Abstract

The aim of the Project is to find out the geo-engineering properties of fly ash, which can act as a stabilizer to many soils in geo-engineering field. The project describes the use of local fly ash in construction industry in a way to minimize the industrial waste. Their been serious shortage of natural material, which are used in Highway or Earth dam construction. Due to soil excavation, deforestation occurs which affects the bio-diversity. Industrial waste such as fly-ash, slag etc can be effectively used in soil stabilization. Several geo engineering Labrotory experiments were performed on fly ash to determine its properties, which may be used in road construction, earth dam construction, soil stabilization etc. If these materials can be used in highway or dam construction, it will be a great effort in minimizing the industrial pollution. Fly ash was collected from captive power plant from the dump pad of Rourkela steel plant. These are stored in air tight container after being oven-dried. Experiments such as determination of compaction properties, CBR analysis, Un-confined compressive strength test, permeability etc are done in order to determine the geo-engineering properties of fly ash, which can taken account in the construction field. A brief comparison is made between fly ash and other soil properties which are used as sub-grade, base in Highway construction.

Introduction
Electricity is the key for development of any country. Coal is a major source of fuel for production of electricity in many countries in the world. In the process of electricity generation large quantity of fly ash gets produced and becomes available as a byproduct of coal-based power stations. It is a fine powder resulting from the combustion of powdered coal - transported by the flue gases of the boiler and collected in the Electrostatic Precipitators (ESP).
Conversion of waste into a resource material is an age-old practice of civilization. The fly ash became available in coal based thermal power station in the year 1930 in USA. For its gainful utilization, scientist started research activities and in the year 1937, R.E. Davis and his associates at university of California published research details on use of fly ash in cement concrete. This research had laid foundation for its specification, testing &usages.
Any coal based thermal power station may have the following four kinds of ash: * Fly Ash: This kind of ash is extracted from flue gases through Electrostatic Precipitator in dry form. This ash is fine material & possesses good pozzolanic property. * Bottom Ash: This kind of ash is collected in the bottom of boiler furnace. It is comparatively coarse material and contains higher unburnt carbon. It possesses zero or little pozzolanic property. * Pond Ash: When fly ash and bottom ash or both mixed together in any proportion with the large quantity of water to make it in slurry form and deposited in ponds wherein water gets drained away. The deposited ash is called as pond ash. * Mound Ash: Fly ash and bottom ash or both mixed in any proportion and deposited in dry form in the shape of a mound is termed as mound ash.
As per the Bureau of Indian Standard IS: 3812 (Part-1) all these types of ash is termed as Pulverized FuelAsh (PFA).
Fly ash produced in modern power stations of India is of good quality as it contains low sulphur & very low unburnt carbon i.e. less loss on ignition. In order to make fly ash available for various applications, most of the new thermal power stations have set up dry fly ash evacuation & storage system. In this system fly ash from Electrostatic Precipitators (ESP) is evacuated through pneumatic system and stored in silos. From silos, it can be loaded in open truck/closed tankers or can be bagged through suitable bagging machine. In the ESP, there are 6 to 8 fields (rows) depending on the design of ESP. The field at the boiler end is called as first field & counted subsequently 2 , 3 onwards. The field at chimney end is called as last field. The coarse particles of fly ash are collected in first fields of ESP. The fineness of fly ash particles increases in subsequent fields of ESP.

Physical Properties of Flyash
The fly ash particles are generally glassy, solid or hollow and spherical in shape. The hollow spherical particles are called as cenospheres. The fineness of individual fly ash particle rage from 1 micron to 1 mm size. The fineness of fly ash particles has a significant influence on its performance in cement concrete. The fineness of particles is measured by measuring specific surface area of fly ash by Blaine's specific area technique. Greater the surface area more will be the fineness of fly ash. The other method used for measuring fineness of fly ash is dry and wet sieving. The specific gravity of fly ash varies over a wide range of 1.9 to 2.55.

