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University Visvesvaraya College of EngineeringUniversity Visvesvaraya College of Engineering..

Department of Civil Engineering, BangaloreDepartment of Civil Engineering, Bangalore. .

Seminar on Seminar on

““Steel Fibre Reinforced Concrete””By By

Virupakshappa C.Koti Virupakshappa C.Koti Branch:Pre Stressed ConcreteBranch:Pre Stressed Concrete

Class: M E III Sem Class: M E III Sem

GuideGuideDr. Sadath Ali Khan Zai &Dr. Sadath Ali Khan Zai &

Mr. Kiran TMr. Kiran T

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Introduction

About two decades back,steel fibre reinforced concrete (SFRC) were considered a new technology for the construction industry.However today this technology has found wider acceptance among the construction Industry .

And its now well established that one of the important properties of steel fibre reinforced concrete is its superior resistance to cracking and crack propagation. The fibre composite pronounced post – cracking ductility which is unheard of in ordinary concrete. The transformation from a brittle to a ductile type of material would increase substantially the energy absorption characteristics of the fibre composite and its ability to withstand repeatedly applied, shock or impact loading(1).

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Steel fibre reinforced concrete

SFRC is a concrete mix that contains discontinuous, discrete steel fibers that are randomly dispersed and uniformly distributed. The quality and quantity of steel fibers influence the mechanical properties of concrete(7).

A plasticiser or super plasticiser is often used to enhance mix workability. Steel fibre products are available in a variety of types and sizes from various manufacturers. However, the underlying principle of all SFRC designs is to provide discrete, discontinuous reinforcement and effective crack control.

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Steel fibre concrete

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Steel fibers

Intended for reinforcing concrete are defined as short, discrete lengths of steel having an aspect ratio in the range of 20-100, with any cross section and that are sufficiently small to be randomly dispersed in an unhardened concrete mixture using usual mixing procedures(7).

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FIGURE 1

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Why would we add steel fibres to concrete?

Plain, unreinforced concrete is a brittle material, with a low tensile strength and a low strain capacity. The role of randomly distributes discontinuous fibres is to bridge across the cracks that develop provides some post- cracking “ductility”. If the fibres are sufficiently strong, sufficiently bonded to material, and permit the FRC to carry significant stresses over a relatively large strain capacity in the post-cracking stage.

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Performance(4)

The most important aspects controlling the performance of steel fibres in concrete are the:

. aspect ratio

. volume concentration

. geometrical shape.

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Aspect ratio And Volume Concentration This are important feature of the steel fibre product is its aspect ratio, (or equivalent diameter for non-circular section).The aspect ratio i.e.the ratio of its length to its diameter, for fibre products are normally in the range of 30 to 100; higher range of 65 to 100 is required for more demanding applications such as heavy industrial floors.

Generally speaking, Higher for a fibre type with higher aspect ratio and volume concentration of the fibre as compare to a fibre type with lower aspect ratio and volume concentration of the fibre, which means that the performance of SFRC is better(2).  

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Unfortunately, the higher the aspect ratio and volume concentration of the fibre, becomes more difficult to mix, convey and shoot. Thus there are practical limits to the amount of single fibres, which can be added,with the amount varying with the different geometrical characteristics of the several fibre types.

However, if the aspect ratio exceeds 55, the risk of fibres bunching together (or known as “fibre-balling “) during mixing with concrete shall also increase. Thus,these “high-end” fibres are usually prepared in glued bundle form before being introduced into the concrete mix to avoid fibre-balling and also to facilitate fibre dispersion(8).

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fibre-balling

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Geometrical shape.

It was determined that the continuous-deformed, slit-sheet fiber provided better micro-macro cracking performance, as well as flexural strength enhancement; whereas, the end-deformed, drawn-wire steel fibers performed best post-first-crack. increase the reinforcing efficiency and ductility(2).

