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Chapter No. 1

INTRODUCTION

1.0 BACKGROUND OF THE STUDY

The construction industry has taken considerable strides forward over the last two or three decades with regard to many materials, in particular - High Strength Concrete (HSC) and generally Higher Performing Concrete Materials.

The development of new technology in the materials sciences is progressing rapidly. Advanced composite construction materials and HSC are gaining wide acceptance in the construction industry of today, and are well positioned for increasing proliferation in use in the future. HSC and High Performance Cement-Based Products will continue to make important contributions to the enhanced quality and efficiency in the construction of infrastructure and our communities in coming decades.

Today, most concrete producers worldwide recognize the value of pozzolanic enhancements to their products and, where they are available; they are becoming a basic, even a routine, concrete ingredient.Most pozzolans used in the world today are byproducts from other industries, such as coal fly ash, blast furnace slag, rice hull ash, or silica fume a byproduct of semiconductor industry. As such, there has been relatively little work done with regard to manufactured, optimized and engineered pozzolanic materials which are specifically intended for use in Portland cement-based formulations.

Various naturally occurring materials possess, or can be processed to possess pozzolanic properties. These materials are also covered under the standard specification ASTM C618. Natural pozzolans such as metakaolin and calcined shale or clay are manufactured by controlled calcining (firing) of naturally occurring minerals. Metakaolin is produced from relatively pure kaolinite clay and it is used at 5% to 15% by mass of the cementitious materials. Other natural pozzolans include volcanic glass, zeolitic trass or tuffs, rice husk ash and diatomaceous earth. Among many naturally occurring pozzolans Silica Fume has proven itself on the top and has attained great deal of attention in construction industry over the last few years.

1.1 AIM OF THE RESEARCH To determine the strength properties of concrete produced with Silica fume as admixtures. Although knowledge of pozzolanic concrete is available and a few studies of their characterization have been conducted, their behavior as renders deserves a further insight.

With this intention, Silica Fume a byproduct of semiconductor industry was incorporated into cement-based concrete and testing campaigns were developed to determine its performance when different proportions and curing conditions were used.

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1.2 OBJECTIVES OF THE RESEARCH

1. To determine the effect of addition of Silica fume as a pozzolan on the strength of concrete.

2. To correlate using the means of statistics, the extent to which Silica Fume as pozzolan can improve the strength of concrete.

3. To make recommendations based on the findings of the research work.

1.3 SCOPE OF THE RESEARCH WORK The research work will cover the use of Silica Fume at 1%, 5%, 10%, 20%, 30%, and 40% replacement of Portland cement in the concrete mix. This will give an in-depth knowledge of the affect of varying amounts of silica fume on the compressive strength of the concrete.

1.4 NEED FOR THE RESEARCH Portland cement, the most common building material, is expensive because of the large amount of energy involved in producing it. It must be fired at high temperatures and the transportation costs are high. The need for a viable alternative is immense, since much of the population in developing countries lives in inadequate housing. The prospect of reducing the cost hinges on reducing the price of building materials. Pozzolan cement is proving to be one answer. Moreover, the cement producing industry is one major source of global warming due to which hazardous affects have been produced on our environment. Using pozzolans we can reduce the use of cement in construction.

1.5 METHODOLOGY OF THE RESEARCH The methodology to be used for the work includes the followings,

1. Review of related literatures 2. Review of the previously held research work and industrial findings 3. Casting of Cylinders 12” x 6” diameter containing the constituent’s materials

(Aggregates, different proportion of Pozzolan, Cement, Sand, water, Air).

The curing of the cylinders is done for 7, 14 and 28 days respectively. The crushing strength of the cylinders is tested after 7, 14 and 28 days so as to determine the respective compressive strength of specimen, having a specific quantity of pozzolan.

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Chapter No. 2

LITERATURE REVIEW

2.1 DEFINITION OF POZZOLAN

A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementations properties. Pozzolans are commonly used as an addition (the technical term is "cement extender") to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete and in some cases reduce the material cost of concrete. Pozzolans are primarily vitreous siliceous materials which react with calcium hydroxide to form calcium silicates; other cementitious materials may also be formed depending on the constituents of the pozzolan.

A pozzolan is a siliceous or aluminosiliceous material, which is highly vitreous. This material independently has few/fewer cementitious properties, but in the presence of a lime-rich medium like calcium hydroxide, shows better cementitious properties towards the later day strength (> 28 days). The mechanism for this display of strength is the reaction of silicates with lime to form secondary cementitious phases (calcium silicate hydrates with a lower C/S ratio) which display gradual strengthening properties usually after 7 days.

The pozzolanic reaction may be slower than the rest of the reactions that occur during cement hydration, and thus the short-term strength of concrete made with pozzolans may not be as high as concrete made with purely cementitious materials; conversely, highly reactive pozzolans, such as silica fume and high reactivity metakaolin can produce "high early strength" concrete that increase the rate at which concrete gains strength.

2.2 POZZOLANIC REACTION

At the basis of the Pozzolanic reaction stands a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)2), and silicic acid (H4SiO4, or Si(OH)4).

For simplifying, this reaction can be schematically represented as following:

Ca(OH)2 + H4SiO4 —> Ca2+ + H2SiO42- + 2 H2O —> CaH2SiO4 · 2 H2O

Or summarized in abbreviated notation of cement chemists,

CH + SH —> CSH

The product of general formula (CaH2SiO4 · 2 H2O) formed is a calcium silicate hydrate, also abbreviated as CSH in cement chemist notation. The ratio Ca/Si, or C/S, and the number of water molecules can vary and the here above mentioned stoichiometry may differ.

