Introduction. The existing methods of calculation of reinforced concrete structures are not universal. Each calculation model is suitable only for a limited class of elements. The construction of a general design model that adequately describes the resistance of structures to shear forces in combination with bending is one of the most difficult tasks of reinforced concrete theory. Therefore, in practical calculations of shear strength, contingent models are used.

Aim. Construction of bar models of concrete compressive strength of inclined strips of concrete.

Materials and methods. The proposed rod models of inclined compressed concrete strips of beam support zones with small shear spans a ≤ h₀₁ between load and support pads are based on modeling their physical performance, copying major crack trajectories and failure schemes.

Results. Experimental and theoretical studies have revealed possible schemes of failure of the support zones of beams at the conjugation with columns under the action of concentrated forces with the ratio of the shear span a = h₀₁ (where h₀₁ is the work height of the console).

Two schemes of failure of compressed concrete strips have been experimentally obtained, the first scheme from compression when the main compressive stresses reach the axial compression strength of concrete and the second scheme from the combined action of compression and shearing when the tangential stresses reach the strength limit.

Analytical evaluation of the obtained schemes of failure of compressed concrete strips is given. Limit states are revealed, calculated dependences of strength are developed.

Conclusions. The proposed rod models and calculation dependencies for determining the strength of compressed concrete strips are based on analogies of their physical work, have simplicity of application and allow calculations with a sufficient degree of reliability.

Introduction. The possibility of FRP using in construction was considered back in the 1960s. The relevance of FRP using in concrete structures was associated with the need to increase the durability of road and marine structures. The widespread introduction and study was hindered by the lack of well-established industrial production of composite reinforcement until the 1980s. As of today, the Russian regulatory framework has a set of rules SP 295.1325800.2017 for the design of concrete structures reinforced with FRP. However, due to the insufficient study of the issue, in this set of rules, an approximate method is used to calculate the strength of unaxial compressed elements, which leads to an underestimation of the strength of unaxial compressed elements. This circumstance leads to an increase in the consumption of materials in the design of such structures. In this regard, it seems necessary to investigate the operation of unaxial compressed concrete elements reinforced with FRP and develop proposals for a refined methodology for calculating the strength of normal sections of such elements.

Aim. Conduct experimental studies of the bearing capacity of normal sections of unaxial compressed concrete elements reinforced with FRP.

Materials and methods. Experimental studies were carried out by testing of experimental concrete samples reinforced with FRP, with unaxial static compressive load. The research was carried out taking into account the requirements of the current regulations.

Results. Experimental data on the strength of normal cross sections of unaxial compressed concrete elements reinforced with FRP have been obtained.

Conclusions. According to the results of experimental studies, destructive loads were determined, crack formation patterns were established, and deflections of test samples were determined. During the tests, it was found that the experimental values of the destructive loads of the test samples are 15–32 % higher than their calculated values calculated according to the instructions of the current set of rules.

In the second half of the 50s of the twentieth century, fulfilling the decree of the Government of the USSR on reducing the consumption of metal, necessary for defense needs, in the country's economy, it was decided to expand the use of reinforced concrete in mechanical engineering, which opened a new facet of the use of this material. Scientific support for solving this problem was entrusted to the Research Institute of Concrete and Reinforced Concrete of the USSR State Construction Committee, or rather, the laboratory of special structures of this institute, which was headed by a talented engineer, Doctor of Technical Sciences I.G. Ludkovsky.

The researchers faced the problem of maximum replacement of metal in mechanical engineering with reinforced concrete, including the manufacture of frames for heavy equipment. The researchers of the laboratory of special structures has developed several types of machine frames for various purposes, of different configurations, with different operational impacts: stamping press frames, rolling mill crates, large machine frames, etc. This article is devoted to the description of the study of the work of concrete under stress during operation with characteristics significantly exceeding the traditional physical characteristics of the metal.

Introduction. The methodology and results of research on the effectiveness of the use of new reinforcement in reinforced concrete structures with a multi-row profile of classes: A500SP and A600SP (six-row), Au500SP (four-row). The main attention is paid to the second limit state – the calculation of crack resistance. Using specific numerical examples, the results obtained when using reinforcement with a multi-row profile of classes A500SP and A600SP, Au500SP and class A500 with a double-row crescent profile are compared. The possibilities of a significant reduction in consumption in the case of multi-row fittings have been identified. Calculations were performed in the computer program "OM SNiP Reinforced Concrete".

