## 6 Review of Alternative Methods for Diagnostics of RC Structures' Stress-Strain State
The specific feature of all the methods of non-contact visual control is that they are complex technical systems, which combine special algorithmic and mathematical software with measurement techniques, approaches to transformation and processing of measured data. The common characteristic of these methods is the requirement of thorough statistical analysis of initial data and iterative modelling of processes. However, such approaches are highly dependent on external factors, such as temperature, humidity and atmospheric conditions. Therefore, their usage for constant long-term field monitoring of the structure is rather complicated and in most cases is associated with decreased accuracy of results. Another perspective includes the methods based on the behavior of different types of waves: sound, acoustic, optical and terahertz, which provide the complex information on the stress-strain state and strength of the structure. It is appropriate to highlight the methods, which enable identifying the internal, so-called 'hidden' parameters of the structural element and invisible defects: ultrasonic (UT) method, image correlation methodology, different laser systems, etc. Ultrasonic (UT) methods [99; 100; 101; 102; 103; 104; 105; 106; 107] are based on the behavior of sound waves, which could be interpreted as a defect in a certain solid medium (See Figure 7). The basic principle of the method is the following [99; 100]. An ultrasonic pulse is generated by a piezoelectric material and passes through the structure under investigation. When passing through a homogeneous, undamaged material, the parameters of the sound wave remain stable. Any change in the speed of the pulse is identified as a defect (discontinuity of the material) and is converted from sound energy into a visual signal on the device's display. Ultrasonic flaw detection devices have the ability to work in wide frequency ranges, which makes their measurements extremely accurate and reliable. The effectiveness of such methods depends on the temperature, surface condition and defect orientation [100]. In a number of works [103; 104; 105; 106], an experimental comparison of ultrasonic methods with other technologies is given, which proved the rather high accuracy of UT technologies. Rather interesting comparative research was provided by the authors of [102], who noted a high correlation between a decrease in ultrasound speed and an increase in acoustic emission in damaged areas. In addition, fairly close values were established during the examination of reinforced concrete structures using the Schmidt hammer and ultrasonic methods [107]. It is important to assume, that the most important conclusions on the structure state can be achieved from visual-based information, including its strains and displacements, arising from the load. Measurement of dynamic and static displacements in time provides the possibility to manage serviceability of the structure and to assess the influence of real loads (operational and environmental) on the stressful condition [108]. For such purpose, the most appropriate is continuous monitoring of the structure state with the use of optical systems containing targets, lasers or LEDs. The basic common principle is the combination of measuring equipment (targets, lasers or LEDs) mounted on a controlled structure from one side and a radiation receiver, namely a phototransistor or video camera from the other side [108]. Below are specified f technologies, similar to DIC, and their specifics, which determine their increasing spread in engineering practice  
Projection laser systems (See Figure 8) include two modules facing each other, each with one or two lasers, a camera and a screen [108,109]. According to this concept, the relative displacement between the modules is obtained from calculated positions of the projected laser rays on both screens. [PERSON] et al. [110] proposed the methodology modification with the use of a visually served paired (ViSP) structured light system.