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http://en.wikipedia.org/wiki/Metakaolin

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2.3 HISTORY OF POZZOLAN

It is stated in the literature that there are 1282 volcanoes in the world considered to have been active in the past ten thousand years, and only 3 of these volcanoes deposited high quality natural pozzolan. The first one is Santorini Volcano, Greece, which erupted during 1600 BC - 1500 BC. Mt. Vesuvius, Italy, is the second volcano which erupted in AD 79. Pozzolan was named after the town of Pozzoli where it was deposited. The third, Mt. Pagan is the only one which has erupted in modern times.

Scientists have proven that the ancient Greeks began to use natural pozzolan-lime mixtures to build water-storage tanks sometime between 700 BC and 600 BC. This technique was then passed on to the Romans about 150 BC. According to Roman engineer Vitruvius Pollio who lived in the first century BC: "The cements made by the Greeks and the Romans were of superior durability, because neither waves could break, nor water dissolve the concrete."

Many great ancient structures, such as the Colosseum, the Pantheon, the Bath of Caracalla, as well as other structures that are still standing in Italy, Greece, France, Spain and the islands in the Mediterranean Sea, were built with natural pozzolan-lime mixtures. Many of them have lasted more than two thousand years.

After the invention of Portland cement, natural pozzolan was used as a concrete strengthening additive to improve characteristics, such as durability, compressive strength, chemical resistance, hydration heat, permeability, etc. In Europe and the USA, there have been numerous high rise buildings, highways, dams, bridges, harbors, canals, aqueducts and sewer systems built with natural pozzolan-cement mixtures. Due to the limited supply of high quality natural pozzolan, in the last 30 years or so, the USA, Europe and other countries have resorted to the use of more readily available but poorer quality, waste materials such fly ash that can be used as a substitute for natural pozzolan.

2.4 USE OF POZZOLANS

Pozzolans can be added to cement during the production process or mixed directly into concrete. Previous studies (canpolat et al .2003;cavdar & yetgin 2004b) say that harmful materials in cements such as CaO is less than 2% & the ratio of mgo is less than 5% 7 furthermore, it is known that addition of natural pozzolan materials decreased the amount of “harmful materials” by reaction with them, depending on the fineness, of the natural pozzolans

In addition, natural pozzolans can fill the cement matrix & increase the durability of cements significantly by changing the framework of the matrix. Natural pozzolans also have lubricant effects on cement mortar or concrete because of their fine grain size; they improve the consistency & thus workability conditions of the concrete. However, besides this, natural

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pozzolans can be expected to increase water requirement a little, since they increase the total cement surface.

Studies also state that in mortars & concrete, where Portland cement & natural pozzolan are applied together as a, setting times depend upon the replacement amount, fineness, & reactivity of pozzolan compared with Portland cement.

The mechanism by which pozzolanic reaction exercises a beneficial effect on the properties of concrete is the same irrespective of whether a pozzolanic material is added to concrete in the form of a mineral admixture or as a component of blended Portland cement. Whenever a pozzolanic & or cementitious by product can be used as a partial replacement for a Portland cement in concrete, it represents significance energy & cost saving. Mineral admixtures are finely divided siliceous materials which are added to concrete in relatively large amounts, generally in the range 20 to 70 percent by mass of the total cementitious material.

Power plants using coal as fuel & metallurgical furnaces producing cast iron, silicon, metal, & ferrosilicon alloys are the major sources of by products that are being produced at the rate of millions of tones every year in many countries. Dumping of these by-products into landfills & streams amount to a waste of the material & causes serious environmental pollution. Countries like china, India, the United States, Russia, Germany, South Africa, & the kingdom, are among the biggest producers of fly ash which, in the year 2000, rate of production, some, some 500 million tones year constitutes the largest industrial waste product in the world.

2.5 EARLY TYPES OF POZZOLANS

The first known pozzolan was pozzolana, a volcanic ash, for which the category of materials was named. The most commonly used pozzolan today is fly ash, though silica fume, high-reactivity metakaolin, ground granulated blast furnace slag, and other materials are also used as pozzolans.

2.6 FACTORS CONTROLLING THE STRENGTH DEVELOPED

The extent of the strength development depends upon the chemical composition of the pozzolan: the greater the composition of alumina and silica along with the vitreous phase in the material, the better the pozzolanic reaction and strength display.

2.7 ADVANTAGES OF THE NATURAL POZZOLAN

2.7.1 LITHIFICATION

Once the Natural pozzolan-lime mixture is hydrated, the pozzolanic reaction begins immediately and continues for many years. Eventually, the mass will reach complete lithification, forming a rocky material similar to plagioclase with some content of magnetite. The compressive strength as well as the flexural strength will continue to increase for a long time. This unique

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characteristic is one of the main reasons many great ancient structures have lasted for over two thousand years.

2.7.2 AUTOGENOUS HEALING

A unique characteristic of Natural pozzolan is its inherent ability to actually heal or re-cement cracks within the concrete by means of the continuation of pozzolanic reaction with the calcium hydroxide freed from the cement hydration reaction. This results in the filling up of most of the gaps inside the hardened concrete matrix.

