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.
Dissertation (MEng)--University of Pretoria, 2019.