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CHAPTER 1
TABLE OF CONTENTS
1.1 General 03
1.2 Composition of concrete 03
1.2.1 Cement 04
1.2.2 Aggregates 04
1.2.3 Water 05
1.3 Chemical Admixtures 05
1.4 Mineral Admixtures 06
1.4.1 Pozzolona 06
1.4.2 Metakaolin 06
1.4.3 Silica fume 07
1.4.4Ground granulated blast furnace slag 07
1.4.5 Rice husk ash 07
1.4.6 Fly ash 07
1.5 Effects of Fly Ash on Concrete
1.5.1 Workability 10
1.5.2 Heat of Hydration 10
1.5.3 Rheology of Fresh concrete 10
1.5.4 Durability and Strength 11
1.5.5 Drying shrinkage 11
1.5.6 Thermal cracking 12
1.6 Advantages of Fly ash in Concrete 12
1.7 Disadvantages 13
1.8 Types of Concrete
1.8.1 Regular Concrete 14
1.8.2 High-strength concrete 14
1.8.3 High Performance concrete 14
1.8.4 Fibre Reinforced Concrete 15
1.8.4.1 Steel Fiber Reinforced Concrete 16
1.8.4.2 Polypropylene Fibre Reinforced (PFR) concrete 16
1.8.5 Glass-Fibre Reinforced Concrete 20
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1.8.6 Hybrid fibre reinforced concrete 20
1.9 Need for the present work 21
1.10 Objectives 23
1.11 Organisation of the thesis 23
CHAPTER 1
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INTRODUCTION
1.1 General
Concrete is the most popularly used construction material in the field of civil
engineering. It is an artificial compound generally made by mixing of binding
material, fine aggregates, coarse aggregates, water and admixtures in suitable
proportions. Concrete is used as a construction material to achieve better strength,
tougher flexural structure, better workability and hence, durability. Water is absorbed
by cement, which hardens, gluing the other components together and eventually
creating a stone-like material. Concrete does not solidify from drying after mixing and
placement; the water reacts with the cement in a chemical process known as
hydration. In a concrete mix, cement and water form a paste or matrix which fills the
voids of the fine aggregates and binds them (fine and coarse) together. The matrix is
usually 22-34% of the total volume of concrete. Freshly mixed concrete before set is
known as green or wet concrete whereas after setting and hardening it is known as set
or hardened concrete. The concrete mix in the mould after curing becomes hard like
stone due to chemical actions between the water and binding materials. The increased
demand for the usage of the huge quantity of concrete leads to increase in cost of
binding material (cement) and depletion of natural sources of fine aggregate which in
turn increases cost of concrete. The supplementary cementitious materials (SCMs),
such as fly ash, ground granulated blast furnace slag (GGBS) and silica fume, as a
replacement for Portland cement in concrete present one viable solution with multiple
benefits for the sustainable development of the construction industry. The most
commonly available supplementary cementitious material (SCM) is fly ash, a
byproduct from the combustion of pulverized coal in thermal power stations. The fly
ash is abundently available all over the world.
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1.2 Composition of concrete
The concrete composition is determined initially during mixing and finally
during placing of fresh concrete. The type of structure and method of construction
determines how the concrete is placed and therefore determines the composition of
the concrete mix. The various materials used in the concrete mix proportion is
explained as follows.
1.2.1Cement
Portland cement is used as basic ingredient in concrete, mortar and plaster. It
consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement
and similar materials are made by heating limestone with clay and grinding this
product with a source of sulphate The resulting powder, when mixed with water, will
become solid sate over time.
1.2.2 Aggregates
Aggregates constitute nearly 70 to 75 % of the total volume of concrete and
are essentially inert in nature. The primary functions of aggregates to provide
concrete with a rigid skeletal structure and reduce the void space to be filled by the
cement paste. In India basalt, limestone, sandstone, granite and quartzite are
commonly used for making of concrete. River sand is used as a large scale for fine
aggregate.
Aggregates are fine and coarse and the dividing line is 4.75mm IS sieve size.