Chemical composition and classification

Fly ash material solidifies while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags. Since the particles solidify rapidly while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 μm to 300 μm. The major consequence of the rapid cooling is that only few minerals will have time to crystallize and that mainly amorphous, quenched glass remains. Nevertheless, some refractory phases in the pulverized coal will not melt (entirely) and remain crystalline. In consequence, fly ash is a heterogeneous material. SiO2, Al2O3, Fe2O3 and occasionally CaO are the main chemical components present in fly ashes. The mineralogy of fly ashes is very diverse. The main phases encountered are a glass phase, together with quartz, mullite and the iron oxides hematite, magnetite and/or maghemite. Other phases often identified are cristobalite, anhydrite, free lime, periclase, calcite, sylvite, halite, portlandite, rutile and anatase. The Ca-bearing minerals anorthite, gehlenite, akermanite and various calcium silicates and calcium aluminates identical to those found in Portland cement can be identified in Ca-rich fly ashes. The above concentrations of trace elements vary according to the kind of coal combusted to form it. In fact, in the case of bituminous coal, with the notable exception of boron, trace element concentrations are generally similar to trace element concentrations in unpolluted soils.
Two classes of fly ash are defined by ASTM C618: Class F fly ash and Class C fly ash. The chief difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash. The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i.e., anthracite, bituminous, and lignite).
Not all fly ashes meet ASTM C618 requirements, although depending on the application, this may not be necessary.Ash used as a cement replacement must meet strict construction standards, but no standard environmental regulations have been established in the United States. 75% of the ash must have a fineness of 45 μm or less, and have a carbon content, measured by the loss on ignition (LOI), of less than 4%. In the U.S., LOI needs to be under 6%. The particle size distribution of raw fly ash is very often fluctuating constantly, due to changing performance of the coal mills and the boiler performance. This makes it necessary that, if fly ash is used in an optimal way to replace cement in concrete production, it needs to be processed using beneficiation methods like mechanical air classification. But if fly ash is used also as a filler to replace sand in concrete production, unbeneficiated fly ash with higher LOI can be also used. Especially important is the ongoing quality verification. This is mainly expressed by quality control seals like the Bureau of Indian Standards mark.
Class F fly ash
The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash. This fly ash is pozzolanic in nature, and contains less than 20% lime (CaO). Possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in order to react and produce cementitious compounds. Alternatively, the addition of a chemical activator such as sodium silicate (water glass) to a Class F ash can lead to the formation of a geopolymer.
Class C fly ash
Fly ash produced from the burning of younger lignite or subbituminous coal, in addition to having pozzolanic properties, also has some self-cementing properties. In the presence of water, Class C fly ash will harden and gain strength over time. Class C fly ash generally contains more than 20% lime (CaO). Unlike Class F, self-cementing Class C fly ash does not require an activator. Alkali and sulfate (SO4) contents are generally higher in Class C fly ashes.
At least one US manufacturer has announced a fly ash brick containing up to 50% Class C fly ash. Testing shows the bricks meet or exceed the performance standards listed in ASTM C 216 for conventional clay brick; it is also within the allowable shrinkage limits for concrete brick in ASTM C 55, Standard Specification for Concrete Building Brick. It is estimated that the production method used in fly ash bricks will reduce the embodied energy of masonry construction by up to 90%.

Engineering properties of fly ash

The properties of ash depend primarily on type of coal and its pulverization, burning rate, temperature, method of collection, etc. The significant properties of fly ash that must be considered when it is used for construction of road embankments are gradation, compaction characteristics, shear strength, compressibility and permeability properties. Individual fly ash particles are spherical in shape, generally solid, though sometimes hollow. Fly ash possesses a silty texture and its specific gravity would be in the range of 2.2 to 2.4, which is less than natural soils. Fly ash is a non-plastic material. Fly ash displays a variation of dry density with moisture content that is smaller than the variation exhibited a well-graded soil. The tendency of fly ash to be less sensitive to variations in moisture content than natural soils can be explained by the higher void content of fly ash. Normal soils have 1 to 5 per cent air voids when compacted at maximum dry density. Fly ash contains 5 to 15 per cent air voids at maximum dry density. The higher air voids tend to limit the build up of pore-water pressures during compaction, thus allowing the fly ash to be compacted over a large range of moisture content. For the same reason, fly ash does not experience density increases from the changes in the compactive efforts of the same magnitude as experienced in case of fine-grained soils.
Fly ash exhibits shear strength characteristics similar to those of a cohesionless soil. It has a significant value of undrained angle of internal friction and a minimal cohesion intercept in partially saturated condition. The friction angle for fly ash usually varies from 300 to 350 and some time especially for coarse ash, friction angle can be as high as 400. Any apparent cohesive behaviour displayed will be lost upon complete saturation. Majority of Indian power plants use bituminous coal and hence ash produced does not have significant free lime content. As a result such a fly ash is not hydraulic. Any latent strength development due to self-hardening would be very insignificant and cannot be counted on for design purposes. The compressibility of fly ash can be estimated in the laboratory using the oedometer. The typical values of compression index, Cc, for virgin compression ranges from 0.05 to 0.4 with a majority of values usually from 0.1 to 0.15. Recompression index, Cr ranges from 0.006 to 0.04. These values show that compaction can significantly reduce the compressibility of fly ash fills. The permeability of fly ash ranges from 8 x 10-6 cm/sec to 7 x 10-4 cm/sec. Generally medium to coarse type of ash have permeability values of about 10-4 cm/sec and hence can be considered to have good permeability.

Various usage of Ash

Pulverized Fuel Ash is versatile resource material and can be utilized in variety of application. The pozzolanic property of fly ash makes it a resource for making cement and other ash based products. The Geo-technical properties of bottom ash, pond ash & coarse fly ash allow it to use in construction of embankments, structural fills, reinforced fills low lying area development etc. The physico chemical properties of pond ash is similar to soil and it contains P, K, Ca, Mg, Cu, Zn, Mo, and Fe, etc. which are essential nutrients for plant growth. These properties enable it to be used as a soil amender & source of micronutrients in Agriculture/ Soil Amendment.

The major utilization areas of PFA are as under: -

* Manufacture of Portland Pozzolana Cement & Performance improver in Ordinary Portland Cement (OPC). * Part replacement of OPC in cement concrete. * High volume fly ash concrete. * Roller Compacted Concrete used for dam & pavement construction. * Manufacture of ash bricks and other building products. * Construction of road embankments, structural fills, low lying area development. * As a soil amender in agriculture and wasteland development.

