## 3 Application of the Ultrasonic Method for the Control of Concrete Properties
The experimental procedure and processing of these results corresponded to Russian GOST 17624-2012 \"Concrete. Ultrasonic Method for Determination of Strength\". To measure the propagation time of an ultrasound pulse in the samples, the through sounding method was used, and therefore the transducers of the device were installed on the opposite sides of the sample. To ensure reliable contact between the concrete and the working surfaces of the transducers, technical Vaseline was used. The evaluation of the heterogeneity of the properties of centrifuged concrete was carried out in two stages [2]. At the first and second stages, the properties of a freshly laid concrete mixture and the properties of hardened concrete were studied, respectively. The study of the inhomogeneity of concrete properties was carried out on molded cylindric specimens. The heterogeneity of the properties of centrifuged samples along their height was investigated, which is equivalent to the differentiation of these properties along the wall thickness of the annular section of a reinforced concrete product. The reliability of the results was confirmed by a fivefold repetition of the experiments, and the coefficient of variation of the studied property was used to assess the reliability. Moreover, the mutual influence of technological and manufacture factors on the properties of centrifuged concrete and its heterogeneity along the wall thickness was investigated by means of mathematical planning of the experiment and mathematical modeling. The simplest models that are convenient for analysis are the equations obtained in the implementation of a full factorial experiment. The reliability of the obtained regression equations was checked by the value of the Fisher criterion, which should not exceed the critical value for a given level of significance and a selected number of degrees of freedom. Graphical interpretation of the obtained regression equations allows one to qualitatively and quantitatively assess the influence of variable factors on the assessment criteria [2]. In [3], a technique for assessing the design characteristics of centrifuged reinforced concrete annular columns was investigated based on the propagation velocity of ultrasound in concrete. Using this technique, a comparison was made between one- and two-stage molding processes. In [29], ultrasonic testing was proposed for the quantitative assessment of transverse and longitudinal cracks using the time and energy characteristics of ultrasonic signals and assessment of the parameters of ultrasound propagation. The method was tested in the field conditions of the Latvian Railway by comparative testing of defect-free and cracked sleepers. Non-linear ultrasonic measurements for real-time monitoring of the damage of concrete, caused by various loads, were performed in [30]. This work focused on the efficiency of the proposed method for monitoring micro-damage of concrete elements caused by mechanical loading. The authors of [31] studied the interaction of ultrasonic waves with partially closed cracks on the surface of concrete structures and, based on the measurements, estimated the width of the area of microcracks around the cracks. To optimize the sensitivity of the method, it was proposed to use correctly positioned triangular prisms [32]. The transducers are placed on the correct side of the prism, thus changing their orientation and the direction of the emitted ultrasonic wave relative to the wall surface. In [33], tests were carried out on various concrete cylinders damaged by compression. The growth of damage was monitored using linear and nonlinear ultrasonic methods in order to better understand the processes of mechanical destruction of concrete and to identify the possibilities and limitations of the used non-destructive testing methods. The purpose of the designation method of concrete quality, proposed in [34], is to determine the damage degree of concrete relative to its original and undamaged state.