Grading is usually described in terms of cumulative percentage by mass of aggregate
passing through particular IS sieves. Coarse aggregates are described either as graded
that is having more than one size of particles, or single sized, that is mainly retained
between adjacent sieves in the upper part of the list. Fine aggregate depending on its
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fineness modulus (FM). The finenes modulus is divided into fine (FM between 2.2
to 2.6), medium (FM between 2.6 to 2.9) and coarse (FM between 2.9 to 3.2).
1.2.3Water
The w/c ratio is the one of important factor to determine the strength of
concrete. A lower w/c ratio will yield stronger concrete, but difficult to work. A
higher w/c ratio yields lowere strength concrete. Cement paste is the material formed
by combination of water and cementitious materials. Water is used for mixing of
concrete ingredients should be free from impurities. The pH value of the water shall
not be less than 6.
1.3 Chemical Admixtures
Chemical admixtures are organic or inorganic material, solid or liquid. These
can be added to the normal concrete mix, interact with the cementitious system and
modifying properties of the mix in the fresh, setting, hardened state. Water reducing
admixtures, set controlling admixtures, air-entraining admixtures, accelerating
admixtures and superplasticing admixtures are commonly sed in the concrete.
1.4 Mineral Admixtures
Mineral admixtures are divided into two groups as follows
i) Reactive minerals which are either pozzolanic or cementitious or both.
ii) Inert mineral fillers which have neither pozzolanic nor cementitious properties.
The first group materials are called as replacement for cement. They react
with calcium hydroxide in the hydrated cement paste and form complex compounds.
These complex compounds reduce the permeability and enhance the ultimate strength
and durability. These materials improve the economy of the mix. IS: 456-2000
recommends the use of fly ash, silica fume, rice husk ash, metakaoline and the ground
granulated blast furnace slag admixtures in making concrete.
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1.4.1 Pozzolona
When the pozzolana reaction takes place, the lime produced during C3S and
C2S hydration is transformed into calcium silicate hydrate. This can be considered as
the weakest link of the hydration products of Portland cement in terms of mechanical
strength properties and durability. The hydration reaction of blended cement with
pozzolan produces C-S-H gel and Sulfoaluminates. At ambient temperature, the
development of a pozzolanic reaction is much slower than the rate of Portland cement
hydration, but a water-cured concrete that contains a pozzolan has a strength that
increases and permeability that decreases with time. Pozzola categorise into two
gropus namely, artificial and natural pozzolanas. The artificial group consists of fly
ashes, silica fume, calcined clay and shales, metakaolin and rice-husk ash and the
natural gropu includes volcanic ashes, volcanic tiffs, trass and zeolites. The heating to
a temperature between 500 and 8000
C improves the pozzolanic reactivity of natural
pozzolans (Malhotra and Malhotra, 1996). The mineral admixtures used as
pozzolaonas are discussed in the following sections.
1.4.2 Metakaolin
Metakaolin is unique and not a by-product of an industrial process or entirely
natural. Metakaolin is produced by heating at temperature of about 650-900º C of
natural mineral kaolin. Metakaolin is used as a partial replacement for cement and it
may improve mechanical strength properties and the durability of concrete.
Metakaolin is used to replace 5 to 20% by weight of the cement.
1.4.3 Silica fume
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Silica fume, also known as micro silica, and it is used as a mineral admixture
in making of concrete. The usage of this material in making of high performance
concrete. It consists of very fine particles and the size of the particle is pproximately
100 times smaller than the average size of cement particle. Because of its high
fineness and rich silica , it becomes a highly effective pozzolanic material. It is used
as an addition in Portland cement concrete to improve mechanical strength properties
and durability.
1.4.4 Ground Granulated Blast Furnace Slag
Ground granulated blast furnace slag is a by-product of iron and steel plants.
GGBS is used to make durable concrete structures in combination with Ordinary
Portland Cement (OPC) and other pozzolanic materials. GGBS is a desirable
cementitious ingredient of concrete and valuable cement replacement material.
Concrete made with GGBS continues to gain strength over time. The strength of
GGBS concrete at 28 days attain double the strength over the period of 10 to 12
years.,(Piette-Claude Aitcen, 2006).
