behavior of steel fiber reiforced concrete

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2016

EFFECT OF STEEL FIBRES ON CONCRETE

BEHAVIOR

Mechanics of Solids Term Report


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Abstract

Since the concrete is a brittle material and carries a sudden failure. In order to

improve the ductile behavior of concrete various types of fibers are addressed in

the concrete. Introduction of fibres into the concrete results in post-elastic propertychanges that depends upon a number of factors, including matrix strength, fibre

type, fibre modulus, fibre aspect ratio, fibre strength, fibre surface bondingcharacteristics, fibre content and fibre orientation.

This report explains the effect of steel fibers on the behavior of concrete.In the section one general introduction is given about concrete. Section two

explains the different aspects of steel fiber reinforced concrete and section three

covers the effect of steel fibers on different behaviors of concrete.


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CONTENTS

Abstract……………………………………………………………….i

1. Introduction…………………………………………………………..1

2. Steel fiber reinforced concrete……………………………………….22.1

Steel fibers in various shapes…………………………………………………...2

2.2 Structural Application of SFRC……………………………………………… ...3

2.3 Mechanism of crack formation and propagation in concrete beams……………4

2.4 Fibre Bridging………………………………………………………………………………………….5

2.5 Structural Application of SFRC…………………………………………………63. Characteristics of SFRC Members……………………………………………… ..7

3.1 Compressive Strength………………………………………………………… ...83.2 Tensile strength………………………………………………………………….83.3 Flexural strength…………………………………………………………………8

3.4

Shear strength…………………………………………………………………… .94. Conclusion…………………………………………………………………………105. References………………………………………………………………………… .11


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1.

Introduction

Concrete is characterized by brittle failure, the nearly complete loss of loading capacity, once

failure is initiated. This characteristic, which limits the application of the material, can be

overcome by the inclusion of a small amount of short randomly distributed fibers (steel, glass,synthetic and natural) and can be practiced among others that remedy weaknesses of concrete,

such as low growth resistance, high shrinkage cracking, low durability, etc. Steel fiber reinforced

concrete (SFRC) has the ability of excellent tensile strength, flexural strength, shock resistance,fatigue resistance, ductility and crack arrest. Therefore, it has been applied abroad in various

professional fields of construction, irrigation works and architecture. There are currently 300,000

metric tons of fibers used for concrete reinforcement. Steel fiber remains the most used fiber of

all (50% of total tonnage used) followed by polypropylene (20%), glass (5%) and other fibers(25%) [1].

Concrete is good in compression but week in tension that is, concrete is a brittle material. So,

in order to improve the tensile properties, short fibers are used. [3] Investigated the effects of

steel fibers on flexural performance of reinforced concrete (RC) beams and found that with theaddition of 1% by fraction of steel fibers in concrete increases the first cracking load, ultimate

load, stiffness and ductility of the concrete beams.Concerning the shear failure of concrete members, it is well known that when principal tensile

stresses exceed the tensile strength of concrete, diagonal cracks occur in the shear span. This way, the behavior of a concrete element under shear is fully characterized by the behavior of the material in directtension [4]. The various experimental studies shown that the use of steel fibers improves the crackingcharacteristics and the overall behavior of shear concrete beams under monotonic loading. Further, thishad inspired some investigations to study the possibility of partially replacing stirrups (conventional

transverse steel reinforcement) with steel fibers, especially in cases where design criteria recommend ahigh steel ratio, which leads to limited stirrup spacing.

Due to the increasing evidence from various research results, the 2008 ACI Building Code

allows engineers to use steel fiber reinforced concrete (SFRC) to replace the conventional shearreinforcement (i.e. steel stirrups) even if the design shear force is greater than half of the concrete

shear strength. Though the new ACI provisions, marked a significant transfer from research to

practice, beams constructed of steel fiber reinforced concrete are required to have a minimum

amount of steel fibers of 0.75% in volume (100 pounds per cubic yards) and compressivestrength not greater than 6 ksi. Shear failure in plain concrete members is brittle in nature and

consequently predisposes structures to sudden collapse without any advance warning [2]. One

measure to protect concrete members from brittle shear failure under excessive loads is to usefiber reinforced concrete (FRC) with proper mixture design and with fibers selected for the

appropriate material properties, FRC is capable of considerably increasing performance in terms

of shear strength and ductility when compared to plain concrete [ACI Committee 544, 2002].


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2.

Steel fiber reinforced concrete

Steel Fibre reinforced concrete (SFRC) may be defined as a composite materials made with

Portland cement, aggregate, and incorporating discrete discontinuous steel fibres.

Compared to the conventional reinforcement, the fibre reinforcement is: Distributed throughout a cross section (whereas bars are only place where needed).

Relatively short and closely spaced (while bars are continuous and not as closely

spaced).

