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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.