1.4.5 Rice husk Ash
It is obtained from the waste product of rice mills. It is highly reactive
pozolanic admixture. It could be produced by controlled combustion of husk
retaining silica in the non-crystalline form, with cellular structure. The fineness of the
rice husk is in the range 50-60 m2/gm.
1.4.6 Fly ash
Fly ash is used to describe the fine particles that are collected in the dedusting
system of thermal power plants. The fly ash covers a large family of powders having
approximately the same grain size distribution as Portland cement and containing
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more or less vitreceous particles. The carbon content of fly ash is very important
when using admixtures.
ASTM C618 standard recognizes two main classes of fly ashes based on the total
addition of SiO2, Al2O3 and Fe2O3.
If the value of SiO2+ Al2O3 + Fe2O3 is more than 70%, the fly ash is Class F
category.
If the value of SiO2+ Al2O3 + Fe2O3 is less than 70% and the CaO is more than
10% , the fly ash is Class C category.
The most frequently used specifications for fly ash are ASTM C 618 and
AASHTO M 295.
There are wide differences in properties within each class of fly ash. The main
difference between Class C and Class F fly ash is the chemical composition of the ash
itself. While Class F fly ash is highly pozzolanic, Class C fly ash is pozzolanic and
also can be self cementing. ASTM C618 requires that Class F fly ash contain at least
70% pozzolanic compounds (silica oxide, alumina oxide, and iron oxide), while Class
C fly ashes have between 50% and 70% of these compounds.
1.4.6.1 Class C fly ash
It is normally produced using sub-bituminous coal. Class C fly ash, in addition
to having pozzolanic properties, has some hydraulic properties. Class C fly ash con-
sists primarily of calcium alumino silicate glass, as well as quartz and tricalcium
aluminate. Class C fly ash is also referred to as high calcium fly ash because it
typically contains more than 20 percent CaO, primarily in mineral combination with
other components. Next to the mineralogical composition of the pozzolonic
admixtures, including the amount of crystalline and non-crystalline phases present,
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the present shape and size distribution determine the effect of the admixture on the
behaviour of the concrete.
Fly ash which is collected using electrostatic precipitators has a finer texture
when compared with mechanical collectors are used alone (Berry and Malhotra,
1980). Since there is no grinding step involved in the production of fly ash, as in the
preproduction of Portland cement, most of the particles occur in the form of solid
spheres of glassy material. Additionally, a small number of hollow spherical particles
exist, called cenospheres. Occasionally the cenospheres are packed with many smaller
spheres and are then called plerosphears (Mehta and Monteiro, 2006). The range of
diameter of spherical grains in between 1 and 150 micro meter, with the majority of
them under 45 micro metre (Berry and Malhotra, 1980). The average particle
diameter of cement grains is of the order of 15 to 20 micro metre while particles exist
that are several times larger (Bentz and Garboczi, 1992).
1.4.6.2 Class F fly ash
Class F fly ash is designated as per ASTM C 618. It mainly consists of
alumina and silica and loss of ignition (LOI) content is more than Class C fly ash.
Class F fly ash also contains lower calcium content than Class C fly ash. Class F fly
ash can be used as a Portland cement replacement ranging from 20-30% of the mass
of cementitious material. When using Class F fly ash as a Portland cement
replacement, it is having certain precautions like time of set may be slightly delayed,
and the early compressive strengths (before 28 days) may be decreased slightly. Also,
the fine aggregate fraction of the concrete will need to be modified because fly ash
has a lower bulk specific gravity than Portland cement, and therefore occupies a
greater volume for an equal mass.
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1.5 Effects of Fly Ash on properties of Concrete
1.5.1 Workability
First, fly ash produces more cementitious paste in the concrete mix. The
greater the percentage of fly ash “ball bearings” in the paste, the better lubricated the
aggregates are and the better concrete flows. Second, fly ash decreases the amount of
water needed to produce a given slump. Water demand of a fly ah concrete mix is
decreases by 2% to 10%. Third, fly ash reduces the amount of fine aggeagte needed in
the concrete mix to produce workability. Fly-ash creates more paste and makes the
slippery paste, so that the amount of fine aggregate portion could be reduced.