Fly ash utilization in brick manufacturing

India is second largest producer of the brick in the world after China. The brick production in India is estimated at 140 billion bricks, consuming 24 million tonnes of coal along with huge quantity of biomass fuels. The total CO2 emissions are estimated to be 41.6 million tonnes and it accounts for about 4.5% of total GHG emissions from India. Box 1 represents the fact sheet of Indian brick industry. In the manufacture of bricks, fly ash can be an alternative material to clay. Fly ash can be used either with clay as part replacement or in combination with other materials like sand, lime, gypsum etc. to produce a substitute to conventional clay bricks.
From the soil to be used for brick manufacturing, India can be divided into three broad regions – Northern Mountanious region, Indo-Gangetic plains and peninsular region. Mountanious soil is coarse and contains parts of partially wathered rocks. The presence of brick industry in mountainous region is negligible. The Indo-Gangetic soil is ‘alluvial’ in nature formed by deposition of river Ganges and its tributaries. The soil is considered good for brick manufacturing and has faint yellow colour and is generally a mixture of fine sand,silt, clay and organic matter. The Peninsular soils are generally black cotton, red or lateritic in nature. They are generally considered difficult for brick making.

When fly ash is used as an admixture to plastic soil / clay for the production of fired clay bricks, fly ash reduces the plasticity of the raw-mix (thereby reducing the drying time and shrinkage cracks) and improves the texture of the product. Addition of fly ash also increases the ‘internal burnability’ of the green brick due to the presence of un-burnt carbon (proportionately reducing the requirement of ‘external’ fuel).

The technologies for the manufacture of fly ash bricks can be classified into the following main technology routes: Clay – fly ash bricks Red mud – fly ash bricks Sand fly ash bricks Fly ash – lime bricks Fly ash – lime / gypsum bricks

In general the soil is composed of different fractions namely, clay (particle size less than 2 microns), silt (particle size between 2 – 20 microns) and sand (particle size more than 20 microns). As per IS: 13757:1993 Burnt Clay Fly Ash Building Bricks: Specifications), clay fly ash bricks shall be hand or machine moulded and shall be made from the admixture of suitable soils and fly ash in optimum soils and fly ash in optimum proportions. As per IS
2117:1991(Guide for manufacture of hand-made common burnt clay building bricks), the clay or mixture of clay selected for brick manufacturing should preferably conform to the following mechanical composition:

Clay 20 to 30 percent by mass
Silt 20 to 35 percent by mass
Sand 35 to 50 percent by mass

The total content of clay and silt may preferably be not less than 50 percent by mass.

A number of measures are being taken to encourage various sectors to utilize fly ash. The targets of ash utilization are primarily governed by the MoE&F Notification dated 14th September, 1999 and its amendment Notification dated 27th August, 2003 & 3rd November, 2009 as well as Hon’ble High Court of Delhi directions vide its judgments dated 4th December, 2002, 10th March, 2004 as well as 5th August, 2004.

Suitability of mixing fly ash with clay for producing fired bricks

Two cases are being presented here related to production of clay-fly ash fired clay bricks:

Case 1: TIFAC in their report on Fly ash bricks has mentioned that with regard to requirements of fly ash for brick manufacture, it is desirable that the oxide composition should be similar to ordinary brick clays – the silica content should be over 40%, aluminium oxide not less than 15%, iron oxide not less than 5% and sulphide and soluble sulphite content should be insignificant. The report further mentions that the range of chemical compositions of Indian fly ashes indicates that they are suitable for brick making. However, not all the clays and fly ashes are suitable for brick making. Fly ash addition as a smaller constituent (8% - 20%) to the alluvial soil or as a larger constituent (25% - 40%) with sticky plastic alluvial red and black soils has been found advantageous to improve workability, green and fired strength and to modify drying behaviour of soils so as to check the incidence of cracking and fuel value of residual carbon. The characteristics of clay-fly ash bricks manufactured using alluvial, black and red soil region/ sites is provided in the following table:

Soil Type | Brick manufacturing site / soilgroup | Proportion of fly ash (w/w %) | Properties of bricks fired at 1000 + 20oC | | | | Compressive strength (kg/cm2) | Water absorption (%) | Bulk density(g / cc) | Alluvial Soil | Haridwar | 8 - 10 | 140 - 170 | 10 - 16 | 1.6 – 1.75 | | Roorkee | 8 - 10 | 135 - 170 | 10.5 - 16 | 1.62 – 1.75 | | Faridabad | 15 - 20 | 100 - 150 | 12 - 15 | 1.6 – 1.7 | | Delhi | 15 - 20 | 116 - 160 | 11 - 16 | 1.6 – 1.75 | | Kanpur | 15 - 20 | 170 - 200 | 10.5 - 12 | 1.68 – 1.77 | Red soil | Korba | 30 – 35 | 160 - 175 | 11.3 - 16 | 1.6 – 1.74 | | Ramagundam | 30 - 35 | 65 - 82 | 13.5 – 18.5 | 1.62 – 1.7 | | Obra | 20 - 25 | 150 - 160 | 16.4 – 18.3 | 1.65 – 1.72 | | Singrauli | 25 - 30 | 100 - 150 | 12 - 15 | 1.75 – 1.85 | | Barauni | 40 | 250 - 300 | 14 – 15 | 1.41 | | Patratu | 25 | 120 - 125 | 16 - 17 | 1.45 – 1.6 | | Bokaro | 40 | 100 - 125 | 20- 21 | 1.2 | | Kargali | 25 - 40 | 75 - 120 | 19 - 20 | 1.5 – 1.7 | | Haldia | 10 | 180 - 200 | 15 – 17 | 1.65 – 1.7 | | Patherdih | 25 – 40 | 85 - 100 | 16 - 20 | 1.5 – 1.7 | | Durgapur | 25 - 35 | 85 - 120 | 15 – 17 | 1.45 – 1.65 | Black soil | Nasik | 33 – 40 | 130 - 155 | 13.5 – 15.7 | 1.65 – 1.75 | | Indore | 33 - 35 | 65 - 80 | 14.5 – 18 | 1.58 – 1.7 |

