A model describing the variation in autogenous shrinkage and drying shrinkage of portland cement concrete, depending on the volume of aggregates and the shrinkage of hardened cement paste, is presented. The equation to calculate shrinkage of concrete as a function of the volume of aggregates and shrinkage of a hardened cement paste was proposed. Formulas are proposed that describe the change in the shrinkage of hardened cement paste as a function of water/cement. The results of studies of the effect of superplasticizers and mineral additives on the autogenous shrinkage and the drying shrinkage of hardened cement paste are presented. Concretes made with superplasticizer and mineral additive may have the potential lower the value of drying shrinkage. The shrinkage value can be lowered from 30% till 70%. Concretes containing superplasticizers and mineral additives can potentially have the autogenous shrinkage reduced to 75%, or increased to 180%.
Strength of concrete is a function of several variables, such as: water / cement ratio, portland cement activity, filler quality, etc., adjusting which you can achieve the required strength. However, the introduction of the concrete admixtures, changing holding conditions of the samples, the volume restriction can affect the strength of the due to exposure impacts both the kinetics of hydration processes and on the structure formation of cement paste. The article presents the results of investigation the influence of the amount of gas-forming admixture, the presence, type and amount of superplasticizer, the ratio sand / cement, aggregate size and water-cement ratio on gassing and expansion of the mixture in the first stage and propose coefficient of effectiveness of gassing and of expansion. The authors proposed a formula to describe the dependence of the relative deformations of expansion of the concentration of aggregate. In order to assess the conditions in which there is a mixture, it is proposed to use the indicator - coefficient of congestion of expansion, to account for the influence of hardening conditions on the strength of the concrete - coefficient hardening conditions. The authors proposed to consider the impact of prescription and technological factors by introducing system of correction coefficients.
Conditional quantitative criteria characterizing the shrinkage crack resistance of various concretes and a model describing the change in the proposed criteria depending on the magnitude of shrinkage deformation, creep coefficient, tensile strength kinetics and shrinkage strain kinetics for ordinary concrete and self-compacting concrete are proposed. The proposed criteria for the class C40/50 concrete have been calculated and it was shown that self-compacting concrete can potentially have higher crack resistance during shrinkage. To ensure high cracking resistance during shrinkage when choosing superplasticizers and mineral additives, attention should be paid to their effect on shrinkage, creep and E-modulus of the cement stone. It should exclude additives that increase the shrinkage and E-modulus and reduce creep of cement stone.
Often, when it is necessary to fill formwork space in confined working conditions with limited operational control in some engineering solutions the special materials for injection are used. In this case, it is advisable to use high-fluidity concrete mixes made with К-cement, regulator of the kinetics of structure formation, superplasticizer and gas-forming admixture. The gas-forming admixture obtains of expansion of fresh-concrete during first stage. The special sulfo-alumina additive in K-cement obtains of expansion of hardening concrete during second stage. The main regularities in the formation of the structure and properties of cement stone are considered. The dependences "composition-technology-structure-properties" for concretes with two-stage expansion are clarified. The main provisions for determination of mix proportion for concrete are proposed. The quantitative relationships between the construction properties of concretes and prescription and technological factors are obtained.
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