Not comparable, in term of area, to the one of the bars. When the fibre reinforcement is in the form of short discrete fibres, they act effectively as

rigid inclusions in the concrete matrix. Physically, they have thus the same order of magnitude as

aggregate inclusions; steel fibre reinforcement cannot therefore be regarded as a direct

replacement of longitudinal reinforcement in reinforced and prestressed structural members.However, because of the inherent material properties of fibre concrete, the presence of fibres in

the body of the concrete or the provision of a tensile skin of fibre concrete can be expected to

improve the resistance of conventionally reinforced structural members to cracking, deflectionand other serviceability conditions.

The real contribution of the fibres is to increase the toughness of the concrete (defined as

some function of the area under the load vs. deflection curve), under any type of loading. That is,

the fibres tend to increase the strain at peak load, and provide a great deal of energy absorption in post-peak portion of the load vs. deflection curve.

2.1.

Steel fibers in various shapes

Fig 01: Fibres in various shapes


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2.2.

Effect of Fiber geometry and volume on properties of concrete

The properties of SFRC in its freshly mixed state are influenced by the aspect ratio of the

fibres, fibre geometry, fibre volume fraction, matrix proportions and the fibre-matrix interfacial

bond characteristics. Post-cracking load-deformation characteristics greatly depend on the choiceof fibre geometry and the volume percentage of the specific fibres used.

D. V. Soulioti et.al investigated the effect of geometry and volume of fiber on the flexural

properties of concrete. Two geometries of fibres, waved fibres and fibres with hooked ends wereused in this work. For each of the two geometries, three different fibre volume fractions were

used in the concrete mixes; 0.5, 1, and 1.5% by concrete volume. Incorporation of fibres in

concrete had small effect on the compressive strength. It was also observed that concrete

mixtures with waved fibres exhibited higher compressive strengths than concrete mixtures withhooked-ended fibres, with the exception of the mixtures W0.5 and H0.5 which showed

comparable compressive strengths. Specimens with hooked-ended fibres (H0.5, H1, H1.5)

exhibited higher values of toughness and residual strength than specimens with waved fibres

(W0.5, W1, W1.5). On the contrary, concrete mixtures W0.5, W1, W1.5 showed higher first- peak strength and peak-strength than the mixtures H0.5, H1, H1.5.

Figure 2 shows the variation of the mean value of the specimen’s toughness T100, 2.0 for

each concrete mixture, as a function of the fibre volume fraction. It is obvious that an increase infibre content leads to significant improvement of energy absorption in the specimens. The

mixtures with hooked ended fibres appear to have improved toughness properties compared to

the mixtures with waved fibres. This behaviour is similar for each one of the three volumefractions [9].

Fig 02: Effect of geometry and volume of fiber on the flexural toughness


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2.3. Mechanism of crack formation and propagation in concrete beamsThe first crack that appears in a beam normally is in correspondence of the region where the

bending moment is maximum and the shear force is small; the cracks are aligned whit each otherand, more or less, perpendicular to the flexural stress; they are, therefore, in mode I condition. As

visible in a normal load‐deflection plot, at a certain point, the behavior from linear becomes non‐

linear [8]. Increasing the load more cracks are formed away from the region of maximum bending moment and also the non‐linearity increases; these further cracks are along region wherethe shear forces are no longer small, for this reason they are in mixed mode condition (mode I +

II), but always normal to the major tensile principal stress.

Fig 03: A longitudinal reinforced concrete beam in three‐ point bending. First flexural cracks appear

in the region of maximum bending moment (a) accompanied by nonlinearity in load‐deflection response(denoted by an asterisk in (b)). More flexural cracks appear away from the region of maximum momentunder increasing load (c), and a dominant crack propagates towards load point until ultimate failure by

crushing of compressive concrete (d)

These cracks, growing, follow a bent path (due to mode II) and they are no more parallel to thedirection of the applied load (sometimes these are incorrectly called shear cracks). Mode II is

also responsible of the sliding of the crack faces. Longitudinal bars, transversal bars and fibres

counteract the opening of the crack, but it is difficult to separate their effects for quantifying thecontribution of each element.

Further increasing of load does grow a dominant crack towards the reduced compression

zone until failure take place. The response is generally ductile. The most significant effect of

the presence of steel fibres is the cracking behavior, the beams made out of FRC display an

increased number of both flexural and shear cracks at closer spacing than the

corresponding beams without fibres. Normally, also a reduction of spalling in the vicinity of

the support and bond cracking can be found. The addition of fibres could (not always) lead


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to achieve the flexural failure; but, although fibres beneficially and substantially improve

the crack and deformational behaviour as well as the ultimate strength, this does not

always happen.

