The article discusses the strength of concrete and other brittle materials in the case of nonuniform biaxial type of compression (σ1 > σ2 > 0) and triaxial compression of σ1 > σ2 = σ3 > 0 type (it was assumed that σ> 0 corresponds to compression). It is noted that, when considering the biaxial loading in the accepted model, probabilistic nature of distribution of stresses along the contour of pores and inclusions, i.e. stress causing formation and propagation of cracks in the material, plays an important role. Moreover, the stress across the circuit pores was regarded as a three-dimensional random field of S (α,β,γ,ω),where ω -is a random argument. Considering the average number of overshoots NR we believed that the random field of S is not homogeneous (not stationary): its expectation is not constant, but is a function of nonrandom arguments Мs = Мs(α;β;γ). External load, corresponding to NR = const (and at the same time constant of level exceedance probability), first increases and then decreases a little. Heating up to 300 о С (573K) and 400 о С (673K) leads to violations, and long-term load leads to significant changes in the macro-and microstructure of concrete.
Рассматриваются результаты теоретических и экспериментальных исследований процесса разрушения твердого тела (бетона) методами механики разрушения. Представлены результаты экспериментальных исследований характера развития трещин в образцах тяжелого бетона, которые подвергались кратковременному температурному воздействию до 300 °С (573 К) и 400 °С (673 К) и остыванию или же вовсе не нагревались (20 °С). Установлено, что длительная прочность бетона после кратковременного нагрева до 300 °С снижается весьма мало, а температурное воздействие до 400 °С приводит к резкому снижению длительной прочности бетона. Ключевые слова: прочность, деформация, температурное воздействие, трещина, ползучесть, разрушение, мера деструкции. The paper considers the results of theoretical and experimental studies of the process of solid body (concrete) destruction by the fracture mechanics methods. The results of experimental studies of the crack propagation nature in heavy concrete samples which were exposed to short-term temperature effects up to 300 °C (573 °K) and 400 °C(673 °K) and cooling or not heated at all (20 °C) are presented. It has been established that the long-term strength of concrete after short-term heating up to 300 °C decreases very little, whereas the temperature effect up to 400 °C leads to a sharp decrease in the long-term strength of concrete.
The results of experimental and theoretical studies of the process of destruction of concrete by the methods of fracture mechanics are considered. Results of studies of long-term strength, durability and deformability of concrete subjected to a preliminary short-term temperature action up to 300° C and 400° C under load and without load are presented. It is shown that after short-term heating up to 300о С the long-term strength of concrete decreases insignificantly. It is established that heating up to 400° C can be considered the boundary of the structural integrity of concrete. The conditions for using the results of these studies in determining the values of a function that characterizes the change in the long-term strength of a material in the mechanics of heterogeneous structures are formulated. The function of the material destruction measure is introduced to describe the nature of the structural changes in the material at a given constant continuous load, and its change for different levels of a continuous load is considered.
The number of researches on steel fiber reinforced concrete (SFRC) fire resistance is insignificant. For the calculation of building structures for fire resistance, it is necessary to use the thermophysical characteristics of concrete: thermal conductivity, heat capacity and thermal diffusivity. The physicomechanical characteristics of SFRC depend on the volumetric content of the fiber in it. This paper presents the results of studies of thermophysical properties of SFRC. The studied SFRC had a high-strength self-compacting cement-sand matrix and a different percentage of fiber content (from 0 to 6%). The experiments were carried out for SFRC with steel wavy fiber 15 mm long and 0.3 mm in diameter. As a result of experimental studies, it was discovered that with an increase in the volumetric content of the fiber, a decrease in the values of heat flow, thermal conductivity and thermal diffusivity coefficients, specific heat capacity is observed and the thermal resistance of SFRC increases.
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