Aim. To identify the advantages of using A500SP, A500SP, A600SP reinforcement with a new multi-row profile in the design of reinforced concrete structures. To show the possibilities of practical realization of advantages in the calculations of the HPE program "OM SNiP Reinforced Concrete".

Materials and methods. The studies were carried out for two types of structures. The first type included eight sections of elements working on bending, oblique bending, off-center compression, oblique off-center compression. In these sections, reinforcement of A500 and A500SP (Au500SP) classes were selected to ensure regulatory requirements for crack resistance. The second type was the reinforced concrete floor of the building. The calculations took into account the requirements of strength and crack resistance. Two types of calculations were performed: efforts were determined and A500 fittings were selected according to the LIRA-SAPR program"; rebar A500SP (Au500SP) was selected according to the "OM SNiP Reinforced Concrete program for the efforts received in the LIRA-SAPR program.

Results. The use of reinforcement of the A500SP, A500SP, A600SP classes with a new multi-row profile allows for metal savings of up to 25 %, compared with the currently used A500, A600C classes with a double-row crescent profile.

Conclusions. Reinforcement rods of A500SP, Au500SP, A600SP classes are recommended for wide implementation using the OM SNiP Reinforced Concrete program in the design.

Introduction. This article presents research work on the further development of the technology of accelerated carbonation of steelmaking slags in the direction of manufacturing full-scale samples of building products (building bricks, paving slabs).

Aim. Obtain products with physics-mechanical characteristics that are not conceding to small-piece road construction products (tiles, bricks) without the use of binders and other types of slag activation (grinding, introduction of alkalis, etc.), having a system of three components: steelmaking slag, water and carbon dioxide (CO₂).

Materials and methods. Steelmaking slag of PJSC NLMK with fractions up to 10 mm and gaseous carbon dioxide in a gas bottle in accordance with State Standard 8050-85 were used in the work. The influence of the following technological factors affecting the physical and mechanical characteristics of the obtained products has been studied:

1. In the process of molding products: granulometric composition of the mixture; humidity of the molding mixture; density of freshly molded products; pre-exposure and drying for reducing of humidity before carbonation.

2. In the process of accelerated carbonation: pressure, temperature, CO₂ concentration, carbonation time.

Results. The possibility of obtaining of small-piece road construction products based on steelmaking slags by hyperpressing using accelerated carbonation with strength characteristics that are not conceding to the requirements for small-piece road concretes has been experimentally proved.

Conclusions. The possibility of manufacturing of highquality small-piece products without the use of binders and other types of slag activation (grinding, introduction of alkalis, etc.) has been proved. Technological patterns for obtaining products with the following characteristics have been established: compressive strength – 15–85 MPa, average density – 1700–2450 kg/m³, frost resistance – up to F₂200.

Introduction. Buildings and structures are exposed to groundwater to varying degrees. The design of the expansion joint in the deepened part of the building must reliably protect it from the penetration of water and aggressive environments.

Waterproofing keys are among the primary measures for protecting concrete and reinforced concrete structures, but methods for assessing the effectiveness of this kind of materials have not yet been proposed. Also in the scientific literature there is no data on the water resistance of keys depending on its cross-section and the number of anchors.

Aim. To propose a method for assessing the waterproofness of waterproofing keys and to determine the dependence of the waterproofness of a key in a concrete sample on its width and the number of sealing anchors.

Materials and methods. Two keys of different widths and with different numbers of anchors were chosen as the test object: 240 mm wide key (number of anchors – 4 pcs); 320 mm wide key (number of anchors – 6 pcs).

For testing, the keys were welded in the form of a rectangular parallelepiped and concreted on the bottom and top sides, thus forming a closed loop for water inside. Water was supplied using a pump with a frequency pressure converter. The pressure was raised in steps of 1 water column meter for an hour until water leaks.

Results. In the case of testing of key No. 1, leakage occurred at a pressure of 0.07 MPa. Key No. 2 withstood greater pressure and leaked at 0.09 MPa. In both cases, leaks occurred at the junction of the key and the concrete.

Conclusions. Thus, the number of sealing anchors directly affects the water pressure that the key can withstand in real-life conditions. The standard method for determining water resistance according to State Standard 12730.5 for the construction of an expansion joint turned out to be ineffective due to the high initial test pressure of 0.2 MPa.