Shear Strength of Steel Fibres
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Transcript of Shear Strength of Steel Fibres
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SHEAR STRENGTH OF STEEL FIBRE-
REINFORCED CONCRETE BEAMSWITHOUT STIRRUPS
Presented By,
Structural Engineering
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SHEAR FORCE
Shear force is present in beams at sectionswhere there is a change in bending momentalong the span.
Transversely loaded reinforced concretebeams may fail in shear before attainingtheir full flexural strengths if they are not
adequately designed for shear. Shear failures are very sudden and
unexpected
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SHEAR FORCE
Shear force is resisted by the combined
action of
i. the uncracked concrete in compressionregion.
ii. the aggregate interlocking.
iii. the shear acting across the longitudinalsteel bars.
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SHEAR
The unbalanced shear in excess of the three
combined factors is resisted by the shear
reinforcement. The shear reinforcement is generally
provided in the form of vertical stirrups.
This may sometimes cause congestion in
reinforcement.
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SHEAR FAILURE
The failure of beam considered in shear is
induced by cracks outside the central section of
the beam.
The shear failure of reinforced concrete
members without stirrups initiates when the
principal tensile stress within the shear span
exceeds the tensile strength of concrete. It results in initiation of diagonal crack which
later propagates through the beam web.
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MODES OF FAILURE
The various failure modes in shear without
web reinforcement are
a) Diagonal Tension failure
b) Shear compression failure
c) Splitting or true shear failure
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DIAGONAL TENSION FAILURE
It is most common in
shear span when the
a/d ratio is above 2.
It does not lead to
sudden failures.
It may stop at point 1
and with increasedload extend beyond 2
causing failure.
Diagonal tension failure
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SHEAR COMPRESSION FAILURE
This failure occurs at arange of a/d between1.0 and 2.5.
Large shear in shortshear spans mayinitiates approximatelya 45 crack.
A compression failurefinally occurs adjacentto the load.
Shear compression failure
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SPLITTING OR TRUE SHEAR FAILURE
This failure occurswhen a/d is less thanunity.
When the shear span isless than the effectivedepth d, the shearcrack is carried as an
inclined between loadand reaction.
Shear strength is muchhigher in such cases.
Splitting shear failure
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STEEL FIBRES
Steel fibres are filamentsof wire, deformed andcut to lengths, forreinforcement of
concrete, mortar andother compositematerials.
It is a cold drawn wirefibre with corrugated
and flatted shape. Steel fibres are crack
arrestors.
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STEEL FIBRES
The addition of steel fibers to a reinforced
concrete beam is known to increase its shear
strength.
The use of steel fibers is more efficient in
high-strength concrete, which can be
relatively brittle without fibers.
Conventional stirrups can be eliminated,
which reduces reinforcement congestion.
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EXPERIMENT
Experiments conducted by Adebar et alBatson, Jenkins, and Spatney on fibrereinforced beams are illustrated below.
In conventional reinforced concrete beams, theultimate shear strength decreases withincreasing shear span depth ratio a/d andconcrete compressive strength fc .
These effects cause arch and dowel action inbeams with low values of a/d, and diagonal-tension failure mode in beams with highervalues of a/d.
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Experiment
The increase in shear strength depends onthe amount of fibers, expressed as the fibervolume fraction Vf ,aspect ratio and
anchorage conditions for the steel fibers.
Twelve reinforced concrete beams weretested to failure to evaluate the influence of
fiber-volume fraction,a/d
and concretecompressive strength on beam strength andductility.
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Test procedure
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EQUIPMENT
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All beams have identical cross sections.
Stirrups are provided only at the supports.
Two equal loads were applied to the beam. Deflections were imposed by increasing
load in small increments.
The deflection and applied load wererecorded at mid span.
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Force-displacement relation
Typical force-
deflection relationship
are shown in fig.
As the fibre content
increased, both
maximum applied load
and ultimate deflection
increased.
Typical force-deflection histories (a/d = 2)
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FAILURE PATTERN
The presence of steelfibres in concrete greatlyaffected the observedcracking pattern.
The numbers next to thecracks refer to the loadat which cracks wereobserved.
The beam FHB1-2 does
not have steel fibres andsudden failure isobserved. Typical crack patterns (a/d = 2)
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TEST RESULTS
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Shear stress variation
Influence of a/d on shear resistance
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Shear stress variation
The ultimate shear stress at failure
decreased with increasing a/d.
The average shear stress at the onset ofshear cracking decreased with increasing
a/d
The difference in capacity between beams
with a/d = 2 and a/d = 3 was significantly
larger than the difference between beams
with a/d = 3 and a/d = 4.
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Shear resistance
Influence of fiber volume on increased shear resistance.
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Shear resistance
The strength of the fiber-reinforced beams rangedfrom 122 to 180% of the strength of the beamswithout fibers.
The strength increase was particularly large (69 to80%) for the beams with low a/d (a/d = 2.0), whichfailed in a combination of shear and flexure
For larger a/d, the increase in strength ranged
from 22 to 38%. The increase in cracking shear ranged from 13 to
33% of the cracking shear of similar beamswithout fibers.
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Comparison of beams with stirrups and with
steel fibres
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Failure pattern of beams with
stirrups
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Failure pattern of beams with
stirrups The failure pattern of the beams shown in Fig
indicates that for a/d 1 and 2 crack initiated
approximately at 45degrees to the longitudinal axis of
the beam. A compression failure finally occurred adjacent to the
load which is designated as a shear compression
failure.
For a/d 3 and 4 the diagonal crack starts from the lastflexural crack and turned gradually into a crack more
and more inclined under the shear loading.
The failure is designated as diagonal tension failure.
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CONCLUSIONS
Steel fibres are crack arrestors.
The beams with small a/d value carried more load aftershear cracking than the beams with large a/d values.
Of the nine beams that contained steel fibers, only twofailed in pure shear and two failed in a combination offlexure and shear.
Five beams with fibers failed in flexure, provide only alower bound on the shear strength.
Concrete beams without fibers failed in shear whichcorresponds to sudden failure along a single shear crack.
Steel fibres are easily available and cost effective whencompared with polymer fibres.
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REFERENCES
Yoon-Keun Kwak, Marc O. Eberhard, Woo-Suk Kim, andJubum Kim ;Shear Strength of Steel Fiber-ReinforcedConcrete Beams without Stirrups : ACI StructuralJournal/July-August 2002.
Sudheer Reddy.L 1, Ramana Rao .N.V , Gunneswara RaoT.D ; Shear ResistanceofHighStrength ConcreteBeamsWithoutShear Reinforcement: International journal of civiland structural engineering volume 1, 2010.
Dileep Kumar P.G; Shear strengthofR.C.Cbeams without
web reinforcement .
www.steelfiber.org
www.greensteelgroup.com
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