Introduction. Geopolymer technology, which involves the synthesis of inorganic polymeric materials based on aluminosilicate raw materials and alkaline activators, is considered the most promising alternative to Portland cement.
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. Basalt mineral wool production waste served as the base for the geopolymers. Reinforcement was provided by three types of fiber: steel cord, polypropylene, and basalt wool waste. The developed materials were tested for cone settlement, compressive, flexural, and tensile strength, as well as resistance to chloride penetration. A stress-strain curve was constructed.
Results. Despite a significant reduction in workability (due to the fiber), all developed fiber-geopolymer composites maintain a medium workability rating, allowing for efficient formation using vibration compaction methods. An unrein-forced geopolymer sample achieves a strength exceeding that of traditional B30 cement concrete by 28 days. The addition of fiber has little effect on compressive strength. A comparison of the flexural and tensile strengths of 28-day-old samples cured under ambient conditions reveals a similar trend to those of samples tested in compression, demonstrating that polypropylene and metal cord fibers are effective both individually and in combination. Analysis of the stress-strain curves demonstrates that hybridization of fibers of different natures and elastic moduli is the most effective tool for transforming brittle geopolymer rock from man-made raw materials into a highly ductile and energy-absorbing structural material. The diffusion coefficient of chloride ions in the geopolymer. Despite the fact that the diffusion coefficient D_Cl– for the developed geopolymer samples in some cases exceeds the coefficient for traditional cement concrete, the obtained values characterize high durability characteristics.
Conclusions. Thus, the developed fiber-geopolymer compositions based on man-made raw materials, while maintaining average workability sufficient for effective vibration compaction, surpass traditional B30 class cement concrete in compressive strength at 28 days and demonstrate a significant increase in bending and tensile strength due to both mono-reinforcement and the synergistic effect of hybrid reinforcement with multi-modular fibers. This, combined with the transformation of brittle fracture into ductile fracture and high durability indicators, confirms their potential as a highly effective structural material.

Introduction. It is well known that the use of innovative types of reinforcement and advanced design methods can significantly reduce steel consumption in reinforced concrete structures. However, reinforcement consumption is also heavily influenced by the results of structural design calculations. As a rule, all reinforced concrete structure calculations are performed using computer software. The requirements of [1] aim to determine the minimum reinforcement consumption that ensures reliable structural performance. In practice, however, software often fails to comply with these code provisions. The resulting reinforcement amount may be either less or more than required. One of the main reasons for this situation is the ambiguous interpretation of codes by software developers. There are also instances of direct violations of code requirements. As a result, with identical input data, the reinforcement calculated by different software packages can differ by several times.
Aim. To verify the accuracy of reinforced concrete structure calculations performed in various software packages.
Materials and methods. Three test cases were examined: an experimentally verified floor slab, a foundation slab, and a slab from the “Lira-SAPR User Manual”. Reinforcement was determined using Lira-SAPR, Arbat, Lira 10, INZH-RU, Statika software packages and compared with results obtained using the “OM SNiP Zhelezobeton” computer software, which utilizes a non-linear deformation model as well as the ultimate forces method. A characteristic failure case was also analyzed.
Results. The reinforcement consumption obtained from different software packages using identical input data differed significantly, in some cases by several times.
Conclusions. To eliminate significant unjustified overconsumption of reinforcement, as confirmed by the provided examples, and to improve structural reliability, it is advisable to develop a document containing standard benchmark examples for reinforced concrete structure calculations. Such reference examples will help achieve consistency in results across different software packages when using identical input data. They will serve as a guideline for all software developers, structural engineers, and regulatory authorities.