Soil Type | Brick manufacturing site / soilgroup | Proportion of fly ash (w/w %) | Properties of bricks fired at 1000 + 20oC | | | | Compressive strength (kg/cm2) | Water absorption (%) | Bulk density(g / cc) | Alluvial Soil | Haridwar | 8 - 10 | 140 - 170 | 10 - 16 | 1.6 – 1.75 | | Roorkee | 8 - 10 | 135 - 170 | 10.5 - 16 | 1.62 – 1.75 | | Faridabad | 15 - 20 | 100 - 150 | 12 - 15 | 1.6 – 1.7 | | Delhi | 15 - 20 | 116 - 160 | 11 - 16 | 1.6 – 1.75 | | Kanpur | 15 - 20 | 170 - 200 | 10.5 - 12 | 1.68 – 1.77 | Red soil | Korba | 30 – 35 | 160 - 175 | 11.3 - 16 | 1.6 – 1.74 | | Ramagundam | 30 - 35 | 65 - 82 | 13.5 – 18.5 | 1.62 – 1.7 | | Obra | 20 - 25 | 150 - 160 | 16.4 – 18.3 | 1.65 – 1.72 | | Singrauli | 25 - 30 | 100 - 150 | 12 - 15 | 1.75 – 1.85 | | Barauni | 40 | 250 - 300 | 14 – 15 | 1.41 | | Patratu | 25 | 120 - 125 | 16 - 17 | 1.45 – 1.6 | | Bokaro | 40 | 100 - 125 | 20- 21 | 1.2 | | Kargali | 25 - 40 | 75 - 120 | 19 - 20 | 1.5 – 1.7 | | Haldia | 10 | 180 - 200 | 15 – 17 | 1.65 – 1.7 | | Patherdih | 25 – 40 | 85 - 100 | 16 - 20 | 1.5 – 1.7 | | Durgapur | 25 - 35 | 85 - 120 | 15 – 17 | 1.45 – 1.65 | Black soil | Nasik | 33 – 40 | 130 - 155 | 13.5 – 15.7 | 1.65 – 1.75 | | Indore | 33 - 35 | 65 - 80 | 14.5 – 18 | 1.58 – 1.7 |

Characteristics of clay fly ash bricks manufactured in alluvial, black and red soils region

Source: TIFAC report on Fly ash bricks page no. 180

Following advantages of clay fly ash bricks are mentioned in the report: Brick confirming to IS: 3102 – 1976 can be manufactured Fuel saving in the range of 15% - 35% (coal consumption) or coal saving up to 3 –
7 tonne per lakh bricks Drying losses are checked in the case of plastic black and red soils. Excessive linear drying shrinkage is reduced Brick strength in the case of black and red soils is increased by almost one and a half times (30% - 50%)

* Waste material is utilized. 30 – 40 tonne per lakh bricks in case of alluvial soils and 100 – 125 tonnes per lakh bricks in case of red and black soils *

Clay saving in brick manufacture is 10% - 40% by weight

Case 2: Aligarh Muslim university (AMU) carried out a feasibility study for mixing fly ash into the manufacture of fired clay bricks. AMU took samples of soil from the three brick kiln sites of Aligarh and Hathras districts of Uttar Pradesh. The fly ash sample was collected from the Kasimpur TPS that was within the designated distance of brick kilns as per the 1999 notification of MoEF. During the study the fluash was mixed with sol in different proportions 5%, 10% and 20% by weight and the bricks were moulded under usual working conditions (same workmen , drying and firing conditions). The following observations were made: Fly ash and soil mix requires more water and labour in comparison to soil without fly ash for making brick.

Bricks moulded from soil and fly ash mix requires three times the longer duration to dry in comparison to bricks without fly ash

Breakage in handling of green bricks was more in case of soil containing 20% fly ash

A distinct change in colour was noticed in bricks containing 20% fly ash as compared to bricks without fly ash. The reddishness in colour was less in bricks with 20% fly ash. For the three soil samples collected from Uttar Pradesh, the following change in properties were found while comparing with properties of the bricks without fly ash :

| 20% Fly ash | 10 % Fly ash | 5 % Fly ash | Reduction in compressive strength | Sample 1: 72.4 %, Sample 2: 82.5 %,Sample 3: 58.65 % | Sample 1: 42 %, Sample 2: 64.7 %,Sample 3: 1 % | 39 %, 59.57 %, Sample 1: 39 %, Sample 2: 59.57 %,Sample 3: increase by18.6 % | Increase in water absorption | Sample 1: 91.4 %, Sample 2: 119.12 %,Sample 3: 17 % | Sample 1: 66.3 %, Sample 2: 71.98%, Sample 3: Notsignificant | Sample 1: 83.9 %, Sample 2: 87.52 %,Sample 3: Notsignificant | Efflorescence | Nil (All samples) | Nil (All samples) | Nil (All samples) |

| 20% Fly ash | 10 % Fly ash | 5 % Fly ash | Reduction in compressive strength | Sample 1: 72.4 %, Sample 2: 82.5 %,Sample 3: 58.65 % | Sample 1: 42 %, Sample 2: 64.7 %,Sample 3: 1 % | 39 %, 59.57 %, Sample 1: 39 %, Sample 2: 59.57 %,Sample 3: increase by18.6 % | Increase in water absorption | Sample 1: 91.4 %, Sample 2: 119.12 %,Sample 3: 17 % | Sample 1: 66.3 %, Sample 2: 71.98%, Sample 3: Notsignificant | Sample 1: 83.9 %, Sample 2: 87.52 %,Sample 3: Notsignificant | Efflorescence | Nil (All samples) | Nil (All samples) | Nil (All samples) |

During the study, the black cotton soil using fly ash from the Virudhu Nagar district of Tamil Nadu was also included and fly ash was mixed in the proportion of 40 – 70 % by weight. It was found that up to 50 % mixing of fly ash the compressive strength increased and water absorption decreased and with increased proportions of flay ash compressive strength decreased and water absorption increased.

