Fiber Reinforced Concrete can be defined as a composite material consisting of

mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly

dispersed suitable fibers. Fiber reinforced concrete are of different types and
properties with many advantages. Continuous meshes, woven fabrics and long wires
or rods are not considered to be discrete fibers.

Fiber is a small piece of reinforcing material possessing certain characteristics

properties. They can be circular or flat. The fiber is often described by a convenient
parameter called “aspect ratio”. The aspect ratio of the fiber is the ratio of its length to
its diameter. Typical aspect ratio ranges from 30 to 150.

Fiber reinforced concrete (FRC) is concrete containing fibrous material which

increases its structural integrity. It contains short discrete fibers that are uniformly
distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic
fibers and natural fibers. Within these different fibers that character of fiber reinforced
concrete changes with varying concretes, fiber materials, geometries, distribution,
orientation and densities.

Fibre-reinforcement is mainly used in shotcrete, but can also be used in normal

concrete. Fibre-reinforced normal concrete are mostly used for on-ground floors and
pavements, but can be considered for a wide range of construction parts (beams,
pliers, foundations etc) either alone or with hand-tied rebars

Concrete reinforced with fibres (which are usually steel, glass or “plastic” fibres) is
less expensive than hand-tied rebar, while still increasing the tensile strength many
times. Shape, dimension and length of fibre is important. A thin and short fibre, for
example short hair-shaped glass fibre, will only be effective the first hours after
pouring the concrete (reduces cracking while the concrete is stiffening) but will not
increase the concrete tensile strength
 Effect of Fibers in Concrete
Fibres are usually used in concrete to control plastic shrinkage cracking and drying
shrinkage cracking. They also lower the permeability of concrete and thus reduce
bleeding of water. Some types of fibres produce greater impact, abrasion and shatter
resistance in concrete. Generally fibres do not increase the flexural strength of
concrete, so it can not replace moment resisting or structural steel reinforcement.
Some fibres reduce the strength of concrete.

The amount of fibres added to a concrete mix is measured as a percentage of the total
volume of the composite (concrete and fibres) termed volume fraction (V f). Vf
typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fibre
length (l) by its diameter (d). Fibres with a non-circular cross section use an
equivalent diameter for the calculation of aspect ratio.
If the modulus of elasticity of the fibre is higher than the matrix (concrete or mortar
binder), they help to carry the load by increasing the tensile strength of the material.
Increase in the aspect ratio of the fibre usually segments the flexural strength and
toughness of the matrix. However, fibres which are too long tend to “ball” in the mix
and create workability problems.

Some recent research indicated that using fibres in concrete has limited effect on the
impact resistance of concrete materials.This finding is very important since
traditionally people think the ductility increases when concrete reinforced with fibres.
The results also pointed out that the micro fibres is better in impact resistance
compared with the longer fibres.

Necessity of Fiber Reinforced Concrete

1. It increases the tensile strength of the concrete.

2. It reduce the air voids and water voids the inherent porosity of gel.

3. It increases the durability of the concrete.

4. Fibres such as graphite and glass have excellent resistance to creep, while the same is
not true for most resins. Therefore, the orientation and volume of fibres have a significant
influence on the creep performance of rebars/tendons.
5. Reinforced concrete itself is a composite material, where the reinforcement acts as the
strengthening fibre and the concrete as the matrix. It is therefore imperative that the behavior
under thermal stresses for the two materials be similar so that the differential deformations of
concrete and the reinforcement are minimized.

6. It has been recognized that the addition of small, closely spaced and uniformly
dispersed fibers to concrete would act as crack arrester and would substantially improve its
static and dynamic properties.

Factors Affecting Properties of Fiber

Reinforced Concrete
Fiber reinforced concrete is the composite material containing fibers in the cement
matrix in an orderly manner or randomly distributed manner. Its properties would
obviously, depends upon the efficient transfer of stress between matrix and the fibers.
The factors are briefly discussed below:

1. Relative Fiber Matrix Stiffness

The modulus of elasticity of matrix must be much lower than that of fiber for efficient
stress transfer. Low modulus of fiber such as nylons and polypropylene are, therefore,
unlikely to give strength improvement, but the help in the absorption of large energy
and therefore, impart greater degree of toughness and resistance to impart. High
modulus fibers such as steel, glass and carbon impart strength and stiffness to the
composite.

Interfacial bond between the matrix and the fiber also determine the effectiveness of
stress transfer, from the matrix to the fiber. A good bond is essential for improving
tensile strength of the composite.
2. Volume of Fibers
The strength of the composite largely depends on the quantity of fibers used in it. Fig
1 and 2 show the effect of volume on the toughness and strength. It can see from Fig 1
that the increase in the volume of fibers, increase approximately linearly, the tensile
strength and toughness of the composite. Use of higher percentage of fiber is likely to
cause segregation and harshness of concrete and mortar.

