LONG-TERM BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE BEAMS

Abstract

Previous works have shown that the use of steel fibres in reinforced concrete beams leads to enhanced cracking strength improved stiffness after cracking and greater restraint to crack propagation. Steel fibres were also found to reduce the long-term deformations of plain concrete due to creep and shrinkage. It is therefore expected that the addition of steel fibres would reduce the long-term deflections and crack widths in reinforced concrete beams. The main objective of this study was to investigate the long-term behaviour of steel fibre reinforced concrete beams with emphasis on deflections and crack widths. Both analytical and experimental works were carried out. In · the analytical works, methods are proposed for the prediction of .-instantaneous deflections and maximum crack width of steel fibre reinforced concrete beams. The methods account for the enhanced cracking strength, improved post-cracking flexural stiffness, and reduced tensile strains in steel reinforcement due to steel fibres. For the predictions of long-term deflections, two refined analytical approaches based on the Effective Modulus of elasticity Method (EMM) and the Age-adjusted Effective Modulus of elasticity Method (AEMM), are proposed. In the EMM-based approach, the deflection due to creep is evaluated from an elastic analysis using an effective modulus of elasticity, while the deflection due to shrinkage is analysed using the equivalent tensile force method. The AEMM-based approach considers the aging of steel fibre concrete and involves a simultaneous linear elastic analysis for deflections due to creep and shrinkage using an age-adjusted effective modulus of elasticity. In addition to the analytical approaches, semi-empirical formulae were proposed for the prediction of additional deflection and increase in crack widths in steel fibre reinforced concrete beams under sustained loads, using long-term multipliers. The experimental works were carried out to investigate the effect of steel fibre content and sustained load level on both the short-term and long-term deformations of steel fibre reinforced concrete beams. Five beams were tested monotonically to failure. Twelve other beams were subjected to flexural creep tests in which beam deflections, concrete strains and cracking were monitored for over a year. Parallel tests on long-term material properties were also conducted to obtain the creep coefficient and shrinkage strains of steel fibre concrete. Test results indicated that creep coefficient and shrinkage strains of steel fibre concrete were significantly lower than those of plain concrete. Steel fibres were found to considerably reduce both the short-term and long-term deflection and crack width of reinforced concrete beams. They were more effective in reducing long-term deflection in beams under sustained loads greater than the design service load. The effects of concrete aging and creep recovery on the behaviour of steel fibre reinforced concrete beams under varying stress histories were similar to that of reinforced concrete beams. Good agreement was obtained between analytical predictions and test results for both deflections and crack widths. The proposed semi-empirical expressions can be used for speedy calculations of both the long-term deflections and maximum crack width in steel fibre reinforced concrete beams, and are suitable for design purposes.