The following conclusions were drawn from the study:

The percentage of fly ash by weight that can be mixed in soil for manufacturing fired clay bricks depends upon the physical properties of soil and it should not be mixed arbitrarily

On the basis of tests carried out in three sites in Aligarh and Hathras districts, the maximum percentage of fly ash by weight that can be mixed in soils is not more than 5 – 10 %. It may further reduce depending on the physical properties of soil.

Determination of physical properties of soil is essential before mixing fly ash with soil for manufacture of burnt clay fly ash bricks.

The use of fly ash in brick making has many advantages and TIFAC has undertaken many initiatives to promote the use of fly ash in brick making. However, most of the brick kilns in the Indo-Gangetic belt have not used fly ash for manufacturing bricks due to following possible reasons: The hand-moulding process is generally used for green brick making in this region and mixing of fly ash with clay is difficult by hand moulding. Further, brick kiln owners felt that no appropriate technology is available for mixing of clay and fly ash at the scale that is presently being produced by hand moulding. Difficulty in logistic arrangements by individual brick kiln entrepreneur for procurement of less quantity from Thermal Power plant and transportation of fly ash to the brick making sites. Increase in the cost of the product due to increase in transportation and mixing cost with no additional premium being fetched by fly ash product in the market. Market perception of requirement of red coloured bricks also discouraged the brick kiln owners to start producing fly ash bricks that generally have grey colour of final product. Non-availability of reliable and low-cost technology for clay-fly ash brick making

The use of fly ash for brick making is quite popular in few States like Maharashtra where the soil quality is such that the addition of fly ash improves the properties of soil for brick making.

The benefits from the manufacture and use of fly ash bricks result in reduced energy use, conservation of top soil and qualifying under clean development mechanism etc. But in spite of these benefits, barriers on account of non-availability of reliable clay-fly ash bricks making machinery, mindset of users, lack of awareness etc. pose significant barriers in tapping the potential of brick making as a gainful use of fly ash.

Use of Fly Ash in Cement Concrete
Cement concrete - most widely used construction material in the world over, commonly consists of cement, aggregates (fine and coarse) and water. It is the material, which is used more than any other man made material on the earth for construction works. In the concrete, cement chemically reacts with water and produces binding gel that binds other component together and creates stone type of material. The reaction process is called 'hydration' in which water is absorbed by the cement. In this process apart from the binding gel, some amount of lime [Ca (OH) ] is also liberated. The coarse and fine aggregates act as filler in the mass. The main factors which determine the strength of concrete is amount of cement used and the ratio of water to cement in the concrete mix. However, there are some factors which limits the quantity of cement and ratio of water / cement to be used in the concrete. Hydration process of cement is exothermic and large amount of heat is liberated. Higher will be the cement content greater will be the heat liberation leading in distress to concrete.
Water is the principal constituent of the concrete mix. Once the concrete is hardened, the entrapped water in the mass is used by cement mineralogy for hydration and some water is evaporated, thus leaving pores in the matrix. Some part of these pores is filled with hydrated products of cement paste. It has been observed that higher the ratio of water / cement, higher is the porosity resulting in increased permeability.
How fly ash works with Cement in Concrete?
Ordinary Portland Cement (OPC) is a product of four principal mineralogical phases. These phases are Tricalcium Silicate- C3S (3CaO.SiO2), Dicalcium Silicate - C2S (2CaO.SiO2), Tricalcium Aluminate- C3A (3CaO.Al2O3) and Tetracalcium alumino-ferrite - C4AF(4CaO. Al2O3. Fe2O3). The setting and hardening of the OPC takes place as a result of reaction between these principal compounds and water.

The reaction between these compounds and water are shown as under:

The reaction of C3A with water takes place in presence of sulphate ions supplied by dissolution of gypsum present in OPC. This reaction is very fast and is shown as under:

Tetracalcium alumino-ferrite forms hydration product similar to those of C3A, with iron substituting partially for alumina in the crystal structures of ettringite and monosulpho-aluminate hydrate.

Above reactions indicate that during the hydration process of cement, lime is released out and remains as surplus in the hydrated cement. This leached out surplus lime renders deleterious effect to concrete such as make the concrete porous, give chance to the development of micro- cracks, weakening the bond with aggregates and thus affect the durability of concrete.