2.7.3 REDUCED PERMEABILITY AND VOIDS

The leaching of water-soluble calcium hydroxide produced by the hydration of Portland cement can be a significant contributor to the formation of voids. The amount of "water of convenience" used to make the concrete workable during the placing process creates permeable voids in the hardened mass. Natural pozzolan can increase the fluidity of concrete without "water of convenience," so that the size and number of capillary pores created by the use of too much water can be minimized.

2.7.4 REDUCES EXPANSION AND HEAT OF HYDRATION Experiments show that replacing 30% Portland cement with Natural pozzolan can reduce the expansion and heat of hydration to as low as 40% of normal. This may be because there is no heat produced when Natural pozzolan reacts with calcium hydroxide and that the free calcium oxide in the cement can hydrate with natural pozzolan to form C-S-H. Natural pozzolan decreases the heat generated by cement hydration and delays the time of peak temperature. The graphic pattern of Natural pozzolan - Portland cement mixture is extended longer and lowers to form a much more moderate curve than the heat of hydration curve of Portland cement itself.

2.7.5 REDUCES CREEP AND CRACKS

While concrete is hardening, the "water of convenience" dries away. The surface of the hardening mass then begins to shrink as the temperature goes down from outside. This results in the formation of creep and cracks. Natural pozzolan moderates the expansion and shrinkage of concrete. It also helps to lower the water content of the fresh concrete. Therefore, the creep and cracks can be significantly reduced without the process of water cooling.

2.7.6 REDUCES MICRO CRACKING

The expansion and shrinkage mentioned above also create micro cracks inside the hardened C-S-H paste and in-between the aggregate and the C-S-H paste. These micro cracks significantly contribute to concrete permeability as well as other concrete defects. The Natural pozzolan- Portland cement mixture expands these shrinks so moderately that there is no microcracking inside the C-S-H paste after drying. E

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2.7.7 INCREASES COMPRESSIVE STRENGTH

The pozzolanic reaction between natural pozzolan and calcium hydroxide happens after the C3S and C2S in the cement begins to hydrate. At the early stage of curing, 30% Natural pozzolan substituting Portland cement mixture is slightly lower than reference OPC [Ordinary Portland Cement} in regard to compressive strength. As time goes by, natural pozzolan continues to react with the calcium hydroxide produced by cement hydration and increases the compressive strength by producing additional C-S-H. After 21 curing days, the 30% Natural pozzolan/ 70% Portland cement mixture begins to exceed reference OPC in compressive strength. After 28 days, it exceeds reference OPC by about 15%. The pozzolanic reaction continues until there is no free calcium hydroxide available in the mass and the compressive strength exceeds the reference OPC by 30-40%.

2.7.8 INCREASES RESISTANCE TO CHLORIDE ATTACK

Concrete deterioration caused by the penetration of chloride occurs quickly when chloride ions react with calcium. The expansion of hydrated calcium oxy-chloride enlarges the micro cracks and increases the permeability that causes quicker chloride penetration and more damage from freezing and thawing action. The 30% Natural pozzolan added into cement can react with almost all the free calcium hydroxide and form a much denser past. Thus, the penetration of chloride can be minimized and the few penetrated chloride ions cannot find free calcium hydroxide with which to react.

2.7.9 INCREASES RESISTANCE TO SULFATE ATTACK There are three chemical reactions involved in sulfate attack on concrete: 1) Combination of free calcium hydroxide and sulfate to form gypsum (CaSO4-2H2O). 2) Combination of gypsum and calcium aluminate hydrate (C-A-H) to form ettringite (C3A-3CaSO-32H2O). 3) Combination of gypsum and calcium carbonate with C-S-H to form thaumasite (CaCO3-CaSiO3-CaSO4-15H2O).

All these reactions result in the expansion and disruption of concrete. Thaumasite in particular is accompanied by a very severe damaging effect which is able to transform hardened concrete into a pulpy mass.

2.7.10 REDUCES ALKALI-AGGREGATE REACTION Because Natural pozzolan is shattered into such a fine particle size resulting in dramatically increased reactive surface area, it can react quickly with calcium hydroxide and can trap the alkali inside the cement paste. Thus, it helps to form a denser paste with almost no alkali aggregate reaction at all.

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2.7.11 PROTECTS STEEL REINFORCEMENT FROM CORROSION

The preceding discussions make it very clear that concrete made from 30% Natural pozzolan/ 70% Portland cement mixture can protect steel reinforcement because it creates an environment so densely packed that no liquids or gases can penetrate through it to cause corrosion to the steel.

2.7.12 INCREASES ABRASION RESISTANCE

Natural pozzolan increases the compressive strength of concrete and makes the concrete matrix stronger and denser. It also prevents the formation of pulpy, crispy, or water-soluble materials created by chemical attack. Therefore, it helps the concrete to durably resist abrasion.

2.7.13 LOWERS WATER REQUIREMENT WITH HIGH FLUIDITY, SELF-LEVELING, AND COMPRESSION

In normal operations, the bulk volume of concrete in the constructions are placed and compacted by use of high frequency poke vibrators. The rapid vibration induces segregation phenomena of all orders of magnitude in the fresh concrete, e.g., stone segregation, internal bleeding giving bonding failures, and inhomogeneous cement paste and air-void systems. Under proper use of vibratory compaction, Natural Pozzolan minimizes or eliminates these problems due to the amorphous structure of the pozzolan particles. 2.7.14 IMPROVES DURABILITY

The benefits and characteristics of Natural Pozzolan mentioned above clearly explain why the ancient structures built by the Greeks have survived over 2000 years of weathering.

