class. Besides, to obtain a uniform strain field it is necessary to install measurement instrument at a distance equal to the width of the specimen (according to Saint-Venant's principle). Also, the height of the specimen can affect the deformations of the item. Studies [3-5] show that when testing concrete specimens for compression in a strain gauge tube, the graph of stress variance in the specimen with relative strains close to the ultimate has a descending arm. While numerous experiments conducted on different grades of concrete