117]. The author proposed the non-contact procedure improved with advanced measurement techniques using mirrors [117]. A 3D laser Doppler vibrometer (3D LDV) has proved to be rather effective for quality assurance and control. The 3D LDV consists of the measurement of the frequency response of single or multiple points without any time gaps. The work [116] emphasizes the modification of the approach, enabling full non-contact monitoring, which uses acoustic pressure changes as the excitation source and three LDV (PSV-500 by Polytec) as the sensing system. A number of studies [118; 119; 120] have proved the applicability of scanning laser vibrometry for damage detection in different types of materials. The principle of the method is based on indication of damages according to analysis of elastic wave interaction with discontinuities. Thus, cracks, holes and slits are monitored on different scales. For instance, authors in [119] used scanning laser vibrometry  
Figure 8: Principle of projection laser systems application [111]. for detection of damages in aluminum elements with small thickness. Articles [118,120] were devoted to delamination detection in composite materials. However, according to [114], 3D-scanning laser Doppler vibrometry has its limitations due to long measuring time, and reduced signal-to-noise ratio.