1.5.2 Heat of Hydration
The hydration of cement is an exothermic reaction. Heat is generated very
quickly and rise the concrete temperature results accelerating the strength and setting
time of the concrete. Mineral admixtures have been used replace energy consuming
Portland cement. This includes faster or slower rates of setting and hardening, lower
heat of hydration and higher ultimate strength. (Mehta, 2006)
1.5.3 Rheology of Fresh concrete
The surface characteristics and the particle size distribution of the fly ash
affect the behaviour of fresh and hardened concrete. The smooth spherical nature of
the fly ash particles enhances the workability and influences the water requirements of
the fresh concrete.(Mehta,2006). Adsorption of the small fly ash particles on the
surface of the cement grains creates electricity charged envelope results cement grains
to disperse in the mixing water. This could be a important effect on the size
distribution and structure of the individual flocs. Modification of the flocculation
behaviour could effect on the packing pattern of the system. This influences the pore
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size distribution and interconnectivity of the pores and therefore affects the
permeability of the paste at later ages (Gregory J.McCarthy, 1986).
1.5.4 Durability and Strength
Fly ash concrete is less permeability, because fly ash reduces the amount of
water needed to produce for a given slump, and creates more durable CSH as it fills
capillaries. By decreasing concrete permeability, fly ash can reduce the rate of ingress
of water, corrosive chemicals and oxygen, thus protecting steel reinforcement from
corrosion and its subsequent expansive result.
Fly ash also increases sulphate resistance and reduces alkali-silica reactivity.
Class F and class C fly ashes improve the permeability of concrete. Some Class C fly
ashes have been used to mitigate sulphate and alkali-silica expansion reactions..
Cement normally gains the great majority of its strength within 28 days, thus the
reasoning behind specifications normally requiring determination of 28-day strengths
as a standard. Typically, concrete made with fly ash will be slightly lesser in strength
when compared with cement concrete up to 28 days and gain high strength within a
year’s time. Conversely, in straight cement concrete, this lime would remain intact
and over time and it would susceptible to the effects of weathering and loss of
strength and durability.
1.5.5 Drying shrinkage
The drying shrinkage of concrete is directly affected by the amount and the
quality of the cement paste available in the mix. Drying shrinkage increases with
increase in the cement paste-to-aggregate ratio and water content of the paste. Clearly,
the water-reducing property of fly ash can be effectively used to reduce the drying
shrinkage of concrete mixtures.
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1.5.6 Thermal cracking
Thermal cracking is one of the major problems in massive structures. At
present, ordinary Portland cements produces very high heat-of-hydration at early
age when compared with normal Portland cements available 40 years ago. High-
early strength requirements in present construction practice are satisfied by
increase of cement content in the concrete mix. The micro cracks present in the
interfacial transition zone play an important role in determining mechanical strength
properties and durability properties of concrete exposed to extreme weather
conditions. This is because due fluid transport rate in the concrete is much larger than
by capillary suction. The fine particles of fly ash can be reduced by the
heterogeneous in the microstructure of the hydrated Portland-cement paste and large
crystalline products in the transition zone. With the progress of the pozzolanic
reaction, a gradual decrease in capillary pore sizes as well as crystalline hydration
products in the transition zone.
1.6 Advantages of Fly ash in Concrete
The important advantage of fly ash is to reduce permeability to water and
aggressive chemicals. Properly cured concrete made with fly ash creates a denser
product because the size of pores is reduced. The advantages of fly ash are discussed
in the following sections.
1.6.1 Fresh Concrete state
The fly ash particles are spherical in shape and same as cement grain causes a
reduction in the amount of water need for mixing and placing of concrete. This can be
transformed into better workability, resulting in sharp and distinctive corners and
edges with a better surface appearance. It benefits the precast concrete construction by
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reducing permeability. The fine particles present in fly ash help to reduce the bleeding
and segregation and improve pumpability and finishing of concrete.
