Strain parameters of loaded concrete under transient heating conditions

The theory of damage accumulation in concrete as a heterogeneous-brittle material, as applied to conditions of high-temperature, abrupt heating, is supplemented by the premise of the invariance of ultimate structural stresses, which makes it possible to imagine the development of a nonlinear component of deformation and a decrease in strength as a single process. A characteristic of this process is the elasticity coefficient (secant modulus coefficient) of concrete, which in the formulation under consideration acquires the character of an entropy parameter of material damage. The consequences of this premise are formulated in the form of basic thermomechanical relationships, thanks to which it becomes possible to consider the reaction of concrete to the action of temperature and load as a result of the action of two degradation mechanisms: evaporation of moisture from the gel of cement stone and destruction of structural bonds with increasing temperature, which are realized respectively in the form of linear and nonlinear components of force deformation.

A method of normalization (representation in a form relative to the initial value) of the development curves of force deformations of concrete is proposed, which allows us to reasonably separate these components when analyzing the deformation curves and find the temperature parameters necessary for their description under heating conditions in a loaded state. It was revealed that at the same load and temperature, an increase in deformability when heating concrete in a loaded state compared to loading after heating occurs due to an increase in the linear component while maintaining its share in the total deformation, characterized by the elasticity coefficient, constant. It is shown that the assumptions made in existing models of deformation of loaded concrete during unsteady heating are partial solutions, and the conditions under which their use becomes possible are determined.

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