Introduction. The design and technology of continuous concreting of two massive steel-reinforced concrete arches inclined to each other (height 19 m, span 47 and 81 m) with a volume of 184 and 283 m³ in a fixed formwork, the function of which was performed by a steel shell with a diameter of 2 and 2.5 m, with a design class of concrete B70 are presented.

The aim of the complex of design and technological works was the calculation and design of arches, including the calculation of the thermally stressed state of massive structures in the initial period after concreting, and the determination of prescription and temperature-time parameters of concrete production technology and concrete quality control.

Materials and methods. The rigidity of the structure is ensured due to the steel-reinforced concrete section of the arches, which, in comparison with the metal structure, has increased bending stiffness up to 5.5 times and axial stiffness – up to 3.5 times. The features of the technology for the construction and quality control of concrete arches were as follows: a self-compacting concrete mixture with additives of an organomineral modifier of the MB 2-30C brand, a superplasticizer and a hardening retarder was used; unhindered heat exchange of structures with the environment at the air temperature of 28–33 °C was ensured; the strength of concrete in the structure was monitored according to control samples made from samples of the mixture taken during concreting of structures.

Results. The actual values of concrete strength and temperature parameters of arch structures fully correspond to the design requirements and values determined by the calculation of the thermally stressed state of structures, including: the compressive strength of concrete in structures at the design age is 108.1 and 111.3 MPa, corresponds to the actual classes of compressive strength B_f94 and B_f97 and exceeds the requirements of the project; the maximum temperature of concrete in the core of structures was 57–68 °C; the average cooling rate of structures did not exceed 5 °C/day; the temperature difference along the length of the structures was 0.6–0.8 °C/m; the temperature difference between the core and the surface of the steel shell, as well as the surface of the shell and the environment did not exceed 20 °C.

Conclusions. The proposed approaches, taking into account the specifics of calculation, design, technology of construction and quality control of concrete, can be used in the construction of technically complex steelreinforced concrete structures in fixed formwork.

Introduction. In the course of the study, a calculated justification of the strength of a reinforced concrete beam reinforced with composite materials was performed. At the first stage, an elastic finite element model was developed to analyze the stress distribution and calculate the required reinforcement area. At the second stage, a nonlinear deformation model of concrete is included, the cracking process is taken into account. Iterative calculations have shown the achievement of the limiting state of the structure. At the third stage, a model with external composite reinforcement was created. The reinforcement reduced the deflection from 7.36 to 6.47 mm, reduced the stresses in the reinforcement by 17.46 % and increased the bearing capacity by 32 %. Experimental studies are planned, including consideration of dynamic effects and temperature and humidity factors.

Aim. The main aim of this study is to investigate the effectiveness of fiber-reinforced polymer-based composite materials for reinforced concrete structures strengthening. The stress-strain states of reinforced and unreinforced structures were also compared.

Materials and methods. Computational studies of reinforced concrete structures were performed using the finite element method with nonlinear models in the ANSYS software package.

Results. The analysis of the stress-strain state of the reinforced concrete structure with and without external composite reinforcement was carried out, allowing for an assessment of the effectiveness of composite reinforcement for improving the strength and durability of the structure. The study models and analyzes the stress-strain state of such structures when using external reinforcement and evaluates the impact of composite materials on stress and deflection reducing, and load-bearing capacity of the structure increasing.

Conclusions. The study results indicate that using of composite materials for external reinforcement of reinforced concrete structures effectively reduces reinforcement stresses, decreases deflection, and prevents crack formation under composite covering. Maximum tensile stresses in the reinforcement were reduced by 17.46 %, and the load-bearing capacity of the structure increased by 32 % (from 126.75 to 167.31 kN).

Introduction. The replacement of traditional (natural) aggregates with industrial waste in the production of concrete is one of the most important reserves for saving material and energy resources in the construction industry of the Russian Federation. Scrap of concrete, reinforced concrete structures and products, which is formed during the demolition of physically and morally outdated buildings and structures, as well as during their destruction after natural disasters and armed conflicts, is one of the most promising wastes from the point of view of recycling in concrete technology. The use of concrete waste will practically ensure the implementation of the most important principle of waste-free technological processes (in the production of prefabricated and monolithic concrete and reinforced concrete structures) and create conditions for the fulfillment of important social, economic and environmental tasks.

Aim. Selection of compositions and study of the standardized parameters of self-compacting concretes based on recycled (concrete) crushed stone and sand using fine fractions of recycled sand as a mineral additive, as well as comparing their properties with self-compacting concretes based on natural crushed stone and sand of a similar composition.

Materials and methods. For the research, recycled (concrete) and granite (natural) crushed stone according to State Standard 8267-93 were used as coarse aggregates. Recycled (concrete) and natural sand according to State Standard 8736-2014 were used as fine aggregates. Fine fractions of recycled sand were used as a mineral additive. Portland cement according to State Standard 31108-2020 was used as a binder. The superplasticizer Polyplast Target according to State Standard 24211-2008 was used as an additive. The mixing water was used according to State Standard 23732-2011.

Results. The rheological properties, frost resistance and abrasion resistance of fine-grained self-compacting concretes based on recycled aggregates are identical to those of self-compacting concretes based on the traditional aggregates. The strength, density and waterproof grade of fine-grained self-compacting concretes based on recycled aggregates are lower than those of fine-grained self-compacting concretes based on traditional aggregates.