2.4. Fibre Bridging

The tensile fracture mechanism of concrete is a complex phenomenon. The post cracking

behavior is affected by two different mechanisms:

Aggregate bridging that is always present in the plain concrete

Fiber bridging that contributes to energy dissipation in FRC concrete.

The fiber bridging is always predominant, but the final bearing in uni‐axial tension is the

combination of both the two mechanisms; aggregate bridging decays to zero for a crack

opening of around 0.3 mm. The addition of fibers increases the work of fracture

(represented by the area under the stress‐crack opening curve) and the critical crack

opening (from approximately 0.3 mm to half the fibre length – for steel fibers usually 10 to

30 mm). The mechanical behavior of FRC depends surely on the amount of fibre (whichshows benefits from 1 % until 15 %, for engineered cementitious composites ECC), on the

orientation of the fibres and largely on the pull‐out versus load (or load‐slip) behaviour of

the individual fibres. The fibre pull‐out behavior is the gradual debonding of an interface

surrounding the fibre, followed by frictional slip and pull‐out of fibre.

The bond (responsible of the forces transmission between fibre and matrix) has different

components:

the physical and/or chemical adhesion between fibre and matrix;

the frictional resistance;

the mechanical component (arising from the fibre geometry, e.g. deformed, crimped

or hooked‐end);

the fibre‐to‐fibre interlock

2.5. Structural Application of SFRC

As recommended by ACI Committee 544, ‘when used in structural applications, steel fibrereinforced concrete should only be used in a supplementary role to inhibit cracking, to improve

resistance to impact or dynamic loading, and to resist material disintegration. In structural

members where flexural or tensile loads will occur, the reinforcing steel must be capable of

supporting the total tensile load. Thus, while there are a number of techniques for predicting the

strength of beams reinforced only with steel fibres, there are no predictive equations for largeSFRC beams, since these would be expected to contain conventional reinforcing bars as well.

For beams containing both fibres and continuous reinforcing bars, the situation is complex,

since the fibres act in two ways,(1) They permit the tensile strength of the SFRC to be used in design, because the matrix will

no longer lose its load-carrying capacity at first crack; and

(2) They improve the bond between the matrix and the reinforcing bars by inhibiting thegrowth of cracks emanating from the deformations (lugs) on the bars.


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(3) SFRC can be used to improve the shear and flexure behavior of beams [4].

(4) SFRC may reliably be used to reduce the columns damage by preventing the concrete

cover to spall out at earlier stages and increase their initial stiffness and energydissipation, especially for uniaxial loads [5].

Fig 04: Experimental moment vs deflection curves [1]

(5) SFRC can be utilized in Beam-Column joints as investigated by [6],

1. The Beam-Column joint with fibres has high strength than ordinary joint

2. The joint with fibres can take high peak loads3. The joint with fibres give high post-cracking ductile behaviour taking higher loads.


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(6) Rafts and Foundation Slabs of Buildings have also been getting equal attention in

extreme cases where the regular reinforcements are too congested and the bar diameters

are already too high (= 32 mm) to allow for further increase. In such cases, steel fibres

become most suited as they contribute substantially to the moment capacity of the

sections.

(7)

Addition of SFs can provide an increased impact resistance to conventional RC members,thereby enhancing the resistance to local damage and spalling.

3. Effect of steel fibers on concreteIt should now be clear that the properties of an FRC could not be represented by a single

characteristic (compression strength) as it happens for normal concretes. In particular, seeing thatthe addition of fibres increases significantly the toughness leaving the compression strength

almost unchanged, for fibre‐reinforced concrete some sort of toughness property is required, andthus other tests have to be used to characterize it [8]. The presence of fibres mainly affects

ductility and this influence is strongly dependent on the fibre content and fibre type.

The main test‐set‐ups used are:

uni‐axial tension test or direct tensile test (UTT); flexural test:

Three Point Bending Test (3PBT) (notched/unnotched): it is the most

widespread method on beam/prism specimens; it is suggested also by RILEM

TC 162‐TDF (2002b) for SFRC;

Four Point Bending Test (4PBT) (notched/unnotched);

panel test or plate test (used for shotcrete and for specific load condition that

simulates a design situation in a real structure);

wedge‐splitting test (WST) that sometimes is an alternative to the UTT and

the 3PBT.

3.1.

Compressive Strength

Fibres do little to enhance the static compressive strength of concrete, with increases in strengthranging from essentially nil to perhaps 25%. Even in members which contain conventional

reinforcement in addition to the steel fibres, the fibres have little effect on compressive strength.

However, the fibres do substantially increase the post-cracking ductility, or energy absorption ofthe material.


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LONGITUDINAL STRAIN X 10^-6

Fig 05: Stress-Strain curves in compression for SFRC[1]

3.2.

