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For sustainability considerations, the use of recycled aggregate in concrete has attracted many interests in the research community. One of the main concerns for using such concrete in buildings is its spalling in fire. This may be alleviated by adding steel fibers to form steel fiber reinforced recycled aggregate concrete (SFRAC). This paper presents an experimental investigation into the compressive properties of SFRAC cylinders after exposure to elevated temperatures, including the compressive strength, Young's modulus (stiffness), stress-strain curve and energy absorption capacity (toughness). The effects of two parameters, namely steel fiber volume content (0%，0.5%，1%，1.5%) and temperature (room temperature, 200℃， 400℃ and 600℃) on the compressive mechanical properties of concrete were investigated. The test results show that both compressive strength and stiffness of the concrete are significantly reduced after exposure to high temperatures. The addition of steel fibers is helpful in preventing spalling, and significantly improves the ductility and the cracking behavior of recycled aggregate concrete (RAC) after exposure to high temperatures, which is favorable for the application of RAC in building construction.

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Abstract:Experimental and numerical studies into the structural behaviour of reinforced concrete columns confined by circular steel tubes after exposure to standard fire conditions are presented. These elements differ from conventional concrete-filled steel tubular (CFST) columns in that breaks in continuity are present at the member ends, which limit the longitudinal stresses in the steel and maximise the level of confinement afforded to the concrete through the generation of hoop stresses in the tube. The temperature distributions in the specimens were measured during the heating and cooling phases, while the load-displacement relationships and longitudinal and transverse strains in the steel tube were recorded during the subsequent compressive tests. 3D finite element (FE) models were also developed using the program ABAQUS to investigate the post-fire performance of circular steel tube confined reinforced concrete (CSTCRC) columns, including both heat transfer and stress analyses. The FE models were used to identify the influences of key parameters on the residual capacity of CSTCRC columns, following exposure to the ISO-834 standard fire. The considered parameters included heating time, cross-sectional dimensions, strength of the materials, steel tube to concrete area ratio and ratio of reinforcement. Finally, a design method was proposed for predicting the residual load bearing capacity and compressive stiffness of CSTCRC columns after standard fire exposure.

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Tests are reported on twenty-six concrete filled steel tube of rectangular section after being exposed to high temperatures, to investigate the influence of temperature on section capacity and load-deformation behavior. The main parameter varied is temperature, from 20°C to 900°C. A mechanics model is described in this paper for the behaviour of concrete-filled RHS (Rectangular Hollow Section) columns after exposed to high temperatures, and is a development of the analysis (Han et al, 2001a) used when only normal temperatures apply. The predicted load versus axial strain relationship is in good agreement with stub column test results. Simplified models are derived for the section capacities and the modulus of elasticity of the composite sections. It was found in general, that the higher the exposure temperature, the higher the loss of section capacities and elastic modulus which resulted. The tests have shown the importance of the influence of high temperatures on the performance of concrete filled steel tubes. The work in this paper provides a basis for further theoretical study on the residual strength of concrete filled steel tubular columns.