Fibre Reinforced Concrete

Kalyani Quantum 2008
A TECHNICAL PAPER
ON

Submitted
By
Amar J.Thorat
S.E(civil)
Shivaratan Desai Ganesh M.Gholap
S.E(civil) F.E(civil)



Rajarambapu Institute of Technology,Rajaramnagar.
.
Contents
Abstract
Introduction
Role of Fibre
Behavior of FRC
Factor affecting properties of Reinforced Concrete
What do fibermesh
Types of Fiber & Their application
Requirement of good Fibre
Advantages of Fibre
Conclusion
Reference


ABSTRACT
Concrete is an important versatile construction material, used in wide variety of
situation such as sky scrapper building, bridges, tunnels etc. When we talk of a building
material, it is very important to consider its durability as it has indirect effect on
economy, serviceability and maintenance. The lack of durability may be caused by
external environmental reasons of internal causes within concrete itself. Normally
concrete fails due to combined actions of various detrimental agencies.
To increase durability of concrete .We are use so many chemicals,
one newly invented material i.e. not chemicals but affect on durability of concrete name
of that material is fibermesh. Fibermesh is a 100% virgin polypropylene fibers use in
concrete. Also we are using Glass, Steel, Polypropylene ,Nylon.They are specially used
in concrete as micro reinforcement system, which reduce cracks up to 95
1. INTRODUCTION:
Plain concrete possesses a very low tensile strength, limited ductility and little
resistance to cracking. Internal micro cracks are inherently present in the concrete and its
poor tensile strength is due to propagation of such micro cracks, eventually leading to
brittle fracture of the concrete.
In the past, attempts have been made to impart improvement in tensile properties
of concrete by way of using conventional reinforced steel bars and also by applying
restraining techniques. Although both these methods provide tensile strength to the
concrete members, they however do not increase the inherent tensile strength of concrete
itself.
Recently, however the development of fibre-reinforced composites in the plastics
and aerospace fields has provided a technical basis for improving these deficiencies.
Fiber-Reinforced Concrete (FRC) results from the addition of either short discrete
fibers or continuous long fibers to the cement based matrix.
1.1 ROLE OF FIBRES:
When the loads are imposed on concrete failure cracks may propagate sometimes
rapidly; fibres in concrete provide a means of arresting the crack growth. Reinforcing
steel bars in concrete have the same beneficial effect because they act as long continuous
fibres. Short discontinuous fibres have the advantage, however of being uniformly mixed
and dispersed throughout the concrete. Fibres are added to a concrete mix which
normally contains cement, water and fine and coarse aggregate.
As a rule, fibres are generally randomly distributed in the concrete; however,
processing the concrete so that the fibres become aligned in the direction of applied stress
will result in even greater tensile or flexural strengths.
Concrete made with Portland cement has certain characteristics: it is relatively
strong in compression but weak in tension and tends to be brittle. The weakness in
tension can be overcome by the use of conventional rod reinforcement and to some extent
by the inclusion of a sufficient volume of certain fibres.
2. BEHAVIOUR OF FIBER-REINFORCED CONCRETE:
The strength of the fiber reinforced concrete can be measured in terms of its
maximum resistance when subjected to compressive, tensile, flexural and shear loads.
In field conditions, usually some combination of these loads is imposed; however
for evaluation purposes, the behavior is characterized under one type of loading without
the interaction of other loads.
The strength under each individual type of loading is a useful indicator of the
FRC material's performance characteristic for design consideration.
2.1 Toughness:
Toughness is defined as the area under a load-deflection (or stress-strain) curve.
As can be seen from Figure 1, adding fibres to concrete greatly increases the toughness of
the material. That is, fibre-reinforced concrete is able to sustain load at deflections
or strains much greater than those at which cracking first appears in the matrix.


