Fire-resistant geopolymer concrete and its application in concrete filled steel tubes

Abstract

Concrete-filled steel tubular (CFST) columns, composed of core concrete and outer tubes, have been extensively used as main structural elements in high-rise buildings to carry loads. Due to the composite action between the steel and core concrete, this type of composite construction has been reported to have many constructional and structural benefits, such as easy construction from the omission of formwork, restraint to local buckling of the steel tube provided by the core concrete, high strength, stiffness and ductility. Recently, there is increased interest in adopting CFST columns in composite frame systems. In recent years, fire disasters have frequently been reported worldwide, and seriously threatened personal and public safety. Exposure of concrete and steel to fire will lead to serious structural deterioration and possible failure of columns, thus resulting in local or global collapse of a building. However, in most cases, unprotected CFST columns are not able to maintain structural integrity for sufficient time under fire conditions. External insulating coating or internal reinforcing steel are required to improve the fire resistance of CFST columns, but these two methods tremendously increase the cost of CFST columns and the difficulty of construction. Although ordinary Portland cement (OPC) concrete is classified as a fire-resistant construction material, during fire exposure severe spalling may occur, as well as significant deterioration of strength and stiffness. Therefore, there is a need to find alternative materials to replace OPC to further improve the fire resistance of concrete. Geopolymer is aluminosilicate synthesised from a material of geological origin or industry by-products (e.g. fly ash) with alkaline solutions. In Australia, fly ash is abundantly available from numerous thermal power plants. The use of fly ash to manufacture construction materials can promote the better utilisation of this industry by-product, avoiding disposal into landfill. Previous studies have demonstrated that geopolymer can be successfully used as a binder to make geopolymer concrete. The absence of OPC in geopolymer concrete (GPC) can significantly reduce CO2 emissions for the construction industry. Recent research has proved that GPC has substantially better fire performance than OPC concrete. Therefore, GPC has the potential to be used in CFST columns to improve their fire performance. So far, very little research has been conducted to investigate the behaviour of geopolymer concrete-filled steel tubular (GCFST) columns, and this research aims to address this knowledge gap. In this thesis, salient parameter analysis of GPC mix was conducted based on the Taguchi method, and three optimised GPC mix designs were proposed. Material properties such as slump, density, Young's modulus, compressive strength, flexural strength and splitting tensile strength were experimentally studied. The measured values of Young's modulus, flexural strength and splitting tensile strength of GPC are compared favourably to the predictions of existing standards. The measured hot strength and displacement of GPC under combined loading and elevated temperature were reported. Full-range stress-strain curves of GPC and reference OPC after exposed to elevated temperatures were evaluated. The results further confirm that GPC has the potential to improve the fire performance of CFST columns. A total of 15 tests were carried out on GCFST columns and conventional CFST columns to compare their behaviours. The main experimental parameters included: (1) Concrete type (geopolymer concrete and OPC concrete); (2) Curing procedure of GPC (ambient curing and elevated temperature curing); (3) Strength of geopolymer concrete (37.4, 58.6 and 68.4 MPa); (4) Test method (tested at ambient temperature, in fire, or after fire exposure). The axial load, axial strain and lateral strain were measured for ambient temperature tests and residual property tests. The axial displacement versus the temperature curves were recorded for the specimens tested under combined load and temperature increase. The results were compared to predictions of numerical models developed with ABAQUS, and the agreement between them is reasonable. This research confirms that the behaviour of GPC at an ambient temperature is comparable to that of OPC concrete at an ambient temperature. However, GPC has better fire performance and a higher post-fire residual strength than OPC concrete. This study has proved the potential of using GPC as fire-resistant concrete in CFST columns. Thus, the use of external insulating coating or internal reinforcing steel could be potentially eliminated for CFST columns.

Date of Award	2017
Original language	English