1.6.2 Hardened Concrete state
Mainly the strength of concrete depends on water to cement ratio. The
reduction in water leads to improve the strength of concrete. The slow setting and low
early strength of fly ash concrete can cause reduction in the amount of fly ash used in
concrete. Some of the fly ashes could reduce early strength and retard the setting time
and some of fly ash accelerates setting time. This problem can be controlled by
addition of accelerators, plasticizer and/or a small amount of addition condensed
silica fume in the fly ash concrete mix.
The reduction of permeability and calcium oxide in selected fly ash, it should
be least effect to alkali-aggregate reaction. Sulfate attack and other chemicals attacks
are decreases when the fly ash is added to the concrete mix. Fly ash reduce the
alkalinity and permeability and to protect the concrete from chloride penetration. This
chloride penetration is the initiation of rebar corrosion. Fly ash in concrete generally
performs better when compared with plain concrete in drying shrinkage tests.
1.7 Disadvantages
Poor quality of fly ash has a negative impact on concrete. The principle
advantage of fly ash is reduced permeability at a low cost, but fly ash of poor quality
can actually increase permeability. Some fly ash such as that produced in a power
plant is compatible with concrete. Other types of fly ash must be beneficiated and
some types of fly ash cannot be improved sufficiently for use in concrete mix. High
carbon fly ash materials tend to use more water demand and the colour of concrete
becomes dark. It is not recommended to use more water and darken the concrete as
well in the construction industry.
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1.8 Types of Concrete
Various types of concrete have been developed for specialist application and
have become known by these names. The details of different concretes are discussed
in the following sections.
1.8.1 Conventional concrete
Conventional concrete is the lay term describing concrete that is produced by
mixing of cement, typically sand and coarse aggregate. This concrete compressive
strength varying from 10 N/mm2 to 40 N/mm
2. It depends on the purpose and type of
structure.
1.8.2 High-strength concrete
The concrete with a compressive strength higher than 60 N/mm2 is called as
high strength concrete. It is produced by lowering the water-cement ratio below 0.35
or lower. Low water/cement ratios and the use of silica fume makes the concrete
mixes less workability. Super plasticisers are commonly added to high-strength
concrete mixes to compensate the reduction in workability.
Care should be taken for selecting the aggregates for making of high-strength
concrete mixes. The weaker aggregates may not be strong enough to resist the loads
imposed on the concrete, and cause failure occurs in regular concrete. The design
criterion is the elastic modulus rather than the ultimate compressive strength in some
applications of high-strength concrete. For concrete consisting of hardened cement
paste and aggregates of different sizes, micro-cracks or porosity are among the
intrinsic factors which can be overcome by using fibre reinforcement.
1.8.3 High performance concrete
The American Concrete Institute (ACI) defines high-performance concrete. A
high-performance concrete is something more than is achieved on a routine basis and
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involves a specification that often requires the concrete to meet several criteria. At
present the production of high-performance concretes contain materials in addition to
Portland cement to help achieve the compressive strength and durability performance.
These materials include fly ash; silica fume and ground-granulated blast furnace slag
are used separately or in combination. At the same time, chemical admixtures are
needed to ensure that the concrete is easy to transport, place and finish. For high-
strength concretes, a combination of mineral and chemical admixtures is nearly
always essential to ensure achievement of the required strength. The water cement
ratio of high-performance concretes is 0.40 or less. Many trial mixes are usually
executed before fixing the final mix proportion.
1.8.4 Fiber reinforced concrete
Fiber reinforced concrete is a concrete which deals with fibrous substances in
the normal concrete. It increases the structural strength and cohesion. Fiber reinforced
concrete has small distinct fibers and that are dispersed randomly and oriented
haphazardly. Fibers such as steel fibers, synthetic fibers, glass fibers, and natural
fibers are used in concrete. Fiber reinforcement is used to improve/ enhance ductility
and toughness properties when compared with brittle cementitious matrices.
Inherently concrete is brittle under tensile loading and mechanical strength properties
of concrete may be improved by randomly oriented short discrete fibers. These fibres
could control the initiation, propagation, or coalescence of cracks. The performance
and character of fibre reinforced concrete depending on the properties of fibre and
concrete. The properties of fibers are fibre concentration, fiber geometry, fiber
orientation, and fiber distribution.