Fig no.1. Stress-Strain curve
2.2 Compression:
The compressive properties of fiber-reinforced concrete (FRC) are relatively less
affected by the presence of fibers as compared to the properties under tension and
bending.
Important factor is that with the addition of fibers there is an almost negligible
increase in strength for mortar mixes; however for concrete mixes, strength increases by
as much as 23%.The complete stress-strain curves of steel fiber-reinforced concrete with
compressive strengths ranges from 35 MPa to 84 MPa.
2.3 Flexure:

There are a number of factors that influence the behavior and strength of FRC in
flexure. These include: type of fibres, fibre length (L), aspect ratio (L/d
f
) where d
f
is the
diameter of the fibre, the volume fraction of the fibre (V
f
), fiber orientation and fibre
shape, fibre bond characteristics (fiber deformation).
Also, factors that influence the workability of FRC such as W/C ratio, density, air
content and the like could also influence its strength. The ultimate strength in flexure
could vary considerably depending upon the volume fraction of fibers, length and bond
characteristics of the fibers and the ultimate strength of the fibers.
2.4 Tensile and Splitting Tensile:
The failure in tension of cement based matrices is rather brittle and the associated
strains are relatively small in magnitude. The addition of fibres to such matrices, whether
in continuous or discontinuous form, leads to a substantial improvement in the tensile
properties of the FRC in comparison with the properties of the unreinforced matrix.
The stress–strain or load–elongation response of fibre composites in tension
depends mainly on the volume fraction of fibres.
Many of the current applications of fibre reinforced concrete involve the use of
fibres ranging around 1.0 percent by volume of concrete. Recently, it has been possible to
incorporate relatively large volumes (ranging up to 15 percent) of steel, glass, and
synthetic fibers in concrete.
2.5 Shear strength:
Shear failure can be sudden and catastrophic.This is true for critical sections
where, due to construction constraints, little or no reinforcing steel may be placed.
Tests performed to study the shear strength are classified into two general groups:
a)The direct shear tests are required to understand the basic transfer behaviour of
concrete.
b)The tests on beams and corbels are necessary to understand the behaviour of structural
members reinforced with fibres.
It is proven that the addition of fibers generally improves the shear strength and
ductility of concrete.
2.6 Modulus of Elasticity:
The modulus of elasticity of a material, whether in tension, compression, or shear,
is a fundamental property that is needed for modeling mechanical behavior in various
structural applications.
If the modulus of elasticity of the fibre is high with respect to the modulus of
elasticity of the concrete or mortar binder, the fibres help to carry the load, thereby
increasing the tensile strength of the material.
2.7 Creep Shrinkage:
From the studies it was observed that the steel fibres were less effective in
restraining the creep at high stress to strength ratio (equal to 0.55) in comparison with
low stress to strength ratio (equal to 0.33).
Large stress to strength ratios increase the lateral strains and hence decrease the
interfacial pressure between the fibers and the surrounding concrete. This in effect
reduces the restraint to sliding action between the fibers and the concrete matrix and
results in larger creep strains.
2.8 Shrinkage:
The primary advantage of fibers in relation to shrinkage is their effect in reducing
the adverse width of shrinkage cracks. Shrinkage cracks arise when the concrete is
restrained from shrinkage movements.
The presence of steel fibers delays the formation of first crack, enables the
concrete to accommodate more than one crack and reduces the crack width substantially.
The addition of small amounts of steel fibers (0.25% by volume) reduced the average
crack widths by about 20% and the maximum crack width by about 50% in comparison
with unreinforced plain concrete.
High strength concretes with silica fume undergo early cracking when
deformation is restrained. This phenomenon, which occurs even when concrete is
protected against any evaporation, is attributed to shrinkage, because of the exceptionally
low water-cement ratio (about 0.26). This phenomenon can be corrected by the use of
fibers.
2.9 Strain Capacity:
The ability to withstand relatively large strains before failure, the superior
resistance to crack propagation and the ability to withstand large deformations and
ductility are characteristics that distinguish fibre-reinforced concrete from plain concrete.
These characteristics are generally described by "toughness" which is the main reason for
using fiber-reinforced concrete in most of its applications.