A wide variety of environmental circumstance are deleterious to concrete, such as reactive aggregate, high sulfate soils, freeze-thaw conditions, exposure to salt water, deicing chemicals & acids. Typically, these problems have been partially overcome by utilizing special cements, increasing strength, & or minimizing water/cement ratios. But there now exists an overwhelming body of laboratory research & field experience showing that the careful use of pozzolans is useful in countering all of these problems ; pozzolans is not just a “filler” as many engineers think, but a strength &performance improving additive. In general terms, the silicious pozzolans react with the calcium silicate hydrates that yield higher strength & dramatically reduced permeability.

2.8 THE EFFECTS ON ENVIRONMENT

Portland cement requires a significant amount of heat in its manufacture, making it expensive not just to the consumer, but to the atmosphere as well. As mentioned earlier, for every ton of cement produced, roughly one ton of car bon dioxide is released by thr burning fuel, & an additional one ton is released in the chemical reaction that changes the raw material to clinker, making production of cement.

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2.9 OBJECTIVES

In the past few years it has been discovered that pozzolan in proper properties can contribute a variety of desirable properties to concrete, while the undesirable properties contributed, such as somewhat slower development of compressive strength, are almost of negligible importance, except in case of special types of construction.

In spite of generally beneficial effects of pozzolans in concrete, its use has been limited for several reasons:

1. Lack of knowledge among builders concerning the properties induced in concrete by pozzolan.

2. Local unavailability of pozzolan on retail basis.3. Inconvenience to ready mix dealers and contractors of storing and batching an additional

component in concrete.4. Variations among pozzolan and incomplete research for determining properties induced

and hence the proper mix properties desirable.

In this research study various types of pozzolanic materials in Pakistan will be discussed from existing literature. The main objective of this study is to examine the effects of the change in pozzolan content upon the compressive strength of the concrete containing pozzolan. The specimens will be prepared by fixing a definite water cement ratio and other parameters, but changing the percentage of the pozzolan.

The following laboratory tests will be performed:

i. Age versus compressive strength, for concrete containing no pozzolan.ii. Age versus compressive strength, for 1% addition of Pozzolan for cement.

iii. Age versus compressive strength, for 5% addition of Pozzolan for cement.iv. Age versus compressive strength, for 10% addition of Pozzolan for cement.v. Age versus compressive strength, for 20% addition of Pozzolan for cement.

vi. Age versus compressive strength, for 30% addition of Pozzolan for cement.vii. Age versus compressive strength, for 40% addition of Pozzolan for cement.

Compressive strength of concrete will be tested at 7 days, 14 days, and 28 days.

It is hoped that the research for this thesis may help fill in some of the gaps of knowledge as indicated by 1 and 4 reasons listed above, and that use of pozzolan, where beneficial to concrete, may be furthered because of that information presented here.

2.10 EXAMPLES OF COMMERCIAL CEMENTITIOUS AND POZZOLANIC MATERIALS

The most commonly used additives to manufacture blended cement today are waste products from industry, which are described in generally descending order of quality as: -

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2.10.1 HIGH-CALCIUM FLY ASH (CLASS C)

HCFA is the residue collected from the smokestacks of coal-fired power plants generally using lignite and/or sub bituminous coals. Class C fly ashes are in themselves mildly cementitious, and have been combined with lime or even calcium carbonate soils to produce moderately strong concrete.

2.10.2 GROUND GRANULATED BLAST FURNACE SLAG

GGBFS is the ground residue from iron smelters, and is also mildly cementitious in itself, but hugely pozzolanic in combination with water and cement. Its usage dates back more than 200 years and is widely used in Europe..2.11 EXAMPLES OF HIGHLY ACTIVE POZZOLANS

2.11.1 CONDENSED SILICA FUME

CSF is a waste product of the silicon metal industry, and is a super-fine powder of almost pure amorphous silica. Though difficult (and expensive) to handle, transport, and mix, it has become the chosen favorite for very high-strength concretes (such as for high rise buildings). It is often used in combination with both cement and fly ash.

2.11.1 RICE HULL (OR HUSK) ASH

RHA is the least known of the four pozzolans discussed, but considered promising on a global scale. The world's primary staple crop is rice, the milling of which generates 100 million tons of hulls, or chaff, annually. Like the straw, hulls have historically been burned in the fields, but the resulting pollution is increasingly causing health problems.Research in India and the United States has found that if the hulls (or straw) are burned at a controlled low temperature, the ash collected can be ground to produce a pozzolan very similar to (and in some ways superior to) silica fume.

2.12 EXAMPLES OF NORMAL POZZOLANS

2.12.1 LOW-CALCIUM FLY ASH (CLASS F AND GENERALLY LESS THAN 10% CAO)

LCFA is the residue collected from coal-fired power plants using anthracite and/or bituminous coals. Though, generally less effective than the class C fly ashes, class F fly ash is nevertheless an abundant and useful pozzolan.

2.12.2 VOLCANIC ASH

Pozzolanic materials are produced as a result of volcanic activity when silica rich magma meets with large quantities of underground water in volcanic conduits. Under high temperature and pressure the steam reacts with the dissolved carbon dioxide and sulfur gases and during the E

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volcanic eruption the magma produces air borne cinder-like material with excellent pozzolanic characteristics. As mentioned earlier, although rare, this type of pozzolan was extensively used prior to the discovery of Portland cement in Italy and Greece.In the USA and some other countries many experts in concrete research are now recommending the increase use of pozzolans, especially the readily available fly ash. In light of economic conditions and the abundant evidence both from research and in the field, it seems inevitable that regular and high-volume usage of pozzolans will become standard practice in the concrete industry.