Tensile strength

Fibres aligned in the direction of the tensile stress may bring about very large increases in direct

tensile strength, as high as 133% for 5% of smooth, straight steel fibres. However, for more or

less randomly distributed fibres, the increase in strength is much smaller, ranging from as little asno increase in some instances to perhaps 60%, with many investigations indicating intermediate

values, as shown in figure 06. Splitting-tension test of SFRC show similar result. Thus, adding

fibres merely to increase the direct tensile strength is probably not worthwhile. However, as in

compression, steel fibres do lead to major increases in the post-cracking behaviour or toughnessof the composites [1].

Fig 06: Influence of fibre content on tensile strength [1]

3.3.

Flexural strength

An accurate measurement of deflection is very important to characterize the toughness of SFRC.

In flexural toughness tests of SFRC, it is common practice to measure the beam midpoint

deflection between the tension face of the beam and a fixed reference on the machine crosshead.


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The behaviour of plain concrete and SFRC is made clear with the help of a four point beam

bending test as illustrated in Figure 07. It is observed that for plain concrete, a sudden and brittle

mode of failure occurs after the peak load is reached which then is used to calculate the flexuralstrength of the concrete. When sufficient ductility is ensured in the beam with the addition of

steel fibres in concrete, a strain softening phenomenon is observed after the load at first crack or

peak load in the beam. Thus, with this kind of toughening behaviour in the beam, post-crackflexural strength of SFRC is guaranteed.

Fig 07: Four point bending test results

3.4. Shear strength

Reinforced concrete members are normally designed for the limit state of collapse in flexurerather than in shear. Shear failure, which in reality, occurs under the combined action of shearing

forces and bending moments, characterized by very small deflection and lack of ductility. Shearfailure of a reinforced concrete beam occurs when the principal tensile stress within the shearspan exceeds the tensile strength of concrete and a diagonal crack propagates through the beam

web. This failure is usually without any warning due to the brittle nature of plain concrete

behavior in tension. For this reason, shear failure is considered very undesirable and is usually

avoided [10].

It is well known that use of steel fibers raises the ductility of concrete and the fractured energy.

This phenomenon is transferable to the shear strength of concrete. The cracking shear stress isalso directly proportional to fiber content and inversely proportional to shear span to depth ratio.


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4.

Conclusion

Concrete is characterized as a brittle material with low tensile strength and low strain capacity. Itsmechanical behaviour is critically influenced by crack propagation. Problems related to concrete

brittleness and poor resistance to cracking can be improved by reinforcing plain concrete with randomlydistributed fibres. Many of the properties of fibre-reinforced concrete (FRC) can be used to advantage inthe concrete flexural members reinforced with conventional bar reinforcement. The use of steel fibres

along with longitudinal steel bars improves the yielding moment, ultimate moment and post-yield behaviour. Moreover, addition of fibres reduces immediate deflection, long-term deflection and crackwidth of beam.


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5.

References

1. Introduction to Steel Fiber Reinforced Concrete on Engineering Performance of

Concrete, Vikrant S. Vairagade, Kavita S. Kene, IJSTR©2012.

2. Flexural Behaviour of Reinforced Fibrous Concrete Beams: Experiments andAnalytical Modelling, Rashid Hameed, Alain Sellier, Pak. J. Engg. & Appl. Sci.

Vol. 13, July, 2013 (p. 19-28)

3. Shear Strength of Steel Fiber Reinforced Prestressed Concrete Beams, Jae-Sung

Cho et.al, 2009 ASCE

4. Shear Performance of Steel Fibrous Concrete Beams C. E. CHALIORIS, E. F.

SFIRI, © 2011 Published by Elsevier Ltd

5. Experimental behavior of SFRC columns under uniaxial and biaxial cyclic loads

by Federica Germano.

6. Strengthening of beam column Joint by steel fiber reinforced concrete during

earthquake loading, Ph.D. thesis by Dr. Sanaullah Baloch University of Leeds, UK

7. Steel Fibre Reinforced Concrete (SFRC): Areas of Application by Navneet T. Narayan

8. Shear Capacity of Steel Fibre Reinforced Concrete Beams without

Conventional Shear Reinforcement by Stockholm, Sweden 2011

9. Effects of Fibre Geometry and Volume Fraction on the Flexural Behaviour ofSteel-Fibre Reinforced Concrete by D. V. Soulioti, N. M. Barkoula, A. Paipetis

and T. E. Matikas, Department of Materials Engineering, University of Ioannina,45110, Ioannina, Greece.

10 Shear strength of steel fiber reinforced concrete without stirrups, by V.T. Baber,

P.K. Joshi, D.N. Shinde, International Journal of Advanced Engineeringtechnology.
http://www.sciencedirect.com/science/article/pii/S1359836815005454?np=yhttp://www.sciencedirect.com/science/article/pii/S1359836815005454?np=y

behavior of steel fiber reiforced concrete

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