Fig.1: Effect of volume of fibers in flexure

Fig.2: Effect of volume of fibers in tension

3. Aspect Ratio of the Fiber
Another important factor which influences the properties and behavior of the
composite is the aspect ratio of the fiber. It has been reported that up to aspect ratio of
75, increase on the aspect ratio increases the ultimate concrete linearly. Beyond 75,
relative strength and toughness is reduced. Table-1 shows the effect of aspect ratio on
strength and toughness.

Table-1: Aspect ratio of the fiber

Type of concrete Aspect ratio Relative strength Relative toughness

Plain concrete 0 1 1

With 25 1.5 2.0

Randomly 50 1.6 8.0

Dispersed fibers 75 1.7 10.5

100 1.5 8.5

4. Orientation of Fibers
One of the differences between conventional reinforcement and fiber reinforcement is
that in conventional reinforcement, bars are oriented in the direction desired while
fibers are randomly oriented. To see the effect of randomness, mortar specimens
reinforced with 0.5% volume of fibers were tested. In one set specimens, fibers were
aligned in the direction of the load, in another in the direction perpendicular to that of
the load, and in the third randomly distributed.

It was observed that the fibers aligned parallel to the applied load offered more tensile
strength and toughness than randomly distributed or perpendicular fibers.

5. Workability and Compaction of Concrete

Incorporation of steel fiber decreases the workability considerably. This situation
adversely affects the consolidation of fresh mix. Even prolonged external vibration
fails to compact the concrete. The fiber volume at which this situation is reached
depends on the length and diameter of the fiber.

Another consequence of poor workability is non-uniform distribution of the fibers.

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. Size of Coarse Aggregate

Maximum size of the coarse aggregate should be restricted to 10mm, to avoid
appreciable reduction in strength of the composite. Fibers also in effect, act as
aggregate. Although they have a simple geometry, their influence on the properties of
fresh concrete is complex. The inter-particle friction between fibers and between
fibers and aggregates controls the orientation and distribution of the fibers and
consequently the properties of the composite. Friction reducing admixtures and
admixtures that improve the cohesiveness of the mix can significantly improve the
mix.

7. Mixing
Mixing of fiber reinforced concrete needs careful conditions to avoid balling of fibers,
segregation and in general the difficulty of mixing the materials uniformly. Increase in
the aspect ratio, volume percentage and size and quantity of coarse aggregate intensify
the difficulties and balling tendency. Steel fiber content in excess of 2% by volume
and aspect ratio of more than 100 are difficult to mix.

It is important that the fibers are dispersed uniformly throughout the mix; this can be
done by the addition of the fibers before the water is added. When mixing in a
laboratory mixer, introducing the fibers through a wire mesh basket will help even
distribution of fibers. For field use, other suitable methods must be adopted.
 Different Types of Fiber Reinforced
Concrete
Following are the different type of fibers generally used in the construction industries.

1. Steel Fiber Reinforced Concrete

2. Polypropylene Fiber Reinforced (PFR) cement mortar & concrete

3. GFRC Glass Fiber Reinforced Concrete

4. Asbestos Fibers

5. Carbon Fibers

6. Organic Fibers

1. Steel Fiber Reinforced Concrete

A no of steel fiber types are available as reinforcement. Round steel fiber the
commonly used type are produced by cutting round wire in to short length. The
typical diameter lies in the range of 0.25 to 0.75mm. Steel fibers having a rectangular
c/s are produced by silting the sheets about 0.25mm thick.

Fiber made from mild steel drawn wire. Conforming to IS:280-1976 with the diameter
of wire varying from 0.3 to 0.5mm have been practically used in India.

Round steel fibers are produced by cutting or chopping the wire, flat sheet fibers
having a typical c/s ranging from 0.15 to 0.41mm in thickness and 0.25 to 0.90mm in
width are produced by silting flat sheets.

Deformed fiber, which are loosely bounded with water-soluble glue in the form of a
bundle are also available. Since individual fibers tend to cluster together, their uniform
distribution in the matrix is often difficult. This may be avoided by adding fibers
bundles, which separate during the mixing process.
2. Polypropylene Fiber Reinforced (PFR) cement
mortar and concrete
Polypropylene is one of the cheapest & abundantly available polymers polypropylene
fibers are resistant to most chemical & it would be cementitious matrix which would
deteriorate first under aggressive chemical attack. Its melting point is high (about 165
degrees centigrade). So that a working temp. As (100 degree centigrade) may be
sustained for short periods without detriment to fiber properties.

Polypropylene fibers being hydrophobic can be easily mixed as they do not need
lengthy contact during mixing and only need to be evenly distressed in the mix.

