Autogenous Shrinkage of Concretes from Highly Mobile and Self-Compacting Mixtures

Introduction. The influence of the chemical and mineralogical composition of Portland cements and the chemical base of superplasticizers on the magnitude and kinetics of autogenic (contractional) shrinkage of concretes from highly mobile and self-compacting concrete mixtures has been revealed. The relevance of the issue is due to the commonly overlooked role of autogenic shrinkage in the formation of the field of temperature-shrinkage stresses in the early hardening period of massive monolithic structures. In order to calculate intrinsic stresses, data on the magnitude and kinetics of autogenic shrinkage are required, and the insufficiency and some inconsistency of data on the effect of superplasticizing additives on the magnitude and kinetics of autogenic shrinkage, depending on the material composition of cement and the chemical and mineralogical composition of clinker, are critical to the expediency of obtaining new data on the issue. The objective of the work is to develop scientific ideas about the influence of prescription factors and material properties on the quantitative and qualitative parameters of autogenic shrinkage using the example of materials commonly used in the production of monolithic reinforced concrete structures in the Rostov region. Materials and methods. Experimental studies were conducted using six Portland cements from four manufacturers that are fast-hardening according to GOST 31108-2020 classification. Superplasticizing additives based on polycarboxylate esters and naphthalene formaldehydes in a dosage of 0.5% of the commercial product were employed. The properties of cements are identified according to GOST 30744-2001 and GOST 310.5-88. Deformations of the hardening cement paste (stone) were determined by means of the Le Chatelier method. The amount of autogenic shrinkage of concrete was determined by the calculation method based on the amount of autogenic shrinkage of cement, taking into account the true value of the I/C of concrete and the concentration of aggregate in concrete. Results. The ratio of "autogenic shrinkage/total contraction" of the investigated cements with additives at the age of 5 days was 0.37–0.74, the quantitative values of the total contraction of the studied cements in combination with additives at the age of 5 days ranged from 2.93 to 3.43 ml/100 g of cement, which does not contradict the available data. Change in the amount of autogenic shrinkage in the presence of additives at the age of 5 days was from 0.64 to 1.65 relative to the nonadditive standard. The effect of additives on the kinetics of autogenic shrinkage was manifested both in acceleration or deceleration, and in the absence of any effect. The calculated value of autogenic shrinkage of concretes of classes B25–B35 from highly mobile and self-sealing mixtures at the age of 5 days ranged from 0.36 to 1.18 mm/m. Discussion and Conclusion. Scientific ideas about the kinetics of autogenic shrinkage have been developed depending on the type of cements and additives. In order to describe the change in autogenic shrinkage over time, a formula similar to the formula EN 1992-1-1 for the change in concrete strength over time is set forth. The classification of concretes according to the kinetics of autogenic shrinkage is suggested. The patterns of changes in the amount of autogenic shrinkage of concretes from highly mobile and self-compacting