CONNECTION BETWEEN STRESSES AND STRAINS OF CONCRETE UNDER DIFFERENT FORCE MODES OF SHORT-TERM AXIAL COMPRESSION
Authors:   National University of Water and Environmental Engineering, Krus Yurii 
Published: August 2022

Modern design of bended reinforced concrete constructions uses analytical methods of calculation involving deformative models, the main tool of which is the actual diagram of concrete state, which establishes the connection between its stressesband relative strains bin compressed and tensile zones of the cross-section. Currently, there is no a unified methodics of establishing such a diagram, and researchers obtain it by indirect methods that differ from each other. That is why the graph-analytical building and mathematical description of the functional dependence of the diagram of concrete state based on clearand accurate physical representations we consider as an important and urgent problem.The lackof a unifiedmethodics for obtaining a diagram of concrete state cast doubt the reliability of the results of the calculations carried out, does not allow to compare, explain and link into a unifiedwhole the many experimental and theoret-ical data accumulated by generations of researchers.In a number of scientific publications, an opinion is expressed that a diagram of concrete state can be obtained by simultaneous using of diagrams of concrete deforming under different force modes of short-term axial compression –«soft» and «rigid», testing standard concrete specimens to failure at a constant rate of stress and strains growth respectively. If the «soft» force mode without unnecessary difficulties can be realized on traditional hydraulic presses, the creation of «rigid» mode requires the manufacture of different from standard special presses or auxiliary devices, capable of taking efforts and unloading concrete after reaching the maximum of compressive stresses. The lack of a unified approach to obtaining diagrams of deforming under rigid-mode loadings leads to contradictory results.Insufficiently investigated in the general problem are the issues of theoretical justification, development and approbation of diagram of concrete rigid-mode deforming bb~, complex application of which simultaneously with the diagram of de-forming bb~in conditions of «soft»force mode will allow to obtain a reliable and convincing diagram of concrete state.The main purpose of the research is a graph-analytical building and mathematical description of the dependence bb~, based on clear and accurate physical representations, which experimental obtaining causes serious technical difficulties and eco-nomic costs, and the accuracy of measurements made and the reliability of the results obtained causes big doubts.The article develops the criteria to which the dependence bb~should correspond, based on modern ideas about the nature of strength and deforming and following from the essence of physical and mechanical processes in concrete; the ade-quate structure of the functional connection has been established; the boundary conditions have been drawn up and constant coefficients have been determined; a comparative analysis of the calculation results using the proposed dependence with the corresponding experimental data was performed.As a result, an easy-to-use and mathematical processing of an approximate function of the diagram of deforming is proposed, which reflects the dependence between stresses and strains of concrete under the «rigid» mode of homogeneous compression. Its simultaneous use with the previously obtained diagram for soft-mode loading makes it possible to graphically build and mathematically describe a stable and free from excessive influencing factors the diagram of concrete state, suitable for based on the deformative model of cross-sections of modern calculations of bended reinforced concrete constructions for all groups of limiting states and assessment of the heterogeneous stress-strain state of constructional cross-sections at any stage of loading up to fracture.