2.13 SPECIFICATIONS OF POZZOLAN MATERIALS FOR USE IN THE CEMENT INDUSTRY

Because of the importance of quality control on cements used in the construction industry, standard tests and certification requirements have been developed over the years in Europe, USA and other industrialized nations of the world. The most widely used set of standards in the world today are those developed by the ASTM organization in the USA. To meet the accepted standards of chemical and physical properties of Portland cement and supplementary cementitious materials, the cement types must meet defined specifications before they can be used. The most important standards used in the cement industry today are summarized below.

2.14 PAKISTANI COMPANIES

The use of additives and admixtures is very low in the Pakistani construction industry. There are very few firms which prefer to use these materials in civil works. Most of the companies are still relying on the traditional materials of construction. Times have changed and the world has turned on its head. New techniques are being implemented throughout the world. New materials are being used for more effective and durable construction. So there is an urgent need for the revival of the construction sector in Pakistan. Nevertheless some of the Pozzolans which are extensively used in Pakistan include,

1. Silica Fume2. Rice Husk 3. Fly Ash

Silica fume is mainly used in the construction industry while the use of the other two classes of the pozzolan is very limited. There are very few chemical companies which are manufacturing and /or importing the pozzolans from the other countries. Because of the high costs of manufacturing and import the prices of the pozzolans available in Pakistan are quite high. For this reason many engineers do not prefer to use pozzolans with the cement as it affects the economy of the construction. The rate of silica fume at present is 60 Rs/ Kg. whereas the cement can be bought at a much cheaper rate. This indifference in the price is a major cause of reluctance by the engineers for pozzolans. Some companies which have got reserves of pozzolans and deals with pozzolan are SIKKA and IMPORIUM CHEMICALS PRIVATE LIMITED which are based in Lahore. E

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Chapter No. 3

SILICA FUME

3.1 DEFINITION OF SILICA FUME

Silica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica. It is sometimes confused with fumed silica (also known as colloidal silica and pyrogenic silica).

These materials have different derivations, technical characteristics, and applications. Silica fume is a byproduct in the reduction of high-purity quartz with coke in electric arc furnaces in the production of silicon and ferrosilicon alloys. Because of its extreme fineness and high silica content, silica fume is a very effective pozzolanic material.

3.2 COMPOSITION OF SILICA FUME

Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). The individual particles are extremely small, approximately 1/100th the size of an average cement particle. Because of its fine particles, large surface area, and the high SiO2 content, silica fume is a very reactive pozzolan when used in concrete. The quality of silica fume is specified by ASTM C 1240 and AASHTO M 307.

Silica Fume consists of very fine vitreous particles with a surface area ranging from 60,000 to 150,000 ft^2/lb or 13,000 to 30,000 m^2/kg when measured by nitrogen absorption techniques, with particles approximately 100 times smaller than the average cement particle. Because of its extreme fineness and high silica content, Silica Fume is a highly effective pozzolanic material (ACI Comm. 226 1987b; Luther 1990).

3.3 USES OF SILICA FUME:

High-strength concrete is a very economical material for carrying vertical loads in high-rise structures. Until a few years ago, 6,000 psi concrete was considered to be high strength. Today, using silica fume, concrete with compressive strength in excess of 15,000 psi can be readily produced.

The greatest cause of concrete deterioration in the US today is corrosion induced by deicing or marine salts. Silica-fume concrete with low water content is highly resistant to penetration by chloride ions. More and more transportation agencies are using silica fume in their concrete for construction of new bridges or rehabilitation of existing

3.4 ADVANTAGES OF ADDING SILICA FUME

Silica fume is added to Portland cement concrete to improve its properties, in particular its compressive strength, bond strength, and abrasion resistance. These improvements stem from

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FIGURE 3.1 (THE FIGURE ABOVE SHOWS THE SILICA FUME AS USED IN THE PROJECT)

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both the mechanical improvements resulting from addition of a very fine powder to the cement paste mix as well as from the pozzolanic reactions between the silica fume and free calcium hydroxide in the paste.

Addition of silica fume also reduces the permeability of concrete to chloride ions, which protects the reinforcing steel of concrete from corrosion, especially in chloride-rich environments such as coastal regions and those of humid continental roadways and runways (because of the use of deicing salts) and saltwater bridges.

3.5 ENVIRONMENTAL IMPACT

Prior to the mid-1970s, nearly all silica fume was discharged into the atmosphere. After environmental concerns necessitated the collection and land filling of silica fume, it became economically viable to use silica fume in various applications, in particular high-performance concrete.

3.6 SPECIFICATIONS

The first national standard for use of Silica Fume ("microsilica") in concrete was adopted by AASHTO in 1990 (AASHTO Designation M 307-90). The AASHTO and ASTM C 1240 covers microsilica for use as a mineral admixture in PCC and mortar to fill small voids and in cases in which pozzolanic action is desired. It provides the chemical and physical requirements, specific acceptance tests, and packaging and package marking.

2.7 MIX DESIGN

Silica Fume has been used as an addition to concrete up to 15 percent by weight of cement, although the normal proportion is 7 to 10 percent. With an addition of 15 percent, the potential exists for very strong, brittle concrete. It increases the water demand in a concrete mix; however, dosage rates of less than 5 percent will not typically require a water reducer. High replacement rates will require the use of a high range water reducer.

