Experimental and analytical models of longitudinal deformation in pipe-concrete specimens with small cross-sections

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1. Introduction One of interesting engineering solutions is the use of steel-concrete structures in construction, which, unlike classic reinforced concrete, use steel not only as a reinforcing material, but also as a full-fledged load-bearing element. One of the types of the steel-and-reinforced concrete structures is piped concrete, consisting of a closed steel pipe into which concrete mixture is specially placed and compacted, resulting in a complex jointed composite structure combining the main advantages of classical steel and reinforced concrete elements, leveling and significantly reducing the impact of their disadvantages. Many scientists and research groups of the world community have been engaged in the study of strength and stability of pipe-concrete elements. Despite the existence of calculation methods reflected in domestic and foreign regulatory documents, all of them do not allow to objectively describe the stress-strain state of pipe-concrete structures under the action of axial compressive load. Various scientific communities in the last decade conducted experimental [1-8], analytical [9-12] and numerical studies [14-16] in order to determine the bearing capacity and deformability of composite structures. Special attention has been paid to numerical and analytical calculations in a nonlinear formulation [17; 18], however, there is no unified engineering methodology capable of describing the stress-strain state of pipe-concrete rods. Depending on the approach for evaluation of the load-bearing capacity of a pipe-concrete section, two interrelated statements of the problem are conditionally accepted: either the influence of concrete on the increase of the load-bearing capacity of the pipe [15], or the reverse variant is considered, i.e. taking the pipe as a steel shell of a concrete rod [6; 7; 9; 19]. Obviously, pipe-concrete is a composite material with mutual influences of concrete core and a steel shell on each other, and both of the above approaches can be considered as approximated models of work. Dimensional ratios of the steel tube-shell (diameter-to-wall thickness ratio - D∕t [2; 12; 20]) or the type of concrete infilling [3] are usually considered as the main factors affecting the strength and strain characteristics of a pipe-concrete structure. The problem of determining the actual stress-strain state of composite structures made of pipes filled with concrete is being raised by many scientific teams, since the existing design standards underestimate the loadbearing capacity of pipe-concrete, defining it as the sum of the load-bearing capacities of the pipe and the concrete core. For example, articles [9; 20] analyze and compare the existing approaches to determine the ultimate compressive load on pipe-concrete columns. The authors of articles [13; 19] analytically consider the effect of casing by introducing an additional summand, which represents the side pressure at the interface between the pipe and concrete. However, experimental studies show that the nature of deformation of the pipe-concrete rod is more similar to the deformation of a hollow steel pipe, and filling with concrete only enhances the performance of the structure. In this regard, the authors of this paper propose a methodology that allows describing the deformation process of a pipe-concrete specimen on the basis of the deformational characteristics of a steel pipe and considering the contribution of concrete to the structure operation, which is constant at all stages of deformation, by using of a correcting coefficient. 2. Materials and methods For conducting the experiment, 12 specimens of 100 mm length were made using steel pipes with the following cross sections: a pipe with a diameter of 60 mm and a wall thickness of 2 mm, a pipe with a diameter of 76 mm and a wall thickness of 3 mm, and a pipe with a diameter of 102 mm and a wall thickness of 3.5 mm. The dimensions of the experimental specimens were taken to exclude the influence of flexibility on the load-bearing capacity of short pipe-concrete rods, i.e. to exclude the loss of stability. Two pipe-concrete specimens and two hollow specimens, i.e. not filled with concrete, were made from each pipe diameter. Additional reinforcement of the specimens was not used. Conditional marking of the specimens is given in Table 1 for the convenience of processing the results. Each specimen was tested in the laboratory of the Department of Building Structures of Nizhny Novgorod State University of Architecture and Civil Engineering using a P-125 compression machine with maximum compressive load equal to 1250 kN. In this study, the longitudinal deformations of the samples at each stage of loading with an axial compressive load were determined by the convergence of the pipe-concrete cylinders end sections, for the registration of which the plate convergence indicator had been installed. Figure 1 shows the basic scheme of the experimental equipment for testing specimens of 100 mm in length. Table 1 Marking of specimens Specimens mark Specimen characteristic Steel pipe dimensions, mm Diameter of concrete rod crimped by the pipe, mm P1.1 Hollow Steel Pipe 60×2 - P1.2 PC1.1 Pipe filled with concrete (pipe-concrete) 60×2 56 PC1.2 P2.1 Hollow Steel Pipe 76×3 - P2.2 PC2.1 Pipe filled with concrete (pipe-concrete) 76×3 70 PC2.2 P3.1 Hollow Steel Pipe 102×3.5 - P3.2 PC3.1 Pipe filled with concrete (pip-concrete) 102×3.5 95 PC3.2 a b Figure 1. Testing of specimens with 100 mm length: a - general view; b - basic scheme of the experimental setup: 1 - specimen under test; 2 - movable loading plate; 3 - fixed loading plate; 4 - indicator for registration of plates convergence 3. Results of the research On the basis of the experimental results for each specimen, the diagrams of longitudinal deformation were generated in variables P - ∆, where P is the axial compressive load, ∆ is the convergence between the pressing plates. For visual clarity and further analysis, the diagrams of pipe-concrete specimens (hereinafter referred to as

About the authors

Pavel A. Khazov

Nizhny Novgorod State University of Architecture and Civil Engineering

Author for correspondence.
Email: khazov.nngasu@mail.ru
ORCID iD: 0000-0003-1220-6930

Candidate of Technical Sciences, Associate Professor of the Department of Theory of Structures and Technical Mechanics, Head of the Laboratory for Continuous Monitoring of the Technical Condition of Buildings and Structures

Nizhny Novgorod, Russian Federation

Vladimir I. Erofeev

Institute of Mechanical Engineering Problems of the Russian Academy of Sciences - Branch Federal Research Center named after Gaponov-Grekhov of the RAS

Email: erof.vi@yandex.ru
ORCID iD: 0000-0002-6637-5564

Doctor of Physics and Mathematics Sciences, Professor, Director of Institute of Mechanical Engineering Problems

Nizhny Novgorod, Russian Federation

Elena A. Nikitina

Institute of Mechanical Engineering Problems of the Russian Academy of Sciences - Branch Federal Research Center named after Gaponov-Grekhov of the RAS

Email: nikitina.ea.nn@gmail.com
ORCID iD: 0009-0000-1189-1062

Candidate of Technical Sciences, Associate Professor, Senior Researcher of Institute of Mechanical Engineering Problems

Nizhny Novgorod, Russian Federation

Artyom P. Pomazov

Nizhny Novgorod State University of Architecture and Civil Engineering

Email: pomazov.a.p@yandex.ru
ORCID iD: 0009-0009-5465-3692

Postgraduate Student in Department of Structural Theory and Technical Mechanics, Assistant Professor in the Department of Building Structures

Nizhny Novgorod, Russian Federation

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