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Introduction. The co-authors have analyzed a monolithic reinforced concrete piled raft foundation of a multi-storied residential building under construction. The mission of this research effort is to obtain trustworthy information about the internal structure of a foundation slab, to detect and survey internal defects, if any. This research project is to capture potential problems that may accompany the inspection of a foundation, to generate awareness about ground penetrating radar surveys and their methods that can help to optimize operating processes in the process of inspection. Materials and methods. The co-authors have employed a method of ground penetrating radar surveying, performed using a regular mesh of orthogonal projections over an easily accessible surface area of a raft foundation and several antennas producing sounding signals with centre frequencies varying within the range of 1,500 and 2,000 MHz. Results. A number of internal defects has been detected in the structure of a foundation slab, including several horizontal cold joints, cavity pockets and honeycombs. Their presence was later confirmed by control drilling and core material sampling. The information thus obtained was later generalized and entered into surface maps of cold joints, that depicted both the relief and the layout of detected defects in space. The analysis of core material chips has proven that reflecting boundaries are the same as those of the core material destruction; it has also demonstrated the presence of air pockets and the proofs of poor quality concrete mix compaction. Conclusions. The resolution of the ground penetrating radar method is sufficient to identify features of reinforced concrete slabs significant for their structure; it enables researchers to obtain trustworthy information about the internal structure of a foundation slab and make conclusions about the presence or absence of internal defects inside it, including cold joints, cavity pockets or honeycombs.

The article discusses the relevance of developing techniques for confined elements with FRP reinforcement. The method for calculating centrally confined concrete columns with non-metallic GFRP reinforcement (without regard to its compression work) is proposed for the first time in the Russian Federation. The strength model was developed based on the well-known theoretical model of confined concrete. The article considers the effect of strengthening the concrete core of the columns, which is obtained due to the more frequent placement of both transverse and longitudinal reinforcement. The dependence of the bearing capacity of concrete columns on the strength of the transverse reinforcement material is shown. It was proved that with a decrease in the spacing of the longitudinal reinforcement, the area of ​​the effectively confined concrete core inside the reinforcement cage increases.  It was shown that, due to the low compressive modulus of elasticity of the FRP reinforcement, the stress in it will be comparable to the concrete stress. Therefore, the compressive strength of the FRP reinforcement can be neglected in the case of determining the bearing capacity of centrally confined concrete elements. As a result, a strength model for calculating confined concrete elements with FRP reinforcement was proposed, considering the spacing of transverse reinforcement, the longitudinal and transverse reinforcement ratio, and the strength of the material of transverse reinforcement. The developed strength model can be applied not only for square, but also for columns of round and rectangular sections.

The paper contains the results of testing of compressive members reinforced longitudinally and transversally with steel and GFRP reinforcement. The spacing of transverse reinforcement (stirrups) and longitudinal reinforcement ratio varied in the specimens. The stress-strain, stress-Poisson’s ration, stress-volumetric strain relationships for tested specimens are given. It was observed that the appearance of cracks occurred with loading 90% of total. By the results of testing investigated that with the decreasing of spacing of transverse reinforcement the strength of specimens increased. Conclusion is made that by using small spacing of transverse reinforcement transverse strains and Poisson’s ratio decreasing, while modulus of elasticity increasing.