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1.8.4.1 Steel Fibre Reinforced Concrete
A number of steel fibre types are available as reinforcement. Round steel fibre
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 fibres having rectangular
cross sections are produced by silting the sheets about 0.25mm thick. Fibres are made
from mild steel drawn wire. Round steel fibres are produced by chopping the wire,
flat sheet fibres having a typical cross section varying from 0.15 to 0.41mm in
thickness and 0.25 to 0.90mm in width are produced by silting flat sheets. Deformed
fibre, which are loosely bounded with water-soluble glue in the form of a bundle are
also available.
1.8.4.2 Polypropylene Fibre Reinforced (PFR) concrete
In 1965, polypropylene fibres were first utilised into the concrete for the
construction of blast resistant structures for the US corps of Engineers. The
Polypropylene fibre has been developed and at present stage it is used either as short
discontinuous fibrillated material for the development of fibre reinforced concrete or
as a continuous mat for production of thin sheets. The usage of polypropylene fibres
has been rapidly increases in the construction of the reinforced concrete structures.
This is due to the addition of polypropylene fibers in concrete enhances the toughness
and also improves the flexural, impact and tensile strength of the concrete.
Polypropylene twine is cheap and abundantly available polymers. Polypropylene
fibres are more resistant to most of the chemicals and it would be cementitious matrix
which would deteriorate first under aggressive chemical attack. The melting point of
polypropylene fibres is high about 1650C. So that a working temperature as (100
0 C)
may be sustained for short periods without detriment to fibre properties.
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Polypropylene fibres being hydrophobic can be easily mixed as they do not need
lengthy contact during mixing.
I) Structure and properties of Polypropylene fibres
Polypropylene fibres are manufactured from homopolymer. Polypropylene
resin and these fibres are available in different properties with varieties of shapes and
sizes. These fibres have low modulus of elasticity and poor resistance to fire and
sensitivity to sun. These fibre are embedded in the concrete matrix provides a
protective cover and help to reduce the sensitivity to fire and other environmental
effects. Polypropylene fibres were made of high molecular weight isotactic
polypropylene. Polypropylene fibres are now available in three different geometries
namely monofilaments, film and extruded tape. The modulus of elasticity of
monofilament Polypropylene is in the range of 3-5 GPa and tensile strength in the
range 140-690 N/mm2. The chemical structure of polypropylene makes hydrophobic
with respect to the cementitious matrix, such that bonding with cement will reduce
and negatively affecting its dispersion in the matrix.
II) Composite Behaviour
Polypropylene fibres are utilizing in several different ways to reinforced
cementitious matrices. The first application of polypropylene fibres in sheet
components in which the fibre acts as primary reinforcement. The volume of fibres
are exceeding 5% are used in the production of thin sheets. The second application of
polypropylene fibres in concrete at low volume fractions. In this concrete, the fibres
act as secondary reinforcement for crack control and not for the structural load
bearing applications.
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III) Primary reinforcement in thin sheet applications
In this, the volume of fibre content exceed the critical volume in order to attain
the both strengthen and toughening. Different varieties of fibre types have been
studied, including monofilaments of various diameters, fibres with buttons on their
ends, twisted tapes and fibrillated mats. Even with smooth filament fibres, a
considerable improvement in performance obtained by increase in length and reduces
the diameter. The composites consists 6-10% by volume of fibres of monofilament
do not exceed the flexural strength 15N/mm2, while with fibrillated polypropylene
mats the flexural strength values higher than 25-30 N/mm2.
IV) Secondary reinforcement in concrete
Polypropylene fibres are commonly used at much smaller contents than the
critical volume less than 1-3%. These polypropylene fibres act as secondary
reinforcement and to control the cracking due to environmental effects. Very low
fractions less than 0.1% control the plastic shrinkage cracks of fresh concrete.
V) Fresh concrete
The fibrillated fibres are added into to the concrete, they will tend to separate
individual filaments from the parent fibrillated fibres during mixing. The dispersion of
fine fibre units in the concrete mix has significance effect on the properties of the
fresh concrete in particular on workability and plastic shrinkage cracking. As increase
in fibre content will result in reduction in the concrete consistency. This reduction is
higher for longer fibres and high fibre volume content.