If fly ash is available in the mix, this surplus lime becomes the source for pozzolanic reaction with fly ash and forms additional C-S-H gel having similar binding properties in the concrete as those produced by hydration of cement paste. The reaction of fly ash with surplus lime continues as long as lime is present in the pores of liquid cement paste. The process can also be understood as follows:

Ordinary Portland Cement +Water

Surplus Lime

+

Fly ash

Additional Cementitious Material

Cementitious Material

Advantages of using fly ash in cement concrete

* Reduction in heat of hydration and thus reduction of thermal cracks and improves soundness of concrete mass. * Improved workability / pumpabilty of concrete * Converting released lime from hydration of OPC into additional binding material – contributing additional strength to concrete mass. * Pore refinement and grain refinement due to reaction between fly ash and liberated lime improves impermeability. * Improved impermeability of concrete mass increases resistance against ingress of moisture and harmful gases result in increased durability. * Reduced requirement of cement for same strength thus reduced cost of concrete.

Quality of Fly Ash as per Bureau of Indian Standards

To utilize fly ash as a Pozzolana in Cement concrete and Cement Mortar, Bureau of Indian Standard (BIS) has formulated IS: 3812 Part - 1 2003.In this code quality requirement for siliceous fly ash (class F fly ash) and calcareous flyash (classC flyash) with respect its chemical and physical composition have been specified. These requirements are given in table 1 & table 2

ASTM International for Fly ash
ASTM International C-618-03 specifies the chemical composition and physical requirements for fly ash to be used as a mineral admixture in concrete. The standard requirements are given in Table 3 and Table 4.

How Fly ash can be used in Cement Concrete?

The main objective of using fly ash in most of the cement concrete applications is to get durable concrete at reduced cost, which can be achieved by adopting one the following two methods:

1. Using Fly ash based Portland Pozzolana Cement(PPC) conforming to IS:1489 Part-1 in place of Ordinary Portland Cement 2. Using fly ash as an ingredient in cement concrete.

The first method is most simple method, since PPC is factory-finished product and does not requires any additional quality check for fly ash during production of concrete. In this method the proportion of fly ash and cement is, however, fixed and limits the proportioning of fly ash in concrete mixes.

The addition of fly ash as an additional ingredients at concrete mixing stage as part replacement of OPC and fine aggregates is more flexible method. It allows for maximum utilization of the quality fly ash as an important component (cementitious and as fine aggregates) of concrete.

There are three basic approaches for selecting the quantity if Fly Ash in Concrete.

* Partial Replacement of Ordinary Portland Cement (OPC)-The Simple Replacement method

* Addition of fly ash as fine aggregates-The Addition method

* Partial replacement of OPC, fine aggregate, and water- A modified replacement method

How fly ash helps in concrete?
Reduced Heat of Hydration
In concrete mix, when water and cement come in contact, a chemical reaction initiates that produces binding material and consolidates the concrete mass. The process is exothermic and heat is released which increases the temperature of the mass When fly ash is present in the concrete mass, it plays dual role for the strength development. Fly ash reacts with released lime and produces binder as explained above and render additional strength to the concrete mass. The unreactive portion of fly ash act as micro aggregates and fills up the matrix to render packing effect and results in increased strength. The large temperature rise of concrete mass exerts temperature stresses and can lead micro crackes. When fly ash is used as part of cementitious material, quantum of heat liberated is low and staggers through pozzolanic reactions and thus reduces micro-cracking and improves soundness of concrete mass.
Workability of Concrete
Fly ash particles are generally spherical in shape and reduces the water requirement for a given slump. The spherical shape helps to reduce friction between aggregates and between concrete and pump line and thus increases workability and improve pumpability of concrete. Fly ash use in concrete increases fines volume and decreases water content and thus reduces bleeding of concrete.
Permeability and corrosion protection
Water is essential constituent of concrete preparation. When concrete is hardened, part of the entrapped water in the concrete mass is consumed by cement mineralogy for hydration. Some part of entrapped water evaporates, thus leaving porous channel to the extent of volume occupied by the water. Some part of this porous volume is filled by the hydrated products of the cement paste. The remaining part of the voids consists of capillary voids and gives way for ingress of water. Similarly, the liberated lime by hydration of cement is water-soluble and is leached out from hardened concrete mass, leaving capillary voids for the ingress of water. Higher the water cement ratio, higher will be the porosity and thus higher will be the permeability. The permeability makes the ingress of moisture and air easy and is the cause for corrosion of reinforcement. Higher permeability facilitates ingress of chloride ions into concrete and is the main cause for initiation of chloride induced corrosion. Additional cementitious material results from reaction between liberated surplus lime and fly ash, blocks these capillary voids and also reduces the risk of leaching of surplus free lime and thereby reduces permeability of concrete.

