Introduction. Regulatory documents establish two main characteristics of concrete strength: axial compressive strength and axial tensile strength, determined using standard-sized specimens. Concrete strength may be determined using specimens with geometric dimensions that differ, either up or down, from those of standard specimens. In these cases, the concrete strength is adjusted using appropriate scaling factors to bring it into line with the strength of standard-sized specimens.

The scaling factors for converting to the concrete strength of standard samples are also established by the relevant standards. In some cases, experimental determination of concrete strength is a complex task that remains unresolved to this day.

This primarily concerns the strength of concrete in the contact layer of the ribbed reinforcement anchorage zone. Moreover, shear strength parameters for concrete, even for standard specimens, are absent from regulatory documents.

Aim. To determine the scaling factors for calculating the shear and compressive strength of concrete in the contact layer, depending on the strength parameters of concrete in standard specimens.

Materials and methods. Based on an analysis of regulatory documents and open sources, scale factor values were theoretically obtained for determining the actual shear and compressive strength of concrete in the contact layer, taking into account the provisions of the OTS, depending on the concrete strength of standard samples.

Results. Statistical processing of the results allowed us to obtain mathematical relationships between the magnitude of the scaling factors for shear and compression and the height of the transverse ribs of the periodic reinforcement profile.

Conclusions. The scale factor values allow us to determine the ultimate strength of concrete in the contact layer of the reinforcement anchorage zone.

Introduction. The shortage and high cost of granite crushed stone stimulate the search for more affordable aggregates for heavy concrete. This paper presents the results of a laboratory study aimed at comparing the properties of heavy concrete prepared using two types of coarse aggregate granite crushed stone and natural gravel of 5–20 mm fraction.

Aim. Substantiation of the technical and economic feasibility of replacing granite crushed stone with natural gravel in heavy concrete, provided that the strength characteristics are not lower than the required ones. Materials and methods. The main attention is paid to the influence of the shape and surface texture of aggregate particles on the water demand, strength, and cost of the concrete mixture under the same workability (slump class S5).

Results. It was found that the use of gravel instead of crushed stone reduces water demand [1], which leads to a decrease in the water-cement ratio and, consequently, to an increase in strength. In the case considered in this article, concrete with gravel achieved class B20, while its counterpart with crushed stone achieved only class B15. An economic calculation showed a cost reduction of 7.8 % when replacing crushed stone with gravel. 

Conclusions. The experimental results indicate that, with proper mix design, gravel can be not only a cost-effective but also a technically preferable alternative to crushed stone in the production of heavy concrete.

Introduction. Ash dumps put public health at risk, and enterprises forced to store ash and slag waste incur significant financial losses due to the negative impact on the environment, in turn, bottom ash, after some preparation, can serve as the basis for the production of competitive cement-free construction products, which is a scientific task.

Aim. Development of a scientifically based technological solution for the production of geopolymer building materials based on hydro-removed ash and slag waste from thermal power plants.

Materials and methods. The basis for geopolymers were the combined heat and power plants of Vladivostok TPP-2, Primorskaya GRES and Artemovskaya TPP of Primorsky Krai. Modern methods were used for a comprehensive study of raw materials and developed materials: X-ray phase analysis, energy dispersion microanalysis, scanning electron microscopy, differential thermal analysis, and infrared spectroscopy.

Results. The technological solution for the preliminary preparation of ash and slag waste has been improved, on the basis of which an innovative geopolymer binder has been developed. A wide range of geopolymer concrete compositions has been developed through various combinations of ash and slag waste disposal products. Unlike the hydration products of traditional Portland cement concretes, which are represented by the main structure-forming neoplasms of calcium silicate hydrates, the microstructure in the developed geopolymers is composed of calcium and sodium hydroaluminosilicates. In this case, aluminum atoms embedded in the crystal lattice of hydrated neoplasms significantly strengthen it. 

Conclusions. The effectiveness of using ash and slag waste from thermal power plants as a source for producing new environmentally friendly geopolymer concretes of classes up to B60 for compression and Btb10.0 for bending tension has been scientifically substantiated and experimentally proven. The compressive and flexural strength of the developed materials suggests their competitiveness in comparison with traditional Portland cement concretes. The compressive strength of the developed geopolymers is 2 times higher than the same characteristic for materials based on common Portland cement CEM I 32.5N and 1.5 times higher than for high-strength CEM I 42.5N.

