Axial Load-Axial Deformation Behaviour of SCC Columns Reinforced with Steel Tubes
Introduction
Steel sections and concrete are commonly used in the construction of composite columns. There are two main configurations of the composite columns: concrete-encased steel section columns and concrete-filled steel tube (CFT) columns [1]. The advantages of composite columns are high strength, stiffness, ductility, fire resistance and seismic resistance [2, 3]. Steel tubes with different cross-sections (rectangular, square, polygon and circular) are used to construct CFT columns [4, 5]. Circular steel tube sections are usually preferred for the CFT columns because circular steel tubes provide better confinement to the infill concrete [6]. In traditional CFT columns, steel tubes are usually filled with concrete without any internal steel reinforcement [7]. In some cases, internal steel reinforcement is used for higher strengths and better connections between the concrete members [8].
Reinforced concrete (RC) columns are usually constructed of normal-vibrated concrete (NVC). However, the congestion of reinforcement in the construction of columns is a critical issue. Casting concrete in columns with a large amount of longitudinal and transverse reinforcements makes the placement of concrete difficult. For such columns, the self-compacting concrete (SCC) is considered a suitable option to overcome the difficulty of the placement of concrete because SCC possesses good workability with high flowability, passing ability and segregation resistances [9]. The SCC can be easily poured into narrow, complex or novel forms of construction without requiring vibration even in columns containing a large amount of reinforcement [10, 11].
Lin et al. [12] examined the behaviour of axially loaded RC columns constructed of NVC and SCC. Test results showed that the ductility, stiffness and crack control ability of the SCC columns were better than NVC columns. Lachemi et al. [13] examined the performance of axially loaded CFT columns constructed of NVC and SCC. Two series of steel tube confined concrete columns with and without longitudinal and transverse reinforcement were tested. The test results showed that axial load carrying capacities of NVC columns and SCC columns were comparable. Also, the casting of columns with SCC was easier than casting of columns with NVC, as SCC did not require any vibration.
Recently, the authors proposed a new method of reinforcing SCC columns with small diameter steel tubes as longitudinal reinforcement [14]. The behaviour of SCC columns reinforced with steel tubes is different from the behaviour of SCC columns reinforced with conventional steel bars. For the same cross-sectional area, the radius of gyration of the steel tube is higher than the radius of gyration of the solid steel bar. Steel tubes filled with SCC decreased the overall buckling of longitudinal reinforcement and consequently increased the ductility of the SCC columns. Also, steel tubes effectively confined the infill concrete resulting in an increase of the axial compressive strength. Under concentric axial load, steel tubes with a tensile strength similar to that of steel bars used in reinforcing SCC columns increased the maximum axial load of the column [14]. However, no analytical investigations have yet been carried out for the influence of different parameters (e.g., the compressive strength of SCC, tensile strength of steel tube, wall thickness of steel tube and pitch of steel helix) on the behaviour of SCC columns reinforced with steel tubes.
Detailed analytical investigations are required for the wide use of SCC columns reinforced with steel tubes. This paper develops an analytical model to predict the axial load-axial deformation behaviour of SCC columns reinforced with steel tubes. The results of the analytical model have been found well matching with the experimental investigation results. The influences of the compressive strength of SCC, tensile strength of steel tube, wall thickness of steel tube and pitch of steel helix on the axial load-axial deformation behaviour of SCC columns reinforced with steel tubes have been investigated.
Section snippets
Research significance
This study presents a simplified analytical model for the axial load-axial deformation behaviour of SCC columns reinforced with steel tubes. The analytical model takes into account the contributions of the steel tubes, unconfined concrete cover, confined concrete core and confined concrete inside the steel tube. The predictions of the developed analytical model have been found to be in good agreement with the experimental investigation results. The analytical observations, based on a detailed
Analytical modelling
The authors have recently proposed using small diameter steel tubes as longitudinal reinforcement for SCC columns. The innovative use of steel tubes for reinforcing SCC column was found to be highly effective, especially considering the maximum axial load and the ductility of the SCC columns [14]. The conventional SCC columns reinforced with steel bars usually consist of three main components: longitudinal steel bars, unconfined concrete cover and confined concrete core (Fig. 1a). The SCC
Experimental axial load-axial deformation behaviour
A total of four SCC column specimens reinforced with steel tubes were cast and tested under monotonic axial compression. All specimens were tested at the Structural Engineering Laboratories, School of Civil, Mining, and Environmental Engineering, University of Wollongong, Australia. The details of the experimental program including the design of experiments, preparation and testing, failure modes and behaviour of the specimens under concentric, eccentric and flexural loads were presented in
Experimental results of the SCC column specimens
Experimental results of four SCC column specimens tested under concentric axial load in terms of yield axial load and corresponding axial deformation, maximum axial load and corresponding axial deformation and ultimate axial deformation are presented in Table 1. The ultimate axial deformation corresponds to the deformation at the fracture of steel helices. Although the cross-sectional areas of the ST33.7 and ST26.9 steel tubes are similar, the yield axial load of Specimen ST33.7H50C was 5%
Analytical versus experimental results
The analytical and experimental axial load-axial deformation behaviours of the SCC column specimens reinforced with steel tubes are compared (Fig. 4). For all the column specimens, for the ascending part of the curve up to the maximum axial load, the analytical axial load-axial deformation curve correlated well with the experimental axial load-axial deformation curve. This good correlation is particularly because the stress-strain response of the different components of the specimens up to the
Parametric study
The developed analytical model was used to study the influences of different parameters on the axial load-axial deformation behaviours of SCC columns reinforced with steel tubes. The parameters studied were the compressive strength of SCC, tensile strength of steel tube, wall thickness of steel tube and pitch of steel helix.
Conclusions
This study presents analytical investigations on the axial load-axial deformation behaviour of self-compacting concrete (SCC) columns reinforced with steel tubes. Two types of steel tubes were used in the SCC columns as longitudinal reinforcement. The influences of different parameters including the compressive strength of SCC, tensile strength of steel tube, wall thickness of steel tube and pitch of steel helix were investigated. Based on the analytical results of this study, the following
Acknowledgments
The authors thank the University of Wollongong, Australia and technical officers at the High Bay Laboratory, especially Mr. Ritchie McLean, for their help in the experimental work of this study. Finally, the first author would like to acknowledge the Iraqi Government for the support of his PhD scholarship.
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