The effect of steel fibres on moment redistribution in high strength reinforced concrete beams

Abstract

The use of short, randomly distributed fibres in reinforcing concrete has led to improved durability and ductility, the latter of which is a key characteristic in the redistribution of moments. Steel fibres can be specified in the design of structural flexural members, as the only reinforcing, or in combination with traditional reinforcing bars. Practical material tests have been developed, although the added complexity inherent in the behaviour of FRC may result in shortcomings in sufficiently describing the mechanical properties of the structure. Although steel fibres improve the general behaviour of a structure in terms of reducing crack widths and minimising deflections, the variability in the post cracking strength may lead to a less beneficial impact on moment redistribution. The overall change in the structural performance was therefore investigated, with the focus of this study being on the effects of different percentages and combinations of steel fibres and steel reinforcing bars on moment redistribution in statically indeterminate high strength concrete beams. The experimental framework consisted of characterising the material properties of the FRC with fibre volume contents of 0 kg/m3, 80 kg/m3 (1.0%), 120 kg/m3 (1.5%), and 160 kg/m3 (2.0%) in 80 MPa concrete. An inverse analysis technique was used to determine the stress-strain properties of the FRC from the results of FPBTs. A total of fifteen 5.0 m beams were cast, each with a different combination of steel fibres and steel bar reinforcing ranging from 0 to 3 bars. The change in structural behaviour was characterised into three major groups; loads and deflections, moment related results, and energy. An in-depth analysis of the effects of the steel fibres was conducted to determine where the differences in behaviour could be explained by the material properties. The addition of fibres did not lead to significant increases in the load capacity, however deflections at relatively low loads were reduced. The optimum fibre content varied, depending on which aspect of the structural performance was considered. For ultimate relative deflections, the optimum fibre content increased with an increase in the number of reinforcing bars. An optimum steel fibre content resulting in the maximum moment redistribution was found at a fibre content of 1.5%. Significant elastic moment redistribution occurred after cracking before any plastic behaviour occurred. Fibres were found to be less effective when combined with reinforcing bars, however their effectiveness increased with an increase in the number of reinforcing bars. The outcome of this research was to provide greater understanding into the effect of varying amounts of steel fibres on structural behaviour and to clarify the complex inter-related mechanisms at work in the deformation of a beam.