Unlike plain concrete specimens, the presence of fibers imparts considerable
energy to stretch and rebound the fibers before complete fracture of the material occurs.
Toughness is a measure of the ability of the material to mobilize large amounts of post-
elastic strains or deformations prior to failure.
2.10 Impact Resistance:
Impact resistance is essential for applications such as the bridge piers. It is well
recognized that the addition of fibers to concrete enhances the impact resistance.
Compared with plain concrete, the increase in impact strengths at full failure were 640%,
847%, 1,824% and 2,806% respectively for concretes with 0.5, 1.0, 1.5, and 2.0%
(volume) fiber content.
2.11 Abrasion Resistance
When erosion is due to abrasion resulting from high velocity flow and impact of
large debris, steel fiber concretes have provided significant erosion resistance.
2.12 Wet-Dry Exposure:
The effect of addition of polypropylene fiber to concrete mix and adequate curing in
enhancing the degradation resistance of concrete surface skin subjected to cyclic wet/dry
seawater exposure. It indicates that addition of polypropylene fibers effectively retard the
deterioration process of the surface skin of the concrete specimens cured in hot weather
environment.
3.FACTORS EFFECTING PROPERTIES OF REINFORCED CONCRETE:
Its properties would obviously, depend upon the efficient transfer of stress
between matrix and the fibres, which is largely dependent on the type of fiber, fiber
geometry, fiber content, orientation and distribution of the fibres, mixing and compaction
techniques of concrete, and size and shape of the aggregate.
3.1 Relative Fiber Matrix Stiffness:
The modulus of elasticity of matrix must be much lower than that of fiber for
efficient stress transfer. High modulus fibres such as steel, glass and carbon impart
strength and stiffness to the composite.
Interfacial bond between the matrix and the fibres also determine the
effectiveness of stress transfer, from the matrix of the fiber. A good bond is essential for
improving tensile strength of the composite. The interfacial bond could be improved by
larger area of contact, improving the frictional properties and degree of gripping and by
treating the steel fibres with sodium hydroxide or acetone.
3.2 Volume of Fibres:
The increase in the volume of fibres, increase approximately linearly, the tensile
strength and toughness of composite. Use of higher percentage of fiber is likely to cause
segregation and harshness of concrete and mortar.
3.3 Aspect Ratio of the Fibre:
It has been reported that up to aspect ratio of 75, increase in the aspect increases
the ultimate strength of the concrete linearly. Beyond 75, relative strength and toughness
is reduced.
3.4 Orientation of Fibres:
One of the differences between conventional reinforcement and fiber
reinforcement is that in conventional reinforcement, bars are oriented in the direction
desired while are randomly oriented. The fibres aligned parallel to the applied load
offered more tensile strength and toughness than randomly distributed or perpendicular
fibres.
3.5 Workability and Compaction of Concrete
Incorporation of steel fiber decreases the workability considerably. Even
prolonged external vibration fails to compact the concrete. Generally, the workability and
compaction standard of the mix is improved through increased water/cement ratio or by
the use of some kind of water reducing admixtures.
3.6 Size of Coarse Aggregate
Several investigators recommended that the maximum size of the coarse
aggregate should be restricted to 10mm, to avoid appreciable reduction in strength of the
composite. Fibres also in effect, act as aggregate. The inter-partical friction between
fibres, and between fibres and aggregates controls the orientation and distribution of the
fibres and consequently the properties of the composite.
3.7 Mixing
Mixing of the fibre reinforced concrete needs careful conditions to avoid balling
of the fibres, segregation and in general the difficulty of mixing the materials uniformly.
Increasing the aspect ratio, volume percentage and size and quantity of coarse aggregate
intensify the difficulties and balling tendencies. A steel fibre contain in excess of 2% by
volume and an aspect ratio of more than 100 are difficult to mix.
It is important that the fibres are dispersed uniformly throughout the mix. This can
be done by the addition of fibres before the water is added. When mixing in a laboratory
mixer, introducing the fibres through a wire mesh basket, will help even distribution of
fibres.
WHAT DO FIBERMESH?
Fibermesh fibers get uniformly dispersed in the concrete of mortar as millions of
fibers in every cubic meter to reduce-
Plastic shrinkage,
Settlement cracks,
Reduce permeability,
Increase impact and shatter resistance,
Increase abrasion resistance,
Increase resistance to freeze/thaw,
Intrinsic cracking.
There by vastly improve overall quality and durability.