3.8 EFFECTS ON VARIOUS FACTORS IN CONCRETE OF THE SILICA FUME

3.8.1 EFFECTS ON AIR ENTRAINMENT AND AIR-VOID SYSTEM OF FRESH CONCRETE

The dosage of air-entraining agent needed to maintain the required air content when using Silica Fume is slightly higher than that for conventional concrete because of high surface area and the presence of carbon. This dosage is increased with increasing amounts of Silica Fume content in concrete (Admixtures and ground slag 1990; Carette and Malhotra 1983).

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3.8.2 EFFECTS ON WATER REQUIREMENTS OF FRESH CONCRETE

Silica Fume added to concrete by itself increases water demands, often requiring one additional pound of water for every pound of added Silica Fume. This problem can be easily compensated for by using HRWR (Admixtures and ground slag 1990).

3.8.3 EFFECTS ON CONSISTENCY AND BLEEDING OF FRESH CONCRETE

Concrete incorporating more than 10% Silica Fume becomes sticky; in order to enhance workability, the initial slump should be increased. It has been found that Silica Fume reduces bleeding because of its effect on rheologic properties (Luther 1989).

3.8.4 EFFECTS ON STRENGTH OF HARDENED CONCRETE

Silica Fume has been successfully used to produce very high-strength, low-permeability, and chemically resistant concrete (Wolseifer 1984). Addition of Silica Fume by itself, with other factors being constant, increases the concrete strength.

Incorporation of Silica Fume into a mixture with HRWR also enables the use of a lower water-to-cementitious-materials ratio than may have been possible otherwise (Luther 1990). The modulus of rupture of Silica Fume concrete is usually either about the same as or somewhat higher than that of conventional concrete at the same level of compressive strength (Carette and Malhotra 1983; Luther and Hansen 1989).

3.8.5 EFFECTS ON FREEZE-THAW DURABILITY OF HARDENED CONCRETE

Air-void stability of concrete incorporating Silica Fume was studied by Pigeon, Aitcin, and LaPlante (1987) and Pigeon and Plante (1989). Their test results indicated that the use of Silica Fume has no significant influence on the production and stability of the air-void system. Freeze-thaw testing (ASTM C 666) on Silica Fume concrete showed acceptable results; the average durability factor was greater than 99% (Luther and Hansen 1989; Ozyildirim 1986).

3.8.6 EFFECTS ON PERMEABILITY OF HARDENED CONCRETE

It has been shown by several researchers that addition of Silica Fume to concrete reduces its permeability (Admixtures and ground slag 1990; ACI Comm. 226 1987b). Rapid chloride permeability testing (AASHTO 277) conducted on Silica Fume concrete showed that addition of Silica Fume (8% Silica Fume) significantly reduces the chloride permeability. This reduction is primarily the result of the increased density of the matrix due to the presence of Silica Fume (Ozyildirim 1986; Plante and Bilodeau 1989).

3.8.7 EFFECTS ON ASR OF HARDENED CONCRETE

Silica Fume, like other pozzolans, can reduce ASR and prevent deletrious expansion due to ASR (Tenoutasse and Marion 1987). E

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3.9 AVAILABILITY AND HANDLING

Silica Fume is available in two conditions: dry and wet. Dry silica can be provided as produced or densified with or without dry admixtures and can be stored in silos and hoppers. Silica Fume slurry with low or high dosages of chemical admixtures are available. Slurried products are stored in tanks with capacities ranging from a few thousand to 400,000 gallons (1,510 m3) (Admixtures and ground slag 1990; Holland 1988).

3.10 REASON FOR SELECTING SILICA FUME

The reason for selecting silica fume among all the other pozzolans was that it was easily available and it is more used in the construction sector of Pakistan.

REFERENCES

Sections of this document were obtained from the Synthesis of Current and Projected Concrete Highway Technology, David Whiting, . . . et al, SHRP-C-345, Strategic Highway Research Program, National Research Council.

ACI Committee 226. 1987b. Silica fume in concrete: Preliminary report. ACI Materials Journal March-April: 158-66.

Admixtures and ground slag for concrete. 1990. Transportation research circular no. 365 (December). Washington: Transportation Research Board, National Research Council.Bunke, D. 1988. ODOT's experiences with silica fume (microsilica) concrete. 67th annual meeting of the Transportation Research Board, paper no. 870340 (January).

Bunke, D. 1990. Update on Ohio DOT's experience with concrete containing silica-fume. 69th annual meeting of the Transportation Research Board, presentation no. CB 089 (January).Carette, G. G., and V. M. Malhotra. 1983. Mechanical properties, durability and drying shrinkage of portland cement concrete incorporating silica fume. Cement, Concrete, and Aggregates 5 (1):3-13.

Holland, T. C. 1988. Practical considerations for using silica fume in field concrete. 67th annual meeting of the Transportation Research Board, paper no. 87-0067 (preprint) (January).Luther, M. D. 1987. Silica fume (microsilica) concrete in bridges in the United States. Transportation Research Record 1204.

Luther, M. D. 1989. Silica fume (microsilica): Production, materials and action in concrete. In Advancements in Concrete Materials Seminar, 18.1-18.21. Peoria, Ill.: Bradley University.Luther, M. D. 1990. High-performance silica fume (microsilica)—Modified cementitious repair materials. 69th annual meeting of the Transportation Research Board, paper no. 890448 (January) (preprint).