VI) Plastic shrinkage crack control
Low modulus polypropylene fibres have been used for plastic shrinkage crack
control. Monofillament or fibrillated fibres at low volume fractions will effectively
reduce the shrinkage cracks. The effect of fibre on reduction of free shrinkage and
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weight loss on drying was shown to be very low and its influence stems from the
crack bridging ability. Oirv’er Fenyvesi et al., (2003) have developed a relationship
between the dosage of fibres such as polypropylene, E-glass, polyacrylnitrile. They
tested early age shrinkage cracking tendency using these fibres and effectiveness of
fibre types are compared. They reported that the relationship between the fibre
dosage and shrinkage crack is linear in case of all the fibres used.
In fibre concrete settlement cracking occurs due to its rate of bleeding and settlement
combined with restraint to settlement. In case of polypropylene fibre reinforced
concrete, fibres are uniformly distributed. The polypropylene fibers are so flexible, so
that negligible effect on the settlement of aggregates.
VII) Crack control in setting and hardened concrete
Polypropylene fibres at lower fibre volume fraction (0.1%) are not effective to
crack control of hardened concrete. But, it found that low volume fibre content lead to
decreased initial cracking of the concrete in the plastic stage and this resulted in lower
corrosion rates of steel reinforcement in the hardened concrete (Sanjuan et al.,2000 ).
For the purpose of hardened crack control, there is need to use high modulus fibres at
larger content (2%).
The compression strength of concrete has been shown to be only slightly
affected by the addition of fibers, except at very early age, under 24 hours. This is
due to the fact that polymer fibers have a lower modulus of elasticity than does
concrete once the concrete cures. Thus the fibers do not take load until the concrete
cracks. However, at early age, the concrete has a lower modulus of elasticity, and the
fibers take load. Compressive strength and tensile strength of concretes reinforced
with low modulus polypropylene fibres are not significantly different from those of
unreinforced matrix, because the volume content of fibre is below the critical volume.
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The fibre content increases, the compressive strength sometimes reduced due to
difficulty in the compacting the concrete mix.
The flexural strength was increased by using the polypropylene fibres. This
may be due to their ability to enhance the load bearing capacity in the post cracking
zone. The increase in the flexural strength will marginally improve 10-20% when the
specimens will test in air dry. Low fibre volume fraction enhances the energy
absorption capacity of the composite in tension or flexure, as can be observed in the
both static and impact. The fibre content increases the flexural tensile strength
increases. It is also observed that there was increase strength for with aspect ratio
increases. The addition of polypropylene fibres in the concrete increases the splitting
tensile strength by approximately 20% to 50%.
1.8.5 Glass-Fibre Reinforced Concrete
Glass fibre is made up from 200-400 individual filaments which are lightly
bonded to make up a stand. Using the conventional mixing techniques for normal
concrete it is not possible to mix more than about 2% (by volume) of fibres of a
length of 25mm. Glass fibre can be used in reinforcing the cement or mortar matrices
used in the production of thin-sheet products. The commonly used verities of glass
fibres are e-glass used. In the reinforced of plastics & alkali resistant (AR) glass E-
glass has inadequate resistance to alkalis present in Portland cement where AR-glass
has improved alkali resistant characteristics.
1.8.6 Hybrid fibre reinforced concrete:
The addition of two or more fibre types into concrete, can offer more attractive
engineering properties as the presence of one fibre enables more efficient utilization
of the potential properties of other fibre. Hybrid fibres of variable sizes and types may
play important roles in resisting cracking at different scales, and results high
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performance. The incorporation of fibre into cementitious materials can improve their
toughness and ability of resisting crack. Steel fibre has a considerably control on
crack control, although the volumetric density is high.
They provide a system in which type of fibre, which is stronger and stiffer,
improves the first crack stress and ultimate strength, and the second type of fibre,
which is more flexible and ductile, leads to the improved toughness and strain
capacity in the post cracking zone. The presence of the durable fibre can cause
increase the strength and toughness after certain age while another type is to
guarantee the short-term performance.
