## 3 Application of the Ultrasonic Method for the Control of Concrete Properties  
One of the advantages of using non-destructive testing methods (NDTs) for studying concrete is the speed of the measurement of properties with a certain degree of reliability. NDTs are used mainly in the construction industry for inspection control of the concrete strength of buildings and structures, as well as for determining the transfer and tempering strength of concrete in reinforced concrete structures. However, in practice, there is an insufficient number of effective methods for controlling the strength of concrete in exploited structures, in which a decrease in its strength occurs due to the appearance of damage in the structure [29; 40; 41]. In [1], an attempt was made to develop a special method for non-destructive testing of the concrete strength of the operated contact network supports. At the same time, the possibility of using the two most common control methods (chipping and elastic rebound) was investigated. The chipping method is laborious and inefficient; moreover, it damages a structure, which is unacceptable in the case of damage of concrete throughout the structure. By this method, the strength of concrete is estimated at a shallow depth of up to 20 mm. The deeper part, which as a rule has a large amount of microdamage, remains out of reach. For high-precision control of concrete strength using the method of elastic rebound, it is required to construct calibration dependences of strength on the height of the striker bounce, which is almost impossible to obtain for operating supports. In the absence of such dependences, the accuracy of assessing the strength of concrete is within \(\pm\)50% and, as a rule, is overestimated. This conclusion is confirmed by the results of mechanical tests of the construction supports, carried out simultaneously with non-destructive tests. The rebound method is unsuitable for concretes with strength exceeding 50 MPa; it requires adjustment when controlling the strength of concrete with curved surfaces, as well as wet concrete. The rebound method allows one to control the strength of concrete of the surface layers of structures and cannot be used to control the inner layers that are most damaged during operation. This method is suitable for testing the strength of non-cracked concrete. With near cracks or damage, the method does not reflect the actual strength of the concrete. Ultrasonic vibrations are used to assess the damage in the concrete structure [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. The main indicator of damage in this case is the ratio of the propagation time of the ultrasound pulse in the direction perpendicular to the stressed reinforcement to the time of propagation of the ultrasound pulse along this reinforcement. As studies show, when damage accumulates in concrete, the time of ultrasound pulse propagation changes most intensively in the direction perpendicular to the stressed reinforcement [1]. In the centrifuged unstressed construction supports, due to their conical shape and closed cross-section, under the action of temperature and humidity factors, a stressed state arises in which tangential stresses are dominant. Theoretically, these stresses are two times higher than the stresses directed along the generatrix of the supports, and therefore, in these construction supports, the appearance and accumulation of damage occurs mainly on the planes parallel to the generatrix of the support surface. The methods of measurements of characteristics in unstressed centrifuged construction supports are the same as in prestressed ones. The speed of propagation of ultrasound pulse is one of the characteristics of the quality of concrete [2]. For the actual control of the strength of concrete on the basis of the ultrasound transmission rate, a special experiment was carried out, and a calibration dependence \"concrete strength--ultrasound pulse transmission rate\" was constructed.