Introduction. The accumulation of non-degradable plastic waste, in particular polyethylene terephthalate (PET), is an acute environmental problem. Finding ways to recycle PET in building materials can reduce the load on landfills and conserve natural resources.

Aim. Experimental evaluation of the effect of partial replacement of fine aggregate with crushed PET plastic on the physico-mechanical properties of fine-grained concrete.

Materials and methods. Four concrete compositions were studied (cement: sand = 1:3, W/C=0.55): control (without additives) and compositions with the replacement of sand by volume by 5 %, 10 % and 15 % with crushed PET fractions of 2–5 mm. The 70 x 70 mm cube samples were tested for compressive strength, average density and water absorption were determined at the age of 28 days.

Results. It was found that the introduction of plastic leads to a decrease in average density (up to 8 % with 15 % PET) and compressive strength. The highest strength of the modified formulations was recorded at a dosage of 5 % PET (a decrease of 12.5 % relative to the control). A further increase in the plastic content to 15 % causes a sharp drop in strength (by 31 %) and a 2.1-fold increase in water absorption.

Conclusions. The optimal dosage, which provides acceptable characteristics for a number of building solutions, is the replacement of 5–10 % of sand by volume. The resulting material can be recommended for curtain structures, paving slabs and landscaping elements.

Introduction. The article discusses the process of research and implementing fittings with an innovative periodic profile in the practice of modern construction in the Russian Federation.

Aim. Description of constructive solutions, the process of research in laboratory and production conditions of new types of multi-row profiles, and an assessment of the prospects for their mass application in construction.

Materials and methods. Materials on the development, research, production and practical application in the construction of new types of reinforcement with a multi-row periodic profile are presented. The research methods and equipment described in the article are used at the NIIZHB n.a. A.A. Gvozdev and at VNIIzhelezobeton.

Results. According to the research results, the positive qualities of innovative types of fittings with a periodic profile are confirmed, as well as their positive impact on the production and consumption of rebar products in the Russian Federation and abroad.

Conclusions. In Russian Federation, innovative types of periodic profiles of the surface of rods with a multi-row arrangement of transverse ribs have been developed for the production of rebar products. This makes it possible to significantly improve the production and consumer properties of domestic reinforcement for reinforced concrete structures and ensure its high competitiveness in world markets.

The innovativeness of the new fittings is confirmed by domestic and foreign patents for inventions.

New rebar rolled products with profiles for common use of classes A500SP, Au500SP, as well as rolled products with screw profiles of classes Ab500P and higher, have been mastered by the metallurgical industry and used in domestic and foreign (Kazakhstan, Kyrgyzstan, Mongolia) construction in the amount of about 5.0 million tons.

The characteristics of reinforcement with an innovative multirow profile can significantly increase the crack resistance of reinforced concrete structures, as well as the safety of construction sites under emergency dynamic loads (seismic, explosions, impacts, etc.). In low-rise and other types of construction, maintaining the adhesion of new reinforcement to concrete during its plastic deformation in overlapping nonwelded joints and anchoring sections of reinforced concrete elements contributes to the redistribution of forces in building structures, and, consequently, reduces the likelihood of their progressive collapse with catastrophic consequences. The mass introduction of new types of fittings in construction will ensure a multibillion-dollar economic effect in its production and application.

Introduction. Today, the number of publications on concrete with an admixture of graphene and graphene oxide has exceeded 3000. However, there is a drawback to these works (in large quantities), which is that when producing concrete, there is no control over the number of graphene monolayers, and for natural graphite the number of monolayers is three. When the number of monolayers exceeds three, it turns into graphite, the properties of which are different from the unique monatomic graphene. Therefore, there is a large difference in the properties of concrete in published works.

Aim. To develop a technology for producing graphene concrete with a controlled number of graphene monolayers and to investigate the effect of graphene and graphene oxide on concrete wear resistance.

Materials and methods. Standard concrete was prepared as follows: an aqueous solution with graphene oxide (GO) and reduced graphene oxide (rGO) was placed in a bath, where an ultrasonic emitter created an ultrasonic field and the graphite and graphite oxide were separated into monolayers of graphene and graphene oxide; the frequency of the ultrasonic field was selected from the “latent” energy of deformation of the samples; graphite delamination was monitored using UV and Raman spectra; M400 cement; crushed stone with grain sizes of 5–20 mm; sand grade M2.5–2.9; VORONEZHFortis mixer; vibrating table; concrete volume 10 L. The resulting concrete samples were tested on a PGM-2000MG4 press. Results.

