Characteristics of a statistical check of the strength of concrete in hydrotechnical construction

Abstract

The purpose of an active statistical check is to achieve constancy of the established assurance of standard resistances by regulating the required strength depending on the actual coefficients of variation andwith consideration of the number of batches of specimens.The equations or algorithms for checking are based on expressions given in the Construction Norms and Regulations (SNIP) II-21-75 for reinforced-concrete structural elements and structures and in sNiP II-56-77 for mass concrete:Rs -----/~--1.64s,(2) or in other words:where R is the mean standard (grade) strength; R s, standard resistance or assured strength; V s, standard coefficient of variation; s, standard deviation; 1.64 (1.28), z-criterion for a number of tests approaching infinity and determines the assurance of strength R s.In quality-control practice, provided validity of the hypotheses of a normal distribution, the z-criterion will remain invariant, and the values of the assured strength, mean strength, and coefficient of variation will have varying actual values:Ract ~ct and p% =Ra~ (l--a~ct)"The required mean strength Rreq should be assigned on the basis of comparing R~c t with R s .The implementation of such a checking system in hydrotechnical construction under conditions when vast amounts of concrete pass in unit time requires strict fulfillment of the following conditions: validity of the hypothesis of normal distributions, especially for mass concrete, where the standard threshold of significance of the reject minimum is reduced to the value ~ = 1.28; the presence of a stringent system of sampling and testing, excluding undesirable distortions of the estimate of homogeneity due to additional batch and interbatch variations; reliability of predicting the estimate of homogeneity at the standard age on the basis of the parameters of homogeneity at an intermediate age. In addition to these conditions it is necessary to mention the specific problem for large hydrotechnical construction --the period of stabilization of technologies at large concrete plants. During its period --usually the first year of "big concrete" --the compositions of the concrete are in the stage of final corrections and the requirements imposed on concrete should be "sparing" with respect to assigning Rre q .To obtain the final answer to the question about the validity of the hypothesis of a normal distribution of strength of hydrotechnical concrete, long-term investigations were

Concerning the problem of checking the strength of hydrotechnical concrete

“…Thus the law of the water/cement ratio determines the tendency toward positive skewness of the strength distribution, which is confirmed by calculating "long series" [3].…”