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Luther, M. D., and W. Hansen. 1989. Comparison of creep and shrinkage of high-strength silica fume concretes with fly ash concretes of similar strengths. In ACI special publication SP-114. Vol. 1, Fly ash, silica fume, slag and natural pozzolans in concrete. ed. V. M. Malhotra. 573-91. Detroit: American Concrete Institute

Ozyildirim, C. 1986. Investigation of concrete containing condensed silica fume: Final report. Report no. 86-R25 (January). Charlottesville: Virginia Highway & Transportation Research Council.

Pigeon, M., and M. Plante. 1989. Air-void stability part I: Influence of silica fume and other parameters. ACI Journal 86 (5):482-90.

Pigeon, M., P. C. Aitcin, and P. LaPlante. 1987. Comparative study of the air-void stability in a normal and a condensed silica fume field concrete. ACl Journal 84 (3):194-99 (May-June).Plante, P., and A. Bilodeau. 1989. Rapid chloride ion permeability test: Data on concretes incorporating supplementary cementing materials. In ACI special publication SP-114. Vol. 1, Fly ash, silica fume, slag and natural pozzolans in concrete, ed. V. M. Malhotra, 625-44. Detroit: American Concrete Institute.

Tenoutasse, N., and A. M. Marion. 1987. The influence of silica fume in alkali-aggregate reactions. In Concrete alkali-aggregate reactions, ed. P. E. Grattan-Bellew, 711-75. Park Ridge, N.J.: Noyes Publications.

Wolseifer, J. 1984. Ultra high-strength field placeable concrete with silica fume admixture. Concrete International: Design and Construction 6 (4):25-31 (April).

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Chapter No. 4

Tests and calculations

4.1 EXPERIMENTS

Following experiments were carried out in the laboratory for finding the results,

Experiment Number 1: To Determine the compressive strength of concrete having no pozzolan.

Experiment Number 2: To Determine the compressive strength of concrete having 1 % pozzolan by weight of cement.






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4.2 EXPERIMENT NUMBER 1

To determine the compressive strength of concrete, having no pozzolan

OBJECTIVE

The objective of this experiment is to find out the compressive strength of concrete, using the cylinders.

APPARATUS

Compressive strength testing machine, cylinder moulds, taming rods, trowel, spade, non-absorbent plate form, weighing balance, curing tank, set of sieves & pozzolan(Silica Fume), concrete ingredients.

MIXING

The concrete for all cylinders shall be mixed in a mixer machine. They shall be mixed in dry condition for 1 minute & then silica fume & water are added.

CYLINDER CASTING

Specific quantity of the silica fume is added in the sample. The mixed concrete is then transferred to cylinder mould in approximately three equal layers each shall be given 25 strokes of standard tamping rod in a uniform manner over the cross-section of cylinder. After this vibrate it on a vibrating table.

CYLINDER MOULD CASTING

The cylinder moulds including concrete and the silica fume shall be placed in an atmosphere of at least 90% of humidity at a temperature of 66+21.1degree centigrade for 24+1/2 hours, cylinders shall be marked. Then removed from moulds & submerged in water & kept there until taken out just before testing.

TESTING OF SPECIMEN

After the specified period cylinders shall be removed from water, surface water & grit shall be wiped o & any projecting fins removed. Dimensions & weight of each cylinder shall be noted before testing. The bearing surface of testing machine shall be wiped & any loose material removed from surface of cylinders. The test cylinders shall be placed in such a manner that load shall be applied to the opposite sides of cylinders while casting & not on the top & bottom. The load shall be applied continuously without shock at a rate of 2000 psi/min. The maximum load applied to cylinder shall be recorded & as appearance or usual features of concrete is noted down. The compressive strength shall be expressed in psi or Mpa. E

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OBSERVATIONS & CALCULATIONS

Mix Design =1:2:4W/C Ratio = 0.60Amount of Cement =7 kgAmount of Sand =14 kgAmount of Aggregate =21 kgAmount of Water = 4.20 kgAmount of Silica Fume = 0 %

Sr. No.

Casting Date

Testing Date

Crushing Load

(KN)

Crushing Load

(lbs)

Area

(in2)

Compressive Strength

(Psi)

Remarks

1 22-04-10 01-05-10 668 15008 28.28 668 Normal failure

2 22-04-10 07-05-10 86.7 19491 28.28 690 Normal failure

3 22-04-10 21-05-10 28.28 Normal failure

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To determine the compressive strength of concrete, having 1% pozzolan

OBJECTIVE


APPARATUS


MIXING


CYLINDER CASTING




TESTING OF SPECIMEN


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Sr. No.

Casting Date

Testing Date

Crushing Load

(KN)

Crushing Load

(lbs)

Area

(in2)


(Psi)

Remarks

1 22-04-10 01-05-10 148.7 33440 28.28 Normal failure

2 22-04-10 07-05-10 28.28 Normal failure

3 22-04-10 21-05-10 28.28 Normal failure

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OBJECTIVE


APPARATUS


MIXING


CYLINDER CASTING




TESTING OF SPECIMEN

After the specified period cylinders shall be removed from water, surface water & grit shall be wiped o & any projecting fins removed. Dimensions & weight of each cylinder shall be noted before testing. The bearing surface of testing machine shall be wiped & any loose material removed from surface of cylinders. The test cylinders shall be placed in such a manner that load shall be applied to the opposite sides of cylinders while casting & not on the top & bottom. The load shall be applied continuously without shock at a rate of 2000 psi/min. The maximum load E

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applied to cylinder shall be recorded & as appearance or usual features of concrete is noted down. The compressive strength shall be expressed in psi or Mpa.