Conclusions. Based on the results of the work carried out, it was revealed that secondary (recycled) crushed stone and sand, as well as fine fractions of secondary (recycled) sand (with a specific surface area of at least 230 m²/kg), are suitable for the production of self-compacting concretes. Self-compacting concretes based on secondary (recycled) crushed stone and sand can be used in the construction industry of the Russian Federation along with self-compacting concretes based on traditional crushed stone and sand.

Introduction. Ensuring reliable operation of hydraulic structures is inextricably linked with ensuring the strength and durability of their structures. Hydraulic structures are characterized by long service life, therefore ensuring durability is the basis for reliable operation of structures. In the modern implementation of a concrete project, there are several methods for increasing the density, strength and moisture resistance of concrete, incl. the use of plasticizing additives.

Aim. The study of the fluidity of cement paste modified with modern chemical plasticizer additives.

Materials and methods. Portland cement of CEM I 42.5N type was used as a binding material. To determine the fluidity indicators of cement paste, superplasticizers of domestic and imported production were used: BASF MasterGlenium ACE 430, BASF MasterGlenium 808 PAV, Sika Sikament BV 3M, JV Osnovit Safescreen SPP1. The methodology included the comprehensive use of a literature review, standardized methods for determining the spread of cement paste (using the Suttard device) and selection of compositions.

Results. It has been established that the use of effective superplasticizers in optimal dosages can significantly increase the fluidity of cement paste. The use of BASF MasterGlenium ACE 430 at a dosage of 1.5 % increases the initial spread by 5.5 times, and after 120 minutes the diameter of the spread is 4.5 times greater than the control composition. Using BASF MasterGlenium 808 PAV at a dosage of 0.9 % increases the initial spread by 2.9 times, reaching a maximum of 236 mm after 60 minutes.

Conclusions. The use of effective superplasticizers in optimal dosages can significantly improve the fluidity of cement paste, that is important for ensuring high-quality concreting of complex hydraulic structures. The results of the study contribute to the development of technologies for increasing the reliability and durability of hydraulic structures.

Introduction. Additive technologies in construction are currently considered to be technologies for emerging future markets, i. e. technological solutions that have not yet been formed but are potentially capable of providing a significant volume of consumption in the future. The aim of this paper is to analyse the 20-year period of science and practice development in the field of additive technologies in construction, to identify the main achievements and unresolved problems, and to outline the prospects for development.

The potential of additive technologies in construction is associated with the possibility of creating bionic design of construction objects, which implies a combination of freeform and organised internal space of object structures, in which the mass of material is located only along the lines of load. This can lead to a radical reduction of material mass in the volume of the structure and change the principles of design and construction.

The practice of application of construction additive technologies in modern construction. On the basis of the analysis of typical implemented projects it is shown that its introduction so far is limited to low-rise housing construction, creation of objects of functional-decorative and special purpose. At present, the robotic process of 3D printing is used only for the construction of the shell of vertical structures of building objects; other structures of these objects are made by the traditional technology of concrete casting, which determines their high cost.

Unresolved problems and factors hindering the introduction of construction additive technologies consist in the lack of design methods, regulatory framework, effective universal technological complexes, sufficient nomenclature of mixtures for printing. Approaches to solving these problems are presented.

Development prospects. The rational directions of introduction of building additive technologies at the present technological level are: construction of small building objects, especially in territories with severe climatic conditions, where there is no developed base of building industry; introduction of building additive technologies in industrial house-building for finishing of building facades and printing of elements of decorative infrastructure of residential complexes.

Principal part. The article considers the recommendations on preliminary preparation and curing of specimens given in the existing domestic and foreign standards of concrete carbonization tests. The conditions of exposure to accelerated carbonization affect the mechanism of processes and the degree of changes that the material will experience. The concept of “maturity index” should be prioritized in future iterations of standards. There is a need to provide direct comparison of the results of different studies and to improve the understanding of how the internal properties of individual concrete types relate to their resistance to carbonation, to define principles for accurately translating carbonation rates in accelerated tests into natural carbonation rates for different types of concrete. Prescriptive and performance-based approaches to durability design of reinforced concrete structures are reviewed in this article. Often a direct correlation between carbonization ratio and compressive strength in concretes with mineral additives is not revealed, especially when the performance characteristics are determined by accelerated tests. Therefore, models of degradation of such concretes under the influence of carbon dioxide in semi-probabilistic, probabilistic calculations, and service life assessment need some refinement.

Conclusions. The existing standards for carbonation depth determination have significant differences from each other, in particular in the variants of specimen preparation, curing conditions, conditions during testing in the carbonation chamber. This leads to different results when tested to different standards. The performance-based approach to assessing the durability and service life of reinforced concrete structures can be considered an important advancement in structural concrete design. At present, the limitations in this approach are due to the fact that the various failure processes affecting the behavior of reinforced concrete structures are not fully studied and described in all necessary details, laboratory test methods do not always reflect the actual operating conditions, and the variation of concrete quality within a structure are determined by the heterogeneity and anisotropy of properties, the presence of defects, time-dependent parameters (shrinkage, creep), and other probabilistic factors.