Effect of fly ash on Carbonation of Concrete
Carbonation phenomenon in concrete occurs when calcium hydroxides (lime) of the hydrated Portland Cement react with carbon dioxide from atmospheres in the presence of moisture and form calcium carbonate. To a small extent, calcium carbonate is also formed when calcium silicate and aluminates of the hydrated Portland cement react with carbon dioxide from atmosphere. Carbonation process in concrete results in two deleterious effects (i) shrinkage may occur (ii) concrete immediately adjacent to steel reinforcement may reduce its resistance to corrosion.
The rate of carbonation depends on permeability of concrete, quantity of surplus lime and environmental conditions such as moisture and temperature. When fly ash is available in concrete; it reduces availability of surplus lime by way of pozzolanic reaction, reduces permeability and as a result improves resistance of concrete against carbonation phenomenon.
Sulphate Attack
Sulphate attacks in concrete occur due to reaction between sulphate from external origins or from atmosphere with surplus lime leads to formation of etrringite, which causes expansion and results in volume destabilization of the concrete. Increase in sulphate resistance of fly ash concrete is due to continuous reaction between fly ash and leached out lime, which continue to form additional C-S-H gel. This C-S-H gel fills in capillary pores in the cement paste, reducing permeability and ingress of sulphate ions.
Corrosion of steel
Corrosion of steel takes place mainly because of two types of attack. One is due to carbonation attack and other is due to chloride attack. In the carbonation attack, due to carbonation of free lime, alkaline environment in the concrete comes down which disturbs the passive iron oxide film on the reinforcement. When the concrete is permeable, the ingress of moisture and oxygen infuse to the surface of steel initiates the electrochemical process and as a result-rust is formed. The transformation of steel to rust increases its volume thus resulting in the concrete expansion, cracking and distress to the structure. In the chloride attack, Chloride ion becomes available in the concrete either through the dissociation of chlorides- associated mineralogical hydration or infusion of chloride ion. The sulphate attack in the concrete decomposes the chloride mineralogy thereby releasing chloride ion. In the presence of large amount of chloride, the concrete exhibits the tendency to hold moisture. In the presence of moisture and oxygen, the resistivity of the concrete weakens and becomes more permeable thereby inducing further distress. The use of fly ash reduces availability of free limes and permeability thus result in corrosion prevention.
Examples of use of fly ash in concrete

World over

World over fly ash has been successfully utilized in cement concrete and as component of Portland Pozzolana Cement/ Blended cement for more than 50 years. Some of the structures wherein fly ash has been utilized are as under:

* Fly ash concrete was used in Prudential Building the first tallest building in Chicago after World war II. * About 60,000 cum of fly ash concrete with an Estimated saving of 3,000 tonne of Ordinary Portland Cement was used in Lednock Dam construction in UK during the year 1955 * About 60,000 m3 of fly ash concrete with 80/20 Ordinary Portland Cement/fly ash having average slump of 175 mm was used in the piles and the foundation slab to meet the requirement of sulphate resistance concrete of Ferrybridge C power station in UK during 1964 * Fly ash concrete was used for all the tunnel lining and slip formed surge shafts at the Dinorwig Pumped Storage Scheme in the year 1979 & 1980 in UK mainly to provide increased resistance to attack from aggressive water. * In the 1980's, in Sizewell B Nuclear Power station fly ash has been used in about 3,00,000 m3 concrete to improve workability for pumping, reduce temperature rise and increased resistance to chlorides and reduced risk of alkali aggregate reaction In India In India calcined clay pozzolana as a mineral admixture was used in mass concrete work of Bhakra and Rana Pratap sagar dam works in late fifties and early sixties. A special plant was set up to produce calcined clay pozzolana in 1957 at Bhakra dam site to meet requirement of pozzolana for mass concrete work.Some of the examples of application of fly ash as a pozzolana in mass concrete works are Rihand Dam and Narora Barrage in UP, Jawahar Sagar Dam in Rajasthan and Chandil Dam in Bihar when it has became available at thermal power stations. The use was limited because of non-availability of good quality fly ash in thermal power station.

With increasing awareness, availability of good quality fly ash in modern efficient thermal power station and concept of Ready Mixed Concrete, the use of fly ash as part replacement of cement and sand is showing increasing trends. Few examples wherein fly ash has been utilized in cement concrete are as under:

1. Fly ash from NTPC's Dadri Thermal power stations is being utilized in prestigious Delhi Metro Rail Corporation (DMRC) works at New Delhi. : More than 60,000 tonne of fly ash has been utilized in the work so far. In this project, the requirement of cement concrete was high strength, high durability (less shrinkage and & thermal crakes), low heat of hydration, easy placement, cohesiveness and good surface finish. Use of fly ash in concrete has fulfilled the entire above requirements. In this work the concrete of M-35 and above were used in structural works. The typical mix used for M-35 grade concrete is given below :

Ingredients for M-35 grade(Kg/ Cum of concrete) | Concrete without fly ash | Concrete with fly ash | Cement | 364 | 300 | Fly ash | - | 120 | Coarse aggregates 20 mm size | 637 | 611 | Coarse aggregates 10 mm size | 421 | 406.6 | Fine aggregates | 546 | 442.4 | Stone dust | 237 | 300.9 | Water (liter) | 163.8 | 168 | Superplasticizer (liter) | 3.64 | 2.52 |

Replacing cement by fly ash has (i) reduced the peak temperature by 80C, (ii) the time attaining peak temperature has been extended and (iii) heat generation pattern was more uniform and gradual.

2. Fly ash concrete (M-30 grade and high performance M-60 grade) was utilized for tremie seal concrete and pile cap concrete in Bandra Worli Sea link project. Fly ash was taken from Dahanu thermal power station, Mumbai.
The mixture proportion used in this work are as under:

Ingredients | Mix Proportion (Kg/m3) | | Tremie sealConcrete | Pile cap Concrete | Cement (53 grade) | 180 | 300 | Fly ash | 220 | 196 | Micro silica | - | 40 | Water | 135 | 136 | W/cm | 0.34 | 0.25 | 20 mm | 550 | 577 | 10 mm | 450 | 500 | Crushed sand | 465 | 423 | Natural Sand | 465 | 327 |

Admixture | 2.5% HRWRA | 13.4 | Slump initial (mm) | Collapse | Collapse | Slump after 3 hours | 180 mm | 165 | Compressive Strength (MPa)3 days28 days56 days | 10.236.547.2 | 39.374.6680.89 |

3. Self-Compacting concrete using fly ash from Kota thermal power station has been utilized for structural members of Rajasthan Atomic Power Project. Self-compacting concrete was used due to difficulties in placing concrete in structures having heavily congested reinforced bars and openings.

