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Project Title: Behaviour of Elliptical Tube Columns filled with Self-Compacting Concrete

PhD Student: Munir Mahgub

Supervisors: Dr Ashraf Ashour, Professor Dennis Lam and Dr Xianghe Dai

Introduction

Self-compacting concrete SCC is a very fluid concrete; it is compacted under its own weight and flows so that it completely fills all spaces. In the early stages of development, SCC included high cement paste. These mixes required special conditions in order to avoid segregation due to inaccurate placing, and shrinkage caused by the high levels of cement (Okamura and Ouchi, 2000). Initially, the overall cost of SCC was higher than that of conventional concrete, but with high levels of research and development, it became cheaper than conventional concrete. It consists of the same components as normal concrete (NC), along with some admixtures. The admixtures are principal materials of SCC that result in better workability.

Columns are an important structural element for researchers to study and investigate, as they carry and transfer loads to foundations. Beams and slabs have been investigated by most researchers since SCC came into use. However, columns have only attracted attention at the beginning of this century. Columns made from SCC are less effective, due to segregation and bleeding, prompting the development of SCC-filled steel tube columns. This type of column came into existence at the beginning of the last century, and was tested with NC before SCC was introduced.

Concrete-filled steel tube columns are being used increasingly in many modern structures, as they provide high strength, ductility and stiffness, with full usage of material. The steel tubes assist in carrying axial loads and provide confinement for concrete. Concrete confinement depends on the properties of the steel column, such as diameter, thickness and yield stress, as well as the concrete strength. While the steel tube provides confining pressure to the concrete, the confined concrete delays buckling of the steel tube. Furthermore, such columns have the advantage of requiring no formwork during construction; thus, reducing construction costs.

A new elliptical shaped steel tube column has been used in few places, but is not yet popular. An elliptical hollow section can provide greater efficiency than a circular hollow section, particularly when subjected to eccentric loading (generating a bending moment about the stronger axis) or when differing end restraints or bracings exist about the two principal axes (changing the effective column buckling length) (Chan and Gardner, 2009).

Project significance

Several researchers have introduced SCC to the construction industry as a new type of concrete used with tube columns. The overall success of this introduction paved the way for others to investigate the possibility of using SCC with different shapes of columns, including elliptical columns.
In comparison with the investigations carried out in this field, the current research will contribute the following:

  • The limited number of previous research projects in the field of using SCC with elliptical tube columns only investigated the behaviour of short columns under varying loading. The present research project goes further in that the length of the column has been considered in investigating the behaviour of elliptical columns.
  • Some design mix techniques of SCC are still not clear to follow; therefore, phase one of the project uses ANNs to predict the compressive strength of SCC.
  • Elliptical columns offer more advantages than circular and square columns; these elliptical columns can carry greater loads and bending moments than circular columns. Moreover, elliptical columns can potentially use a smaller space compared with other shapes and are aesthetically more appealing.
  • The effect of elements such as different dimensions of the section, length and compressive strength of concrete on the behaviour of elliptical tube columns filled with SCC have not been studied yet owing to the limited extent of previous research. 

Aims

The main goal of this PhD project is to study the behaviour of SCC-filled elliptical tube columns. The principal objectives of this research are categorized into three phases as below:

  • Concrete properties: In this stage, collected data from existing research in the literature will be used to predict the compressive strength of the SCC design mix using ANNs.
  • Experimental work: In this stage, the mix design of columns will be carried out using ANN, then the fresh properties will be tested and the behaviour of loaded SCC-filled elliptical columns will be investigated experimentally.
  • Computational analysis: In this stage, results from experimental work and other research will be collected and used to develop a computer simulation to predict the behaviour of SCC-filled elliptical columns.  The compressive strength of elliptical tube columns filled with SCC will be predicted by the Column Deflection Curvature method.

Methodology

The behaviour of SCC-filled elliptical tube columns will be investigated. Ten tube columns will be tested in a parametric study, where they are divided into two groups. The first group of 5 columns will use dimensions 15x7.5cm and the second group of 5 columns 25x12.5cm. The same thickness of elliptical columns will be used, 6.3mm, and two lengths 2m and 2.5m. Moreover, two concrete compressive strengths will be used: 38 MPa and 76 MPa. Elliptical tube columns without concrete will be tested as control columns for each group.

The SCC specimens will be made using sand, ordinary Portland cement, FA, SP, coarse aggregate with values of water-to-binder (w/b) ratio to produce the desired compressive strength after 28 days, as predicted by the ANN. Therefore, small trial mixes will be made. Testing of the composite columns will be carried out using a 250 and 100 tonne capacity PACT testing machine and the experimental set up. Both ends of the specimens will be milled flat and capped with rigid steel plates in order to distribute the applied load uniformly over both the concrete and steel sections for the compositely loaded columns. The specimens will be loaded at 20kN intervals at the beginning of the test (i.e. in the elastic region) and at 10kN intervals after the column begins to yield, in order to have sufficient data points to delineate the “knee” of the stress-strain curve. A linear variable differential transducer (LVDT) will be used to monitor the vertical deformation.

All of the operation and the change of loading rate will be operated manually and all the readings will be recorded when both load and strain stabilize. After the immediate drop of the load due to local buckling, the test will continue until excessive deformation of the column will be observed. The purposes of this stage are to understand the behaviour of the elements and to add more data to the literature. The data from the experiments will be used to validate the computational analysis.

The top hinge plate.   The bottom hinge plate.

Column in test rig – close-up view.   Column in test-rig – full view.

Column at the buckling load.

References

Okamura. H, Ozawa. K., Ouchi. M.(2000). "Self-compacting concrete." Structural concrete 1(1):3-17.

 Chan, T. M. Gardner, L. (2009). "Flexural Buckling of Elliptical Hollow section Columns." Journal of Structural Engineering 135(5): 546-557.

Yu, Q., et al. (2008). "Experimental behavior of high performance concrete-filled steel tubular columns." Thin-Walled Structures 46(4): 362-370.

Yu, Z.-w., et al. (2007). "Experimental behavior of circular concrete-filled steel tube stub columns." Journal of Constructional Steel Research 63(2): 165-174.

Zhao, X. L. and J. A. Packer (2009). "Tests and design of concrete-filled elliptical hollow section stub columns." Thin-Walled Structures 47(6-7): 617-628.

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