Test results of concrete under the conditions of simultaneous action of multiply repeated loads of axial compression and cyclic freezing at different temperatures are given. It is established that such variable stress state of concrete under the condition σmax<R0T (where σmax is the maximum stress of the loading cycle and R0T is the stress level determining the lower boundary of the microcrack formation region of concrete) can increase its frost resistance. This is only true if the degree of water saturation of gel pores and pore capillaries of concrete does not exceed a critical value – more than 90% by volume. The results of the investigations of the structural and technological characteristics of concrete (water-cement ratio, moisture content, differential porosity, ice content, determined by calorimetric and ultrasonic methods), frozen to temperatures down to minus 70 °C, confirm the validity of the conclusions about the effect of the variable stressed state of concrete on its frost resistance.
Research object is constructional and thermal-insulating lightweight concrete (STILC) with a modified matrix in the direction improving of thermotechnical quality and durability indicators of such concrete, produced with use of porous aggregates of various petrogenesis. Motivation: the development relevance of STILC new modifications with high performance of thermotechnical quality and durability is determined by the fact that the STILC usage in the enclosing structures of buildings will significantly improve their thermal insulation function, operational reliability. Research method: the developed physical and chemical bases, structural and technological models of the modified matrix of STILC are used in researches. Research results: physical and chemical bases, structural and technological models for creating of STILC with high indicators of thermotechnical quality and weather resistance; technologies for manufacturing STILC with rational usage of technogenic formations recycling products; results of determination of thermophysical characteristics, indicators of weather resistance and other indicators of STILC operational quality. Conclusion: fundamentally new modifications of STILC with high indicators of thermal quality and weather resistance, and the main provisions of their manufacturing technology also have been developed. Based on the results of STILC samples testing, the thermophysical and mechanical properties, as well as weather resistance indicators, were determined for use in the calculations and design of enclosing structures with high thermal protection function and reliability in operation.
Diagrams of axial, lateral and volumetric strains of concrete elements under axial compression in the conditions of low negative temperatures (in the frozen state) are built. It has been established that concrete specimens in the frozen state have increased strength and increased limits of axial, lateral and volumetric strains under axial compression. The diagrams of lateral strains coefficient are built. The dependencies connecting the changes in the values of coefficient of lateral strains of the concrete with variable parameters of its stress-strain state under negative temperatures action are investigated. Based on the analysis of change patterns in lateral, axial and volumetric strains, patterns of microcracks forming in concrete elements in a frozen state are established. The influence of low negative temperatures on the change of parametric points (levels) of stress-strain state of concrete, characterizing the lower and upper boundaries of microcracks forming regions is established. The results of these investigations are used for the development for the diagram method calculating of reinforced concrete structures, designed for use in conditions of low negative temperatures.
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