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(a) (b) (c)
Figure 1: Disturbed regions; (a) Transfer girder, (b) Typical reinforcement detailing of a deep beam

2. STEEL FIBRE REINFORCED CONCRETE Steel fibres can be used as a replacement to minimum shear
[8]
reinforcement in beams . Previous research studies reported
(SFRC)
that, adding steel fibres slightly increased the shear strength
The studies on steel fibre reinforced concrete (SFRC) started of beams when compared to conventional RC beam with shear
[1]
as early as the 1960s . Steel fibres are the most commonly reinforcement, whereas a significant increase in ductility is
used fibre reinforcing material for structural applications to observed [12,15-16] . Providing a steel volume fraction of around
[17]
control crack widths and increase ductility due to increased 1 % could sufficiently enhance the ductility and strength .
fracture toughness and post-cracking strength [1,11] . Additionally, The fibres also redistribute the tensile stresses across the
increased tensile strength, flexural, and shear strength have diagonal cracks by fibre bridging action, thus reducing crack
[6]
also been reported [1,12] . SFRC consists of short steel fibres with propagation .
a typical aspect ratio (α f ) ranging from 20 to 100, and lengths 2.1 Summary of code provisions
[1]
ranging from 6.4 mm to 76 mm .
The design provisions for SFRC members are given in RILEM ,
[7]
Previous studies have shown that the compressive and tensile FIB Model code and Eurocode 2 . A modified stress-strain
[10]
[6]
strength of SFRC is only marginally higher than conventional behavior for concrete in tension is recommended considering
concrete. However, the stress-strain behavior of SFRC showed the post-peak behavior in tension, including parameters
[1]
a significant post-peak behavior in tension . The area under obtained from a notched beam test. The slight increase in
the load-crack mouth opening displacement curve is much flexural strength can be captured using this modified stress-
[6]
higher for SFRC indicating higher fracture toughness, thereby strain behavior. In the case of shear strength, an expression
increasing ductility. SFRC beams possess a significantly higher for the fibre contribution in shear strength is specified, which
ability to sustain load at high crack widths [7,13] . Also, the high is based on the results from the notched beam test. Also, the
fracture toughness of SFRC contributes to the redistribution of condition for minimum reinforcement has been modified based
stresses in indeterminate structures resulting in an increased on the steel fibre content.
strength . The primary factors that influence the strength of FRC ACI 318 (2019) does not allow any strength increase for SFRC
[1]
[8]
are the fibre volume fraction, aspect ratio, and bond between members. Whereas, code provides relaxation in minimum shear
the fibre and the matrix . These factors are usually considered reinforcement if the shear force is less than the specified value,
[11]
[11]
in design through the fibre factor, F given by , only for beams with depth less than 600 mm. Also, a minimum
steel fibre content of 0.75 % with fibre length-to-diameter
(1)
ratio of at least 50 and not exceeding 100, has to be adopted.
where α f is the aspect ratio which is the ratio of the diameter Table 1 shows the summary of design provisions incorporated in
of fibre, d f to the length of fibre, lf; V f is the fibre volume international standards for slender beams.
fraction; η f is the fibre efficiency factor depending on the fibre Currently, design guidelines for SFRC deep members are
configurations and surface conditions. Fibres having high aspect limited. Eurocode 2 provides modifications in the strut-and-
ratios are observed to exhibit improved post-peak behavior due tie provisions to include the fibre contribution, which may be
to their higher pull-out strengths than fibres with smaller aspect adopted in the design of SFRC deep beams. As per other
ratios [1,11] . The tensile stress and crack width relationship SFRC standards, the design using strut-and-tie method/empirical
may be obtained reliably using standard tests or approximately method for SFRC deep beams do not consider any relaxation in
[7]
[11]
from theoretical models . main or distribution reinforcement.

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