Andrii Klym

The paper presents the results of an experimental investigation into the stress–strain state of an reinforced concrete (RC) slab subjected to two concentrated forces. The relevance of the study is driven by the need to refine the deformation characteristics of thin slabs experiencing large deflections. The aim of the research is to analyse the evolution of strains in concrete and reinforcement at various loading stages up to failure. The digital image correlation method was employed to measure displacements and strains. The tests established that the bending moment corresponding to the attainment of the limiting deflection is Muls=1.18 kN·m; at this stage, strains in the compressed concrete zone remain at a relatively low level. Immediately prior to failure, a redistribution of deflections along the span and localisation of strains in the vicinity of one of the applied forces were recorded. The obtained results confirm the effectiveness of the DIC method for analysing the stress–strain state of RC slabs.

This study investigates the repair effectiveness of reinforced concrete (RC) beams with crushed compression zones and reduced tensile reinforcement. The aim was to evaluate the restoration of load-bearing capacity and flexural performance after repair using polymer-modified mortar. The experimental program included four-point bending tests, high-precision displacement monitoring, and digital image correlation (DIC). Experimental results were compared with predictions obtained from a nonlinear sectional deformation model and with reference beams having reduced reinforcement but no damaged compression zone. The repaired beams reached a reinforcement yield moment of 8.62-8.97 kNm and an ultimate bending moment of 9.5 kNm. The strain in the repaired compression zone remained below the ultimate strain capacity of the repair mortar, indicating stable composite action between the original concrete and the repair material. The repair method increased the reinforcement yield moment by approximately 35%, confirming effective restoration of the compression zone and improved structural performance. The results demonstrate the usefulness of DIC for assessing repaired RC beams and support the application of polymer-modified repair systems for beams with damaged compression zones.

The article presents a comprehensive study of the influence of strain hardening (work hardening effect) on the mechanical properties of thermally strengthened A500C reinforcement in reinforced concrete structures. The use of stepwise testing with the digital image correlation (DIC) method allowed detailed data to be obtained on local stress–strain processes, accumulation of residual plastic deformations, and changes in the yield strength of reinforcement under repeated loading. Experimental results showed that after primary overloading, the reinforcement retains significant residual deformations, which shift the stress–strain curve along the strain axis, while the elastic properties of the steel remain practically unchanged. It was found that the yield strength of the reinforcement under repeated loading increases by 10–20% due to strain hardening and partial stabilization of the dislocation structure of the crystal lattice. The interaction of reinforcement with concrete in the crack zone, including mechanical interlock and adhesion effects, was studied, contributing to local stiffness increase and actual load-bearing capacity of the elements. The proposed methodology allows quantitative assessment of the reinforcement’s residual load-bearing capacity after overloading, partial damage, or restoration, which has practical significance for reconstruction, strengthening, and modernization of reinforced concrete structures. The study results confirm the necessity of considering the work hardening effect and residual deformations when evaluating residual capacity and safety of structures, especially bridges, high-rise buildings, and objects in seismically active areas.

This review analyzes key factors contributing to damage in reinforced concrete (RC) structures, including reinforcement corrosion, chemical attacks, cyclic and long-term mechanical loads, and extreme temperature effects. The study highlights crack formation as a primary damage mechanism, leading to structural degradation. Advanced computational modeling techniques, such as finite element analysis, offer valuable insights into crack propagation and corrosion processes but require further refinement. Future research should focus on developing high-performance materials, improving corrosion protection methods, and refining predictive models. Additionally, sustainable rehabilitation techniques and experimental validation of damage mechanisms are essential for enhancing the durability and serviceability of RC structures.

Restoring and strengthening existing bridges is more economically and environmentally feasible, as cement production in new RC bridges significantly contributes to CO₂ emissions. Additionally, the production of composite carbon materials for strengthening RC structures does not require a large amount of energy, unlike the production of steel for reinforcement, which requires a significant amount of electricity and, accordingly, causes a significant amount of CO₂ emissions. This is why this article presents a comprehensive review of the damage, calculations, and strengthening of RC bridge structures. It examines the main types of damage, including mechanical impacts, material fatigue, corrosion processes, seismic actions, and thermal loads. The mechanisms of their formation, correlations with environmental factors, and operational conditions are detailed. Examples of damage from real engineering objects are provided to assess the scale of the problem. Approaches to the calculation of RC bridge structures are analyzed, particularly methods for modeling the stress–strain state, considering crack formation and material degradation. Key studies by Ukrainian and foreign researchers are highlighted, identifying areas for further methodological improvement. Special attention is given to traditional and modern strengthening methods, including the use of steel elements, composites, and carbon strips. A comparative analysis of the effectiveness of different strengthening approaches is conducted. The conclusion emphasizes the need for further development of existing diagnostic, calculation, and strengthening methods. The integration of innovative materials and technologies is particularly relevant for enhancing the durability of bridges under modern operational loads.

The article presents the results of adapting the digital image correlation method for the possibility of diagnosing reinforced concrete structures. Reinforced concrete (RC) bending elements are the most widely used in construction practice, which determines the importance of reliable estimation of their stress-strain state. The purpose of this study includes reliable theoretical and experimental investigation of the strength and deformability parameters of the RC beam. The experimental study was conducted using digital image correlation and sub-micron contactless gauges. Experimental data was verified with the calculation of the stress-strain state of the RC beam according to DBN V.2.6-98:2009 and Eurocode 2 and the finite-element modelling (FEM). As a result, the values of deflections, concrete and rebar strains were obtained and presented as corresponding diagrams. The results of all the methods are within the same ranges. Also, the form and character of corresponding diagrams are very similar. The indicated deviations were within acceptable limits. It was noted that the theoretical calculation generally provides lower strain values, which is a satisfactory result, as it indicates the bearing capacity reserves provided by the current regulations. The propagation of cracks was monitored during the experiment and the measured cracks opening was compared with theoretical assumptions. Theoretical values are higher than experimental, which shows certain conservativity of valid normative regulations. The experimental and theoretical results were in good correspondence, which confirms their reliability. It was concluded, that the proposed in the study complex theoretic-experimental approach provides essential information about the strength and deformability of the structure.