Polypropylene short fibers in small volume fractions between 0.5 to 15 commercially

used in concrete.

Fig.3: Polypropylene fiber reinforced cement-mortar and concrete

3. GFRC – Glass Fiber Reinforced Concrete
Glass fiber is made up from 200-400 individual filaments which are lightly bonded to
make up a stand. These stands can be chopped into various lengths, or combined to
make cloth mat or tape. Using the conventional mixing techniques for normal concrete
it is not possible to mix more than about 2% (by volume) of fibers of a length of
25mm.

The major appliance of glass fiber has been in reinforcing the cement or mortar
matrices used in the production of thin-sheet products. The commonly used verities of
glass fibers are e-glass used. In the reinforced of plastics & AR glass E-glass has
inadequate resistance to alkalis present in Portland cement where AR-glass has
improved alkali resistant characteristics. Sometimes polymers are also added in the
mixes to improve some physical properties such as moisture movement.

Fig.4: Glass-fiber reinforced concrete

4. Asbestos Fibers
The naturally available inexpensive mineral fiber, asbestos, has been successfully
combined with Portland cement paste to form a widely used product called asbestos
cement. Asbestos fibers here thermal mechanical & chemical resistance making them
suitable for sheet product pipes, tiles and corrugated roofing elements. Asbestos
cement board is approximately two or four times that of unreinforced matrix.
However, due to relatively short length (10mm) the fiber have low impact strength.

Fig.5: Asbestos fiber

5. Carbon Fibers
Carbon fibers from the most recent & probability the most spectacular addition to the
range of fiber available for commercial use. Carbon fiber comes under the very high
modulus of elasticity and flexural strength. These are expansive. Their strength &
stiffness characteristics have been found to be superior even to those of steel. But they
are more vulnerable to damage than even glass fiber, and hence are generally treated
with resign coating.

Fig.6: Carbon fibers

6. Organic Fibers
Organic fiber such as polypropylene or natural fiber may be chemically more inert
than either steel or glass fibers. They are also cheaper, especially if natural. A large
volume of vegetable fiber may be used to obtain a multiple cracking composite. The
problem of mixing and uniform dispersion may be solved by adding a superplasticizer.

Fig.7: Organic fiber

There are various factors which affects the durability of fiber reinforced concrete such
as temperature, weathering, corrosion, freezing and thawing etc. are discussed.

Durability of concrete element is the ability of the member to resist aggressive

environment, accidental event, and impact effect and maintain the structural integrity.

In this article, the durability of fiber reinforced concrete (FRC) will be discussed.
 Factors Affecting Durability of Fiber
Reinforced Concrete (FRC)
Following are the factors which affects the durability of fiber reinforced
concrete:
 Extreme temperature and fire

 Freezing and thawing

 Degradation and embrittlement due to alkali attack and bundle affect

 Weathering and scaling

 Corrosion resistance

Effect of Extreme Temperature and Fire on

Durability of FRC
Generally, concrete has a reasonable resistance to severe temperature because of its
low thermal conductivity, great heat capacity, and it is not burn easily while exposed
to fire.
Concrete constituents for example specific aggregate types and cement clinker are not
influenced by high temperature both chemically and physically. However, there are
others concrete constituents that affected by temperature changes such as hydration
product. It is influenced by loss of water, micro-cracking, and damage by differential
expansion.

The addition of steel fiber, synthetic fiber, or combination of both to concrete

enhances structural concrete elements resistant against substantial temperature and
fire.

The strength of conventional concrete is decreased considerably if it exposed to fire

for long time. Cement paste and aggregate bond in concrete is damaged at a
temperature of 202oC and about half of the concrete strength is decline at temperature
of 427 oC, and 90% of concrete strength is lost at temperature of larger than 927 oC.
Fiber provision do not impede concrete failure under this sever condition but it
increases fire exposure safe time. The extension of fire exposure safe time provides
more time during which evacuations and the fire extinguishment can be proceeded
safely.

It is reported that, the application of hybrid combination of steel and polypropylene

fibers in precast concrete fireplace hearths produced small or not explosive spalling.

Regarding concrete spalling, when concrete exposed to fire, excess water inside
concrete, which used to provide workability during construction, changes to steam
pressure. If the pressure inside concrete is not released and surpass concrete tensile
strength, explosive spalling will occur.

The concrete spalling depends on the amount of free water and its distribution while
concrete element is exposed to fire.

The damage caused by spalling may penetrate concrete to about 6 cm.

Spalling is a serious problem because it may expose steel reinforcement to high
temperature. Hence, steel reinforcement is deteriorated quickly which in return
ultimate load carrying capacity of concrete member is declined.

