R = R (Z, bi), i = 0, 1, 2, …, n

The form and character of this expression depend on technological, physical and other factors (xi) affecting both non-destructive characteristic Z and material property R. However, the numerical values of the influencing factors xi can significantly differ: when the correlation dependence is determined in the laboratory and when it is utilized in practice. Moreover, the influencing factors xi can sometimes have an opposite effect on the material property. Such a paradoxical effect triggered the authors to develop a special algorithm of obtaining correlation relationships «Z - R». The obtained models are fundamental, but should be proved and adapted for the specific testing conditions.

The suggested approach is illustrated by the example of controlling concrete strength using ultrasonic pulse velocity method. Using the methods of experimental design, the quantitative dependencies of the coefficients regression on the technological factors (water/cement ratio, water content and logarithm of concrete age) and physical factors (temperature and relative humidity) are determined. Approbation of the suggested approach shows that the variation coefficient of the dependence «concrete strength -ultrasonic pulse velocity» significantly decreases.