Sr. No.

Casting Date

Testing Date

Crushing Load

(KN)

Crushing Load

(lbs)

Area

(in2)


(Psi)

Remarks

1 22-04-10 01-05-10 28.28 Normal failure

2 22-04-10 07-05-10 28.28 Normal failure

3 22-04-10 21-05-10 28.28 Normal failure

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OBJECTIVE


APPARATUS


MIXING


CYLINDER CASTING




TESTING OF SPECIMEN


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Mix Design =1:2:4W/C Ratio = 0.60Amount of Cement =7 kgAmount of Sand =14 kgAmount of Aggregate =21 kgAmount of Water = 4.20 kgAmount of Silica Fume =10 %

Sr. No.

Casting Date

Testing Date

Crushing Load

(KN)

Crushing Load

(lbs)

Area

(in2)


(Psi)

Remarks

1 22-04-10 01-05-10 28.28 Normal failure

2 22-04-10 07-05-10 28.28 Normal failure

3 22-04-10 21-05-10 28.28 Normal failure

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OBJECTIVE


APPARATUS


MIXING


CYLINDER CASTING




TESTING OF SPECIMEN


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Sr. No.

Casting Date

Testing Date

Crushing Load

(KN)

Crushing Load

(lbs)

Area

(in2)


(Psi)

Remarks

1 22-04-10 01-05-10 28.28 Normal failure

2 22-04-10 07-05-10 28.28 Normal failure

3 22-04-10 21-05-10 28.28 Normal failure

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OBJECTIVE


APPARATUS


MIXING


CYLINDER CASTING




TESTING OF SPECIMEN


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Sr. No.

Casting Date

Testing Date

Crushing Load

(KN)

Crushing Load

(lbs)

Area

(in2)


(Psi)

Remarks

1 22-04-10 01-05-10 28.28 Normal failure

2 22-04-10 07-05-10 28.28 Normal failure

3 22-04-10 21-05-10 28.28 Normal failure

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To determine the compressive strength of concrete, having no pozzolan

OBJECTIVE


APPARATUS


MIXING


CYLINDER CASTING




TESTING OF SPECIMEN


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Sr. No.

Casting Date

Testing Date

Crushing Load

(KN)

Crushing Load

(lbs)

Area

(in2)


(Psi)

Remarks

1 22-04-10 01-05-10 28.28 Normal failure

2 22-04-10 07-05-10 28.28 Normal failure

3 22-04-10 21-05-10 28.28 Normal failure

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4.9 SCOPE

4.9.1 This test method covers the determination of strength of cylindrical concrete specimens that have been molded in place using special molds attached to formwork. This test method is limited to use in slabs where the depth of concrete is from 5 to 12 in. [125 to 300 mm]. 4.9.2 The values stated in either inch-pounds or SI units shall be regarded separately as standard. SI units are shown in brackets. The values stated may not be exact equivalents, therefore each system must be used independently of the other. Combining values of the two units may result in nonconformance.

4.9.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to consult and establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. (Warning—Fresh hydraulic cementitious mixtures are caustic and may cause chemical burns to skin and tissue upon prolonged exposure.)

4.10 SIGNIFICANCE AND USE

Cast-in-place cylinder strength relates to the strength of concrete in the structure due to the similarity of curing conditions since the cylinder is cured within the slab. However, due to differences in moisture condition, degree of consolidation, specimen size, and length-diameter ratio, there is not a constant relationship between the strength of cast-in-place cylinders and cores. When cores can be drilled undamaged and tested in the same moisture condition as the cast-in-place cylinders, the strength of the cylinders can be expected to be on average 10 % higher than the cores at ages up to 91 days for specimens of the same size and length-diameter ratio.

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COMBINED GRAPHS UPTILL NOW FOR SEVEN AND FORTEEN DAYS

0 1 5 10 20 30 400

200

400

600

800

1000

1200

1400

1600

1800

Figure 2 Compressive Strength After 7 Days

% of silica Fume

Com

pres

sive

Stre

nght

(Psi)

0 1 5 10 20 30 400

200

400

600

800

1000

1200

1400

1600

Figure 4 Compressive Strength After 14 Days

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FIGURE 4.1 (WEIGHING BALANCE)

FIGURE 4.2 (CONCRETE MIXER USED IN LABORATORY) Eff

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FIGURE 4.3 (WEIGHING SILICA FUME TO MIX WITH CEMENT)

FIGURE 4.4 (ASSEMBLING THE MOULDS FOR THE CASTING OF CUBES

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FIGURE 4.5 (OILING THE MOULDS)

FIGURE 4.6 (FILLING THE CUBES AND CYLINDERS)

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FIGURE 4.7 (COMPACTION OF FILLED MOULDS)

FIGURE 4.8 (CURING OF TEST SPECIMEN)

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FIGURE 4.9 (SPECIMEN READ TO TEST AFTER CURING)

FIGURE 4.10 (WORKING UNDER SUPERVISION OF THE PROJECT ADVISOR)

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FIGURE 4.11 (TESTING CONCRETE CUBE)

FIGURE 4.12 (TESTING CONCRETE CYLINDER)

Chapter No. 5

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Conclusions and recommendations

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final project edited

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