shear strength of steel fibres

8/7/2019 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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shear strength of steel fibres

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