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STATIC MECHANICAL PROPERTIES

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Compressive strength Fibres do little to enhance the static compressive strength

of concrete, with increases in strength ranging 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 of the material . …

Stress-Strain curves in compression for SFRC (5)

LONGITUDINAL STRAIN X 10-6

 FIGURE 2

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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 as no increase in some instances to perhaps 60%, with many investigations indicating intermediate values, as shown FIGURE 3. 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 toughness of the composite.

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Influence of fibre content on tensile strength (5)FIGURE 3

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Flexural Strength

Steel fibres are generally found to have aggregate much greater effect on the flexural strength of SFRC than on either the compressive or tensile strength, with increases of more than 100% having been reported. The increases in flexural strength is particularly sensitive, not only to the fibre volume, but also to the aspect ratio of the fibres, with higher aspect ratio leading to larger strength increases. Figure 4 describes the fibre effect in terms of the combined parameter Wl/d, where l/d is the aspect ratio and W is the weight percent of fibres. It should be noted that for Wl/d > 600, the mix characteristics tended to be quite unsatisfactory. Deformed fibres show the same types of increases at lower volumes, because of their improved bond characteristics.

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The effect of Wl/d on the flexural strength of mortar and concrete (5)

FIGURE 4

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Fatigue Resistance - the fatigue strength of steel fibre concrete is far greater (1.6 times) than that of conventional concrete.

Impact– steel fibres greatly increase (1.5 to 5.0 times) concrete’s resistance to damage from heavy impact.

Shear Strength – shear strength is much improved (1.25 to 2.0 times) over unreinforced concrete.

OTHER PROPERTIES

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•Shrinkage– although the steel fibres themselves do not affect shrinkage rate, they can minimise and help eliminate shrinkage cracks, particularly in a restrained situation.

• Permeability - again, steel fibres do not directly affect concrete permeability. But by effectively controlling micro-cracking – and the resulting susceptibility to moisture and chemical penetration– SFR can help reduce the overall porosity of the matrix.

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Advantages And Disadvantages

Advantages

•Creates more ductile concrete with reduced cracking •Reduces the effect of shrinkage curling •More economical than conventional steel solutions •Fast installation thereby reducing schedule time •Easy materials handling •Supported by large manufacturers •Very durable •Does not interfere with guidewire signals •Does not cause concrete delaminations •Can replace wire mesh in most elevated slabs

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Disadvantages

•May sometimes be visible at the surface.

•Will rust in corrosive environments (does not cause delaminations) .

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Applications

Steel fibre reinforced concrete has gained widespread use in applications such as the following:

•. Rock slope stabilisation and support of excavated foundations, often in conjunction with rock and soil anchor systems;

•. Industrial floorings, road pavements, warehouses

•. Channel linings, protect bridge abutments and stabilise debris flow prone creeks;

•. Rehabilitation of deteriorated marine structures such as light stations, bulkheads, piers, sea wall sand dry docks;

•. Rehabilitation of reinforced concrete in structures such as bridges, chemical processing and handling plants.

•. Support of underground openings in tunnels, mines, drainage adits and exploratory adits,

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References

1) Determination of Early-Age Ductility of Steel Fiber-Reinforced Shotcrete Lining System at INCO’s Stobie Mine by Jean-François Dufour, J. Denis P. O’Donnell, Sr., and Michael Ballou.

2) Fibers in Concrete, The Indian Concrete Journal , March 2003.3) Influence Of fiber type on handened properties of Steel Fiber-Reinforced

concrete by Klaus Holschemacher, Torsten Müller.4) Steel Fiber-Reinforced concrete by Nguyen Van CHANH.5) “Steel fiber reinforced mortar and concrete”, A review of mechanical properties

by C.D. Johnston . In fiber reinforced concrete ACI – SP 44 – Detroit 1974 .6) Compressive behaviour of steel fibre reinforced concrete by R. D. Neves and J.

C. O. Fernandes de Almeida.7) Experimental Investigation on Behavior of Steel fiber reinforced mortar and

concrete by Chuanbo Wang of University of canterbury.8) How to choose Required Steel fibre by Timuran Engineering Sdn. Bhd

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THANKTHANK you. you.