Introduction. Foam concrete is a complex highly porous system, the structuring process of which (sol-gel transition) determines its physical, mechanical, and operational characteristics. Fractal percolation theory is a modern tool for describing and modeling the formation of the fractal pore structure and the cement matrix.
Aim. To analyze the phase transition of the foam concrete matrix from a colloidal solution to a solid body from the perspective of fractal percolation theory using computer aggregation models (RLA, DLA, DLCA), as well as to experimentally study the effect of surfactants on the setting time.
Materials and methods. The experimental part used Portland cement CEM I 42.5N, technical water, and a synthetic silicone foaming agent “Penta Pav 430A”. Cement paste (W/C=0.24) was prepared in a control mix and with a surfactant additive. Setting times were determined according to GOST 30744–2001 using a Vicat device. The theoretical part was based on the analysis of diffusion-limited (DLA) and reaction-limited (RLA, DLCA) aggregation models.
Results. It was established that the introduction of a synthetic foaming agent in the range of 0.3–3 % increases the setting time of the cement paste by an average of 30 minutes. It is shown that the percolation transition (onset of setting) occurs when the concentration of chemically bound particles reaches 22 to 30 %. The presence of surfactants shifts the onset of setting, but the critical ratio of free to bound particles remains practically unchanged.
Conclusions. It is concluded that to prevent sedimentation and form a strong framework, it is necessary to control the setting kinetics through the water-solid ratio and the introduction of chemical additives.  

Introduction. Lightweight high-strength concrete is considered a promising material for transport and infrastructure structures; however, data regarding its resistance to repeated loading remain limited.
Aim. To experimentally evaluate the fatigue strength of lightweight high-strength concrete and compare it with that of two normal-weight concrete mixes, as well as with the standard loading levels previously established in the SP 35.13330.2011 code of practice.
Materials and methods. The study investigated lightweight high-strength concrete, normal-weight high-strength concrete, and normal-weight concrete of medium strength. Cyclic tests were conducted on 100 × 100 × 400 mm prisms at a frequency of 5 Hz. Testing followed the provisions of State Standard 24545–2021, involving a stepwise reduction of the relative maximum stress level (η). The minimum load was set at 10 tf, causing the cycle asymmetry coefficient to vary depending on the loading level. A test duration of 2 × 10⁶ cycles was adopted as the baseline.
Conclusions. Experimental results demonstrate that, under the applied cyclic loading regimes, the lightweight high-strength concrete exhibited superior fatigue strength compared to the normal-weight concretes tested. The lightweight high-strength concrete reached the 2 × 10⁶-cycle baseline at a relative loading level of η = 0.70, whereas the corresponding values for normal-weight high-strength concrete and normal-weight concrete of medium strength were 0.50 and 0.40, respectively. These findings confirm the potential of lightweight high-strength concrete for use in transport structures subject to repeated loading and highlight the necessity of considering the concrete type and its strength characteristics when assessing fatigue strength.

Introduction. The specifics of reinforced concrete manufacturing technology mean that the physical and mechanical properties of concrete are often distributed unevenly within the structure. Furthermore, the placement of the reinforcement cage within the structure depends on its specific function as a load-bearing element and often varies across different zones. Accordingly, individual zones of reinforced concrete structures may be reinforced with different reinforcement ratios, with more or less dense arrangements of the working and structural reinforcement. In densely reinforced zones of structures, the risk of developing defects in the internal structure of the concrete (areas of insufficiently compacted concrete, cavities, caverns, etc.) increases. All this leads to an uneven distribution of concrete properties throughout the product, which manifests itself in an uneven distribution of ultrasonic signal velocities and amplitudes.
Obviously, when inspecting such reinforced concrete structures, the problem of uneven ultrasonic wave velocity distribution can lead to errors in determining defect coordinates and product thickness if the ultrasonic velocity value measured at a single point on the reinforced concrete structure is used when measuring at different points on the same product.
Aim. To improve the accuracy of determining reinforcement parameters and the position of reinforcement cage elements in reinforced concrete structures using ultrasonic pulse echo. Development of a measurement algorithm and data processing method for determining the thickness of the concrete cover of steel reinforcement, followed by mathematical processing (creation of a calibration curve).
Materials and methods. The article uses the method of experimental research of full-scale structures. Theoretical research is conducted using algorithms and methods of mathematical statistics. Quantitative assessments, configuration, and testing of measurement systems and algorithms are conducted using modern computational automation tools.
Results. The author proposes a method for improving the accuracy of determining reinforcement parameters and the position of reinforcement cage elements in reinforced concrete structures using ultrasonic pulse echo. A measurement algorithm and data processing method are proposed for determining the thickness of the concrete cover of steel reinforcement, followed by mathematical processing (construction of a calibration curve). The reliability of the obtained results and conclusions is confirmed by experimental verification of the research materials.
Conclusions. The use of the proposed data processing method for determining the thickness of the concrete protective layer of steel reinforcement using the ultrasonic echo method, followed by mathematical processing (construction of a calibration dependence) for reinforced concrete structures with high heterogeneity in the distribution of concrete properties in the structure’s massif, allows for increasing the accuracy of determining the reinforcement parameters and the position of the reinforcement cage elements.