In the period from 1 to 7 days, the compressive strength of the samples relative to the control samples increased from 13 to 63 %, and up to 14 days from 16 to 62 %. Regarding the effect of the graphene suspension, the following conclusion can be made: stratification of graphite and graphite oxide into monolayers of graphene and graphene oxide occurs in an ultrasonic field of a certain frequency; the large specific surface area of graphene and graphene oxide serve as a source of crystallization and hardening of concrete.

The above effects are especially pronounced at concentrations of graphene or graphene oxide in the range of 0.05 0.07 % of the concrete mixture. At higher concentrations, coagulation of graphene monolayers and their transformation into graphite occurs.

To obtain highly wear-resistant graphene concretes in production, it is necessary to adhere to the regulations for obtaining a suspension in an ultrasonic field, as described above.

Conclusions. The article proposes a model of cracks in graphene concrete based on the theory of catastrophes, from which it follows that both graphene and graphene oxide enter the interlayer region, destroying the structure of the C–S–H gel due to a sharp decrease in the molar mass of the compound. This is a brake on the movement of nanocracks that occur in any solid when its surface is formed. It has been shown that modifing concrete with graphene oxides increases its wear recistence by 4-5 times compared to standard concrete.

Introduction. The design of civil buildings in modern conditions is accompanied by an increase in technical risks due to the complexity of design solutions, regulatory uncertainty and limited introduction of digital technologies. An urgent task is to increase the reliability and safety of design solutions, taking into account the systemic problems of the industry associated with insufficient integration of risk analysis and a shortage of qualified personnel.

Aim. To identify areas for improving the methodology of designing civil buildings in the context of technical risks, including the development of risk-oriented approaches, the introduction of BIM technologies and improving the quality of the regulatory and technical framework.

Materials and methods. The paper uses methods of systematic, structural and comparative analysis, as well as a synthesis of scientific publications, regulatory documents and practical materials reflecting current trends in engineering and construction design.

Results. The key factors influencing the occurrence of technical risks in the design of civil buildings are identified, as well as the main limitations of existing practice are identified: the fragmentation of the regulatory framework, insufficient integration of risk-oriented techniques and limited use of digital modeling. Promising directions for the development of design activities are formulated, including the introduction of digital building doubles, the expansion of the use of information modeling and the development of competencies of engineering and technical specialists.

Conclusions. The necessity of forming an integrated riskbased design system that ensures the reliability, safety and economic efficiency of civil buildings at all stages of their life cycle is substantiated. The research results can be used in the development of project documentation, improvement of regulations and the introduction of innovative technologies in construction practice.

Introduction. The increase in the number of accidents and emergencies noted in the state reports of the Russian Ministry of Emergency Situations in recent years highlights the need to study the operation of building structures under impacts with varying rates of load application. One of the most interesting is the medium-speed loading range, where the stress-strain state of reinforced concrete elements differs significantly from static and high-speed modes.

Aim. Investigation of the influence of medium-velocity impacts on the strength and crack resistance of bendable reinforced concrete elements using an experimental and analytical approach.

Materials and methods. The study used test data from 27 concrete prisms of classes B25 and B45, destroyed at three speed modes: static, accelerated (1–3 s) and moderately accelerated (8–12 s). The results were processed using the three sigma criterion and the 5 % quantile to form statistically representative diagrams «stress (σ) relative strain (ε)». Analytical modeling was carried out using fractional-rational dependencies and the energy criterion of destruction.

Results. It was found that an increase in the loading rate leads to an increase in the ultimate strength of concrete (by 9–20 % for B25 and 6–12 % for B45) and a decrease in the corresponding ultimate deformations (by 10–19 % and 5–13 %, respectively). For bent elements, an increase in the moment corresponding to the exhaustion of the element strength was found by 1–2.3 % (for concrete B25) and 0.9–1.4 % (for concrete B45), as well as an increase in the moment of cracking by 6.6–13.8 % and 4.7–7.1 %, respectively.

Conclusions. Medium-speed loading leads to an increase in the strength and crack resistance of bendable reinforced concrete elements while reducing their deformability. The results obtained refine the parameters for calculations based on the nonlinear deformation model and can be used in the analysis of special effects.

