Assessment of Thermally Stressed State of Concrete Massif

The paper describes a technique for assessing the thermally stressed state of a concrete massif of a foundation slab made of a self-compacting concrete mixture. The proposed method consists in a preliminary calculation of temperature fields in hardening concrete. The objects of research have been self-compacting concrete mix and structural concrete in the structure mass. The choice of materials for the preparation of a concrete mixture is given and substantiated. The composition of self-compacting concrete has been used to assess the thermally stressed state. A binder with a reduced exotherm has been used in order to reduce the self-heating of concrete. Studies have been carried out to assess the specific heat release of the recommended cement depending on the initial water-cement ratio. The effect of a chemical additive on the rate and magnitude of the specific heat release of cement has been studied. The paper presents the main theoretical provisions and an algorithm for calculating the thermal stress state of a concrete massif. The finite difference method has been used to calculate the expected temperatures and their distribution in the structure mass, and the temperature stresses in the sections of the concrete mass have been calculated to assess the thermally stressed state. The performed calculations of the temperature fields have made it possible to estimate the maximum possible temperatures and temperature differences over the sections of the concrete massif depending on the initial temperature of the concrete mixture and the average daily temperature of the outside air. Analysis of the temperature distribution has revealed the most dangerous sections of the concrete mass. An assessment of the thermal stress state of the concrete mass has been made on the basis of the results pertaining to calculation of temperature fields. The calculation of temperature stresses in the most dangerous sections of the concrete massif has been performed. It is shown that the calculated value of the temperature stress can serve as a characteristic of the thermally stressed state of the concrete mass. The formation of temperature cracks in a concrete mass is possible when the calculated value of the temperature stress exceeds the actual tensile strength of concrete. Comparison of the calculated and actual values of temperatures in the sections of the foundation slab has made it possible to conclude that the calculations of the temperature fields and, as a consequence, possible temperature deformations are correct.

KINETICS OF CEMENT HYDRATION, PLASTIC STRENGTH OF CONCRETE OF CHANNEL CLADDING AND ITS THERMAL STRESS STATE

The presented research results show that the development of the technology of laying monolithic concrete under various temperature and humidity conditions of the external environment is associated with the acceleration of the hardening process of freshly laid concrete6and the acquisition of the necessary strength before reaching dangerous values of moisture loss and freezing. Only in this case can a high-quality material with high potential properties be obtained. The studies were carried out under isothermal conditions at different temperatures.The obtained data allow us to calculate the temperature fields of concrete that arise due to the exothermy of cement in non-stationary modes. The lining of the channel is considered as an isotropic plate unlimited in the horizontal direction, lying on a solid ground base. When solving the problem, the following prerequisites are accepted: – the temperature regime in the lining and the underlying layer of the soil changes according to the damped harmonic function; – the thermal stress state of the cladding is affected by the creep of the concrete. As a result, the study of the kinetics of heat emission of cement during its hydration under various temperature conditions is mportant and necessary, especially for laying monolithic concrete in the lining of channels.

Investigation of the thermally stressed state of shallow shells on a rigid base with a sliding contact layers using analytic solutions of equations оf elasticity theory

Modern calculations of layered plates and shells in a three-dimensional formulation are based on a technique where the distribution of the desired functions over the thickness of a structure is sought by the method of discrete orthogonalization. In this article, based on the approaches developed by the authors, the thermally stressed state of layered composite shallow shells with a rigidly fixed lower surface is analyzed. The distribution of the desired functions over the thickness of the structure is found based on the exact analytical solution of the system of differential equations. An approach to the study of the thermal stress state of shallow composite shells is considered, and an analytical model is constructed for calculating the thermal stress state of shallow shells on a rigid base with a sliding contact of the layers. Currently, this is a very urgent task when calculating the pavement of bridges. A feature of this approach is the assignment of the desired functions to the outer surfaces of the layers, which allows one to break the layer into sublayers, reducing the approximation error to almost zero. Using the model in question, an analysis of flat layered composite shells on a rigid base with a sliding contact of the layers under the influence of temperature loading was carried out. To build a spatial model, a load option is selected with temperature loads (according to the sine law) and boundary conditions (Navier), which lead to the distribution of the desired functions in terms of a plate with trigonometric harmonics of the Fourier series. A polynomial approximation of the desired functions by thickness is involved. Using the model in question, an analysis of flat layered composite shells on a rigid base with a sliding contact of the layers under the influence of temperature loading was carried out. The considered example showed that the proposed model provides sufficient accuracy in the calculations of layered shallow shells when considering each layer within one sublayer. The proposed approach can be used as a reference method for testing applied approaches in calculating various stress states of layered flat composite shells.

MODELING THE THERMAL STRESS STATE OF THE FOUNDATION AND DEVELOPMENT OF MEASURES

The modeling of the thermal stress state of the foundation slab of the building of a general educational institution is considered. The performed calculations took into account the occurrence of stresses as a result of heating of the concrete mass during cement hydration. To simulate the thermal stress state, the specialized Midas Fea software package is used. At control points, diagrams of changes in temperatures, stresses, and crack formation coefficients depending on the periods of curing are obtained. The isopoles of temperature distribution and crack formation coefficients over the volume of the studied array were obtained. Measures have been developed to improve the operational properties of concrete, including engineering and technical recommendations to reduce the negative impact of temperature differences on the manufacturing process of concrete mixtures and foundations.