The problem of concrete strength control when using permanent formwork is considered. The results of the analysis of the experience of using the dependences of concrete strength on temperature-time parameters for forecasting and assessing the strength of concrete during its hardening, including the limits of application of the proposed dependencies, are presented. It is established that the methods of assessing the strength of concrete in a structure by its maturity allow for effective control of the strength of concrete structures concreted in permanent formwork, when there is no access to the concrete surface for non-destructive testing. Recommendations are given on the use of temperature-time dependencies for predicting concrete strength and the equipment used to implement the method, presented on the Russian market.

Keywords: non-destructive methods of testing concrete strength, temperature-time parameters, strength prediction, maturity

Improving calculation models of concrete based on experimental data, especially when studying the volumetric stress-strain state, is an urgent task. When calculating the containment shells of nuclear power plants under the influence of beyond design basis accidents, it is absolutely important to take into account the available safety margins of the materials used. Under volumetric loading, due to the work of concrete in cramped conditions, its strength and deformation characteristics improve, which is not sufficiently reflected in the regulatory documentation in force in the Russian Federation. Taking into account the analysis of existing test methods, based on the labor intensity and reliability of the results obtained, the optimal method of testing for volumetric loading was identified. The article contains the results of testing the selected test method, taking into account the improvement of the deformation measurement system. They confirmed the effectiveness of using the proposed methodology from the point of view of data reliability and relatively low labor intensity of work.

Keywords: concrete, volumetric loading test, volumetric strain sockets, deformation diagram

The article contains the results of verification of the finite element design model of work on axial compression of reinforced concrete structures with combined reinforcement. Structures with combined reinforcement (with non-removable steel-fiber concrete formwork) are widely used in the construction of nuclear power plant structures. Steel fiber reinforced concrete, in comparison with heavy concrete, has increased strength and deformation characteristics, due to which it performs the function of a bearing element that perceives compressive and tensile forces. The study of the joint work of steel fiber concrete and heavy concrete is a promising area of research. The developed finite element model of a reinforced concrete structure with a fixed steel-reinforced concrete formwork with a high degree of reliability allows us to determine the stress-strain state, the redistribution of forces between the cross-section elements and the parameters of cracking during loading. The described modeling technique is recommended to be used in the study of the operation of such structures.

Keywords: NPP, nonremoveale formwork, structural testing, Ansys, stress-strain state diagram, steel fiber reinforced concrete, finite element method

The article presents dependencies between deformation characteristics of SFRC with a high-strength cement-sand matrix from the type of steel fiber and its volumetric content. It was found that the introduction of fiber imparts plasticity to the matrix. Plasticity depends more on the type of fiber than on its amount per unit volume. Full deformation diagrams of steel fiber reinforced concrete have a descending branch, in contrast to the matrix diagram. The introduction of steel fiber increases the deformations corresponding to the ultimate tensile strength by no more than 75%. Deformations of the descending branch of the diagram, depend on the type of fiber, characterizing the plastic properties of steel fiber reinforced concrete. At the same time, it essential how fiber ensures the adhesion to the matrix: along its entire length due to its fiber profile or due to the design features of the anchors at its ends.

Keywords: axial tension, complete diagram of deformation of steel fiber reinforced concrete, type of fiber, percentage of fiber reinforcement, ductility, residual strength

Determination of the actual category of the technical condition of building structures is the main purpose of the survey. At the same time, the reliability of the survey results depends on the volume of instrumental measurement of the monitored parameters, which should be minimal, but sufficient to obtain data with a given level of security. This article presents a methodology for de-termining the scope of control based on a statistical approach with justification of the necessary and sufficient number of tests for each type of survey. The approbation of the described approach in the survey of precast concrete ele-ments is presented. The proposed methodology is based on a statistical approach adopted at the international level, which allows work to be carried out in other countries.

Keywords: inspection of building structures, instrumental control, statistical justification, precast concrete