1.9 Need for the present work:
“Sustainable development”, the catch phrase of the modern world from the
one and half decade had its impact on all the fields and frontiers known to the
mankind. Consumptions of large quantities of virgin material coupled with major
contribution to the green house gasses in the form of cement production made the
construction industry highly unsustainable. In this scenario, the supplementary
cementitious materials (SCMs), such as fly ash, GGBS and silica fume, as a
replacement for Portland cement in concrete present one viable solution with multiple
benefits for the sustainable development of the concrete industry. The most
commonly available SCM worldwide is fly ash, a byproduct from the combustion of
pulverized coal in thermal power stations.
In order to considerably increase the utilization of fly ash that otherwise is
being wasted and to have a significant impact on greenhouse gas emissions, it is
necessary to use concrete that will incorporate large amounts of fly ash as
replacement for cement. This is particularly important issue for developing country
like India. The production of fly ash is about 131 million tons per year during 2011-12
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from 88 thermal power plants available in India (Md Emamul Haque,2013). This fly
ash content is expected to increase to 300-400 million tons per year by 2016-17.
Hence there is an urgent need to utilize the fly ash content in relatively large
proportions. Generally, the percentage of fly ash as part of the total cementing
materials in concrete normally ranges from 15 to 25%, although it can go up to 30-
35% in some applications. The utilization of fly ash is 55.79% against total
production during the year 2010-12 in India. This could results the gain of acceptance
for utilization of large proportions of fly ash in structural and non- structural
applications in India. The main aspects of the concrete performance that will be
improved by the use of fly ash are increased long-term strength and reduced
permeability of the concrete resulting in potentially better durability. The use of fly
ash in concrete can also address some specific durability issues such as sulphate
attack and alkali silica reaction. However, a few additional precautions have to be
taken to insure that the fly ash concrete will meet all the performance criteria.
Though a lot of research is focused on use of various admixtures in producing
high strength concrete, very little information is available on using Class C fly ash as
admixture in the concrete. The utilization of the polypropylene fibres in different
volume fraction in the fly ash concretes for improvement in the strength and
durability aspects need to be investigated. The limited research available in the
utilization of class C fly ash with polypropylene fibres with variable aggregate binder
ratio encourages the present study. Hence, there is need to study the strength and
durability characteristics of concrete produced with Class C fly ash admixture with
polypropylene fibres with different volume fractions.
The overall objective of this study is to determine and enhance the behaviour
of mechanical characteristics and durability properties of polypropylene fibre
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reinforced concrete containing large volumes of Class C fly ash with different
aggregate binder ratios. The objectives of this present study are listed in the next
section.
1.10 Objectives
To establish the optimum cement, fly ash, water content to yield concrete with
high strength.
To study the compressive strength development of concrete mixtures containing
different levels of Class C fly ash replacement with variable water binder ratios at
the ages 28, 56 and 90 days.
To choose an optimum fly ash /cement ratio based on the compressive strength of
various aggregate binder ratios.
To test the strength properties of the concrete mixture which include compressive
strength, split tensile strength, flexural strength and modulus of elasticity at
different ages of curing.
To test the durability properties of the concrete mixtures using Rapid chloride
permeability test.
To develop an Artificial Neural Network model to predict the compressive
strength, split tensile strength flexural strength and rapid chloride penetration of
concrete mixtures containing various fly ash replacements with variable aggregate
/binder ratio.
1.11. Organisation of the Thesis
A detailed review of literature and the literature summary has been presented
in chapter 2. Experimental investigations including the mechanical and durability tests
conducted on the specimens and the test results for various mix proportions are
presented in chapter 3. The test results of compressive strength, split tensile strength,
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flexure strength and modulus of elasticity have been discussed in detailed manner in
chapter 4. The test results of rapid chloride penetration test have been discussed in
detailed manner in chapter 5. The modeling of the experimental results being done
using the software tool artificial neural network (ANN) for the estimation of
compressive strength, split tensile strength, flexural strength and chloride penetration
for all the concrete mixes and is presented in chapter 6. The final conclusions based
on the experiments and recommendations are presented in chapter 7.
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