Introduction. During construction of the massive monolithic reinforced concrete structures, concrete hardening takes place in the unsteady abnormal temperature conditions, which, along with the massiveness of the structure and concrete recipe features, influence significantly on the concrete strength gaining kinetics. The non-homogeneity of the temperature field leads to the difference of the hardening concrete strength and deformation indicators in the different points of a structure at one and the same time, eventually resulting in formation of the non-homogeneous stress field. Due to the insufficient number of publications dedicated to the topic of forecasting the dependence of the strength of concrete on the time and temperature conditions of its hardening and recipe features, the research in this field becomes a relevant objective. Thus, the aim of the work is to determine a mathematical dependence of the strength gaining kinetics of different concretes on the reduced time of hardening parameter, the dependence which is necessary for calculating the thermal stress of the massive reinforced concrete structures at early-age hardening in the unsteady temperature conditions.Materials and Methods. The data received during the authors’ experiments and calculations, as well as the data available in the existing publications and regulatory documents was used. Theoretical assessment of the concrete strength gaining kinetics, depending on the cement type hardening kinetics, was done. The assessment took into account the actual heat of hydration of the hardening concrete and its temperature, including the unsteady one, and was based on the following provisions: the kinetics of cement heat of hydration can be considered as the cement hydration degree and strength gaining indicator; the cement hydration degree in a given moment of time determines the porosity of a cement brick, and the strength of a cement brick and concrete is determined by the porosity thereof.Results. The dependence of the relative strength of concrete on the reduced time of hardening parameter in the unsteady temperature conditions is determined for the concretes having different kinetics of hardening in the normal conditions. The good convergence of the proposed dependence with the results of experimental studies and some normative documents is shown.Discussion and Conclusion. The scientific concepts on the relative strength assessment of the concrete, hardening in the abnormal temperature conditions, have been further developed based on the hypothesis of the concrete hardening kinetics dependence on the kinetics of cement heat of hydration, taking into account the degree of concrete aging per "reduced time of hardening". It’s worth to use this dependence for calculating the thermal stress of the massive reinforced concrete structures at early-age hardening. The efficiency of assessing the strength gaining kinetics of the various concretes, hardening in the unsteady temperature conditions, by using the "reduced time" parameter, has been shown and substantiated. The heat of hydration kinetics and the total porosity magnitude, determined by the hydration degree indicator, have been taken into account. The application of the "reduced time of hardening", depending on the concrete aging degree to assess the relative strength of concrete, with regard to its recipe features, has been substantiated.