4. TYPES OF FIBRE AND THEIR APPLICATIONS:
Fibre Type Application
Glass Precast panels, curtain wall facings, sewer pipe, thin concrete shell roofs,
wall plaster for concrete block.
Steel Cellular concrete roofing units, pavement overlays, bridge decks,
refractory, concrete pipe, airport runways, pressure vessels, blast-
resistant structures, tunnel linings, ship-hull construction.
Polypropylene,

nylon
Foundation piles, prestressed piles, facing panels, flotation units for
walkways and moorings in marinas, road-patching material, heavyweight
coatings for underwater pipe.
Asbestos Sheet, pipe, boards, fireproofing and insulating materials, sewer pipes,
corrugated and flat roofing sheets, wall lining.
Carbon Corrugated units for floor construction, single and double curvature
membrane structures, boat hulls, scaffold boards.
Mica Flakes Partially replace asbestos in cement boards, concrete pipe, repair
materials.
* Combinations of more than one fibre type can be used for special purposes.

5. REQUIREMENT FOR GOOD FIBRES:
For the effective use in hardened concrete
Fibre length must be sufficient.
Fibres should be significantly stiffer than the matrix, i.e. a higher modulus of
elasticity.
There must be a good fibre-matrix bond.
Fibre content by volume must be adequate.
Fibres must have a high aspect ratio, ie they must be long relative to their
diameter.
It deals with high volume concentrations of fibre. However, for economic reasons, the
current trend in practice is to minimize fibre volume, in which case improvements in
properties can be marginal. For the quantities of fibres typically used (less than 1% by
volume for steel and about 0.1% by volume for polypropylene) the fibres will not have
significant effect on the strength or modulus of elasticity of the composite. It must be
noted that high volume concentrations of certain fibres may make the plastic concrete
unworkable.Generally the workability and compaction standard of the mix is improved
through increased water/cement ratio or by the use of some kind of water reducing
admixtures.
6.ADVANTAGES:
1. Fibres introduced in the concrete acts as an crack arrester.
2. Fibre-reinforced concrete is able to sustain load at deflections or strains much
greater than those at which cracking first appears in the matrix.
3.The addition of fibers generally improves the shear strength and ductility of concrete.
4.The fibres help to carry the load, thereby increasing the tensile strength of the material.
5.The presence of steel fibers delays the formation of first crack, enables the concrete to
accommodate more than one crack and reduces the crack width substantially.
6.It has got the ability to withstand relatively large strains before failure.
7.It has superior resistance to crack propagation.
8.It has got the ability to withstand large deformations and ductility.
9.The addition of fibers to concrete enhances the impact resistance.
7. CONCLUSION:
Innovations in engineering design, which often establish the need for new
building materials, have made fibre-reinforced cements very popular. The possibility of
increased tensile strength and impact resistance offers potential reductions in the weight
and thickness of building components and should also cut down on damage resulting
from shipping and handling.
Although every type of fiber has been tried out in cement and concrete, not all of
them can be effectively and economically used. Each type of fiber has its characteristic
properties and limitations. Some of the fibres that can be used are steel fibres,
polypropylene, nylons, asbestos, coir, glass and carbon.
At this time, no general IS standards for fibre-reinforced cement, mortar and
concrete. Until these standards become available, it will be necessary to rely on the
experience and judgment of both the designer and the fibre manufacturer.
8. REFERENCES
www.irc.nrc.

Concrete technology by M. S. Shetty.
www.aci.us.co

www.icjonline.com

www.ask.com

www.google.com
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