It is demonstrated that, when concrete reinforced with polypropylene fiber exposed to

high temperature, the polypropylene fiber is melted and fine capillary pores will be
emptied and this lead to release the accumulated steam pressure and the concrete
maintain its strength.

The provision of steel fiber increases small concrete slab fire resistance to three to
nine times that of the slab with no fibers.

Finally, fibers can be added to concrete to bridge cracks and keep structure integrity of
the damaged structure.

Effect of Freezing and Thawing on Durability of

FRC
In this section durability of three fiber reinforced concrete namely steel, synthetic, and
cellulose FRC will be explained.

It is demonstrated that, among factors such as fiber content, air content, cement
content, and water to cement ratio, the air content create significant effect on the steel
fiber reinforced concrete resistance against freezing and thawing.

Moreover, the reduction of SFRC modulus of rupture due to freezing and thawing is
smaller than that of concrete with zero fiber.

It is recommended by Rider and Heidersbach that, mix design of SFRC that is used in
marine environment, need to have water content of no greater than 0.45, cement
content should be at least 519 Kg/m3, and air content ranges from 6-7.5%.
Regarding synthetic fiber reinforced concrete, it is pointed out that, not only does the
synthetic fibers improves freezing and thawing resistance of synthetic FRC but also
enhances concrete ability to withstand deicer scaling.

Moreover, freezing and thawing cause reduction of flexural strength of concrete

reinforced with polyolefin micro-fiber by about 15% whereas plain concrete flexural
strength reduced by 30%.

As far as cellulose fiber is concerned, it is found that, fiber reinforced cement board
(FRCB) which is laminated material and consist of cellulose fiber, cement, silica, and
water, is vulnerable to freezing and thawing deterioration due to its high porosity,
hydrophilic and tabular nature of cellulose fibers, and laminated nature of the
composite.

FRC Degradation and Embrittlement due to Alkali

Attack and Bundle Effect
Strength of various fibers for example glass, polymeric, and natural fibers are
decreased in long term because of weathering. It is substantially important to know
time-dependent reduction of durability and strength of those fibers in structurally
related areas.

That is why deterioration mechanism of various fibers will be explained in this

section.

Glass Fiber Concrete

Reinforced concrete commonly contains alkali resistance glass fibers between 3-5% of
the whole composite weight. It is reported that, the corrosion of fiber is the major
degradation mechanism.

However, it is claimed that, apart from the effect of corrosion, there are other factors
that influence the durability of GFRC. Added to that, in most situations, calcium
hydroxide, which is a product of cement hydration, is the agent that is to blame for
decreasing GFRC durability.

That is why attempts made toward the reduction of calcium hydroxide in order t
improve the durability of GFRC. Calcium hydroxide can be reduced by either adding
admixtures for example fly ash, ground granulated blast furnace slag, and silica fume
or avoid the use of conventional Portland cement especially those types which contain
calcium aluminates or sulfo aluminates.

In summary, the glass fiber reinforced concrete damage mechanisms are chemical
attack, mechanical attack, delayed fracture.

Cellulose Fiber Concrete

Cycles of wetting and drying lead to degrade cellulose fiber and this degradation
occur in different mechanism includes change in degree of fiber cement bonding and
fiber mineralization.

In the former mechanism, hydration product transportation specifically lime within the
lumen of fibers and around the fibers lead to reduce interface porosity. This could be
the cause of the increase of fiber cement bond and the decline of composite ductility.

In the latter mechanism, it is claimed that, the embrittlement of fiber occur as a result
of the penetration of cement hydration product into the fiber.

Lastly, the durability of cellulose fiber may be increased by Fiber impregnation with
blocking agents, and water repellent Agents Sealing of the matrix pore system;
Reduction of Ca(OH)2 content in the matrix; and A combination of fiber impregnation
and matrix modification.

Effect of Weathering and Scaling on Durability of

FRC
The deicer salt scaling, which its mechanics is still not clear, is merely affect a thin
layer of exposed concrete which not exceed few centimeters. It is reported that, the
present fiber and the type of fiber do not influence dicer salt scaling resistance.
Moreover, it is pointed out that, steel fibers that in contact with concrete which
suffered scaling, rusts.

Corrosion Resistance of Fiber Reinforced Concrete

Unlike ordinary reinforced concrete beam, FRC is distributed in concrete and some of
them might be close or at the surface of the concrete. Therefore, those fibers which are
not protected by concrete might corrode.

Factor that could lead to corrosion are chloride induced corrosion, corrosion because
of PH reduction in the concrete mix.

It is showed that, low carbon steel and galvanized steel fibers do not corrode in
chloride concentration that greater than 2 percent by weight. Moreover, at much
greater chloride ions, melt extracted fiber does not corrode.