Rheological equations of concrete state and relaxation of stress

Some approaches to the derivation of rheological equations of the mechanical state of concrete are considered and the principle of superposition of fraction deformations is justified in a nonlinear statement. In linear creep theory, this principle is known as L. Boltzmann’s superposition principle of fraction creep deformations. The concept of the strength structure of the constructive material is the basis for substantiating the statements given in this work. The statistical distribution of the strength of the fractions forming a structural element in the union allows the derivation of nonlinear equations of state. At the same time, the so-called structural stresses of fractions that capable to force resistance are considered. The overlay principle of fraction deformations in non-linear statement is justified. This means the modification of L. Boltzmann’s principle of superposition allowing its applicability also under the nonlinear dependence of deformations on stresses. It is established that the integral equation of state, which is nonlinear with respect to calculated stresses, is linear with respect to structural stresses. It is this circumstance that permits its reduction to a simple linear differential equation, which, in particular, simplifies the solution of relaxation problems. These problems are closely related to the calculation of structures for long-term safety.