Introduction. The development of new construction technologies has given rise to new types of prefabricated building elements–large, factory-prepared reinforced concrete modules. One of the key distinguishing features of these modules is the use of thin-walled elements– wall panels–with slinging zones containing specially designed and tested embedded components that absorb and transfer tensile forces to the module during lifting and handling during fabrication and installation. A key objective is to ensure the safety and integrity of these modules, weighing up to 80 tons or more, during the precommissioning phase.
Aim. Determination of the stress state in the design of modules during their lifting and movement during manufacturing and installation, and the formation of a set of design.
Materials and methods. The distribution of forces was determined by spatial calculation of the module in the LIRA-SAPPHIRE software package.
Results. During fabrication and installation, tensile stresses predominate in the longitudinal wall panels. Stress isofield plots for the module's most heavily loaded wall panel, derived from a spatial structural analysis, reveal a concentration of tensile stresses in the lifting zones through which the load is transferred to the module. Compressive stresses predominate during the operational stage. The placement of additional longitudinal and transverse reinforcement in slinging zones, as well as in areas of weakened sections near openings and thin-walled sections of the wall panel, taking into account the resulting complex stress state, ensured the strength and deformation resistance of the module structure under the forces generated during lifting and handling during manufacturing and installation.
Conclusions. The locations of stress concentration zones differ between the pre-commissioning and operational stages of module fabrication, a factor that is important to consider during their design. Taking into account the identified stress state of the modules made it possible to ensure the absence of defects that could have a significant impact on the operational reliability of the modules of the implementation objects.

Introduction. Industrial buildings with a combined reinforced concrete and steel frame, designed and built during the Soviet period, were generally not checked for resistance to progressive collapse, since the relevant requirements were not part of the mandatory calculations of that time. When such facilities are reconstructed and their service life extended, current codes require verification calculations that take accidental actions into account, and part of the reserves that were previously sufficient may prove to be exhausted owing to increased design loads and additional reduction factors.
Aim. Development and verification of a method for assessing the resistance of the load-bearing frame of an industrial building to progressive collapse, using the main building of a heat power plant as an example, and selection of efficient strengthening solutions for overloaded elements.
Materials and methods. A spatial bar finite-element model of the building was developed in the LIRA-SAPR software package with detailed modelling of the joints between reinforced concrete and steel elements. The progressive collapse analysis was performed using the alternate-load-path method with successive removal of elements that exceed their bearing capacity, up to system stabilisation or the development of inadmissible deformations.
Results. The roof trusses of the boiler unit were found to be resistant to the local-failure scenarios considered, whereas the machine-hall trusses of the first and second construction stages proved unstable, with a bearing-capacity deficit of up to two times. Targeted strengthening solutions for the steel trusses were developed, including additional lattice members and longitudinal intercepting trusses, and their efficiency was confirmed by a repeat analysis.
Conclusions. The proposed method makes it possible to identify vulnerable zones of the frames of industrial buildings with a combined structural scheme and to justify minimally sufficient strengthening measures without stopping the building’s operation or completely dismantling the structures.