The article proposes the simplified method for determining stresses in massive monolithic foundation slabs arising from the heat release of concrete during the hardening process. The proposed technique makes it possible to reduce a three-dimensional problem to a one-dimensional one based on the features of the distribution of stresses and strains in the structures under consideration, identified during finite element modeling in a three-dimensional setting. The resulting resolving equations take into account the creep and shrinkage of concrete, the coefficient of reinforcement of the structure. The strength and deformation characteristics of concrete are assumed as functions of the degree of maturity of the concrete, which in turn is determined by the time and temperature of curing. Approbation of the developed model is carried out by comparison with the calculation in a three-dimensional setting in the ANSYS software package. The influence of creep and contraction shrinkage of concrete, the degree of concrete maturity and the coefficient of reinforcement on the stress-strain state of structures is investigated.

Introduction: Due to hydration heating and heat exchange with the environment during hardening, mass cast-in-situ reinforced-concrete structures exhibit non-uniform heating, which can result in early cracking and make the structures unsuitable for further use. One of the main risk factors for early cracking is the temperature difference between the center and the surface of the structure. Purpose of the study: We aimed to study how such factors as the ratio of dimensions, heat transfer conditions on the surfaces, concrete recipe, pauses during concreting and their duration affect the maximum temperature difference between the center and the surface of the structure. Methods: In the course of the study, we applied finite element modeling in one-dimensional and three-dimensional cases using the software in the MATLAB environment that we developed earlier. Results: We established that the most significant risk factors for early cracking are heat exchange conditions on the top surface, structural thickness, and the heat release rate of concrete. Verification and validation of the model were performed based on experimental data and by comparing it with a numerical solution in the ANSYS software.

Based on the analysis of the effect of various superplasticizers on the compressive and tensile strengths of concrete hardened under different conditions for three months, it was concluded that to estimate the effectiveness of superplasticizers it is not enough to compare the relative strength indicators of concrete only (f90/f28; ft = f(f); f = f(W/C)) without taking into account the effect of the superplasticizer on the water content in the concrete mix and the cement content. The assessment methodology is given. It is shown that provided the required design parameters for durability are ensured, analysis of only data on the increase in compressive and tensile strengths in time can lead to erroneous conclusions. A reliable assessment should be made taking into account the influence of the superplasticizer on the composition and cost of the concrete mix, shrinkage, creep and E-modulus of concrete.

Introduction. The shear strength of concrete, while not being an independently standardized indicator of concrete quality, plays an important role in the analysis of reinforced concrete structures. The concepts related to the dependence of the shear strength of concrete on the standardized compressive and axial tensile strength are quite ambiguous. Self-compacting concrete (SCC), which has been widely used recently, is somewhat different from ordinary concrete (OC) compacted by vibration in terms of structure and properties, and data on the shear strength of SCC are sparse. Purpose of the study: We aimed to clarify the dependence of the shear strength of concrete on the standardized compressive and axial tensile strength, and assess the shear strength of SCC in comparison with that of OC. Methods: We compared the shear strength of SCC with that of OC experimentally, by applying the common methodology with the use of a Mörsch specimen and performing modeling in MATLAB with the use of six strength theories. Results: No significant differences were found in the dependence of the shear strength of SCC in comparison with that of OC at the design age of 28 days. In terms of quantity, the excess of the shear strength of SCC relative to OC is less than 12%. The best agreement with the experimental data among those analyzed is provided by the Geniev theory. The shear strength of concretes is most likely described by the equation Rsh=k*(R*Rt)^(1/2) at k = 0.5–0.6.