Introduction. The use of steel fibre reinforced concrete (SFRC) in flat slabs without capitals remains limited despite its potential to improve crack resistance and reduce reinforcement complexity. This study investigates a slab with combined reinforcement, integrating steel fibres with localized bar reinforcement.

Aim. To experimentally verify the structural performance and load-bearing capacity of a column-supported SFRC flat slab with combined reinforcement through full-scale testing.

Materials and Methods. Full-scale tests were carried out on a slab of a local wastewater treatment facility at the age of 30 days. The concrete class was B30, and Hendix® Prime 75/52 steel fibres were used at a dosage of 70 kg/m[3] . The load was applied stepwise at the slab centre over an area of 1.5 × 3.0 m. The maximum load reached 15.5 tonnes-force (3.45 tonnes-force/m[2] ). A finite element model and elastic analysis at the ultimate limit state were used to ensure equivalence between test and design loading conditions.

Results. The slab exhibited elastic behaviour up to approximately 65 % of the maximum load. Further loading resulted in stiffness reduction and nonlinear deflection growth, reaching 2.10 mm at peak load. No visible cracking was observed. Residual deformation after unloading was 0.14 mm.

Conclusions . The results confirm the feasibility of using SFRC with combined reinforcement in flat slabs. The proposed solution ensures adequate load-bearing capacity and crack resistance while reducing conventional reinforcement, indicating strong potential for practical implementation.

Introduction . The article analyses the stress-strain states of reinforced concrete elements when calculating the strength of normal sections using a nonlinear deformation model NDM, proposes a method for describing axial deformations and forced in the form of function parameters, introduces criteria for grouping deformed states based on emerging forces. An analytical method is proposed for determining the limiting forces in the normal section of a reinforced concrete element when calculating strength according to NDM.

Aim. Development of a methodology for finding the limiting forces at which there is no significant degradation of the rigidity of the normal section in a reinforced concrete element when calculating using the NDM, which does not require the use of numerical modeling tools.

Results. The results of the research showed that recording the deformed states of the section in the form of parameters of the axial deformation equation makes it possible to describe the mechanisms of NDM in the form of a system of nonlinear equations, which makes it possible to express direct analytical dependencies for finding deformations from forces. A group of stressstrain states of normal sections has been identified, which can be characterized, from the point of view of forces, as close to the maximum possible (about 90…100 % in bending moment), and from the point of view of rigidity as a boundary, after crossing which a sharp degradation of the rigidity characteristics of the section is observed. Analytical dependencies describing the identified group made it possible to abandon the use of numerical methods to determine limiting forces according to NDM.

Conclusions. A method for determining the limiting forces in the normal section of reinforced concrete elements when calculating according to the NDM has been developed, which does not require the use of numerical models. The essence of the method lies in the established mutual dependencies between the parameters of equation of the deformed state of the section, at which forces close to maximum possible are observed in it, and there is no sharp degradation of the rigidity characteristics of the normal sections.

Introduction. The subject of the study is the methods of embedded monitoring of the moisture and deformation state of envelope structures in timber frame houses and the procedures for validating their digital twins using sensor data.

The aim of the work is to develop a methodology for integrating embedded sensors of temperature, relative humidity, wood moisture content and deformations with a digital twin of a multilayer timber frame envelope structure in order to improve the reliability of its technical condition assessment.

Materials and methods. The research methods include analysis of scientific publications and regulatory sources, including SP 31–105–2002 [13], development of the digital twin structure, design of the sensor placement scheme in critical zones of the structure, planning of laboratory and field monitoring programs, and development of criteria for comparing calculated and measured parameters.

Results. As a result, a comprehensive methodology for monitoring the condition of timber frame envelope structures has been developed, the architecture of the digital twin has been proposed, the most informative zones for sensor placement have been identified, and validation criteria based on time series of controlled parameters have been developed. It has been established that the use of embedded monitoring improves the accuracy of the digital twin through calibration of the parameters of the hygrothermal and mechanical subsystems, while stable discrepancies between calculated and measured values can be considered as a diagnostic sign of a hidden defect. 

Conclusions. The practical significance of the work is associated with early detection of hidden moisture accumulation, prediction of deformation behavior and improved reliability of timber frame buildings designed in accordance with the current regulatory framework.