The details on the mix proportion used in this work are as under:

Ingredients | Mix Proportion(Kg/m3) | Cement (43 grade) | 250 | Fly ash | 200 | Water | 180 | W/cm | 0.4 | 20 mm | 250 | 10 mm | 374 | Crushed sand | 562 | Natural Sand | 426 | Superplasticizer | 3.8 | Retarder | 0.45 | Viscosity –modifying agent | 0.45 | Compressive Strength (MPa)3 days28 days56 days | 11.53541.5 |

Use of Fly Ash in Embankment

For material to be used in embankments construction, the properties of concern are specific gravity, compaction characteristics, workability, internal angle friction, cohesion etc. Indian fly ashes generally have comfortable scores on these properties.
The most important parameter for selection of a material for roads & embankments is compaction behaviour. Ash has a favorable point than soils here. Compaction curves (moisture content v/s dry density curve) for soil & pond ash show good compaction characteristics on addition of moisture. But the curve for soil shows steep rise in dry density with increase in moisture content upto optimum moisture content (OMC) and fall in dry density subsequently. Satisfactory compaction (dry density above 95% of that at OMC) is achieved for a limited range of moisture content (about 2% - 5%). On the other hand similar curve for pond ash is relatively flat and the corresponding range for pond ash is quite large (about 7-8%), which means that it can be compacted over a wide range of moisture content without much variations in dry density. Hence, provides more flexibility for use in different seasons. It may be noted that though the maximum dry density of fly ash at OMC is less than that of soil, but it is not due to loose compaction or presence of the voids, rather it is due to lower specific gravity of ash particles. Further, fly ash is easy to compact and can be compacted by using either static or vibratory rollers. Fly ash has internal angle of friction in the range of 300 to 420, which is quite high as compared to that of soils (280 to 350). Fly ash when moist possesses apparent cohesion too. So, it can provide greater stability of slopes as compared to soil and side slopes steeper than soils can be provided in the embankments.
Specific gravity of coal ash particles ranges from 1.6 to 2.4 as compared to that of soil, which is in range 2.55 to 2.75. Due to lightweight, it imparts less load on sub-grades, hence can be used on weak sub-grades.
Fly ashes have permeability in the range of 10-6 to 10-4 cm/sec. Its high permeability ensures free & efficient drainage. After rainfall, water gets drained out freely, which means its workability is better than soil, especially, during the monsoon. Work on fly ash fills / embankments can be re-started within a few hours of rain while in case of soils, one is required to wait for much longer periods. Further, fly ash gets consolidated at a faster rate and primary consolidation gets over very quickly. So, it has low compressibility & shows negligible subsequent settlements. Thus, it can be used in bridge abutments also. Further, fly ash provides better bonding with geogrid material, as it has more friction angle as compared to soil. Hence, it provides a better & steeper RE wall as compared to soil.

Embankment construction using fly ash

Successful field trials have shown the suitability of fly ash as a fill material for construction of road embankments. Both reinforced as well as un-reinforced type of embankments have been constructed using fly ash. Reinforced embankments, popularly known as Reinforced Earth walls (RE walls) are used in urban areas for approaches to flyovers and bridges. RE walls have several advantages like faster rate of construction, economy, aesthetic look and saving the land required for construction of an unreinforced embankment. Fly ash is an ideal backfill material for RE wall construction because of its higher angle of internal friction and better drainage property. Geosynthetic materials like geogrids or geotextiles can be used as reinforcement for construction of reinforced fly ash embankments.
The most distinguishing feature of un-reinforced fly ash embankment would be use of fly ash as core material with earth cover. In case of un-reinforced embankments, side slope of 1:2 (Vertical: Horizontal) is generally recommended. Providing good earth cover using loamy soil should protect the slopes of the embankments. The thickness of side cover would be typically in the range of 1 to 3 m. The thickness of cover depends on the heightof the embankment, site conditions, flooding if expected, etc. This cover material can be excavated from the alignment itself and reused as shown in the Fig below. Stone pitching or turfing on this cover is necessary to prevent erosion due to running water. Intermediate soil layers of thickness 200 to 400 mm are usually provided when height of embankment exceeds 3 m. These intermediate soil layers facilitate compaction of ash and provide adequate confinement. Such intermediate soil layers also minimise liquefaction potential. Liquefaction in a fly ash fill generally occurs when fly ash is deposited under loose saturated condition during construction. To avoid the possibility of any liquefaction, fly ash should be properly compacted to at least 95 per cent of modified proctor density and in case water table is high, it should be lowered by providing suitable drains or capillary cut-off. Fly ash can be compacted using either vibratory or static rollers. However vibratory rollers are recommended for achieving better compaction. Compaction is usually carried out at optimum moisture content or slightly higher. The construction of fly ash core and earth cover should proceed simultaneously. High rate of consolidation of fly ash results in primary consolidation of fly ash before the construction work of the embankment is completed. The top 0.5 m of embankment should be constructed preferably using selected earth to form the subgrade for the road pavement.

Earlier fly ash embankment projects

Delhi PWD in association with CRRI pioneered in the construction of first reinforced flyash approach embankment on one side of the slip roads adjoining NH-2 in the Okhla fly over project. The length of the approach embankment is 59 m while the height varied from 7.3 m to 5.3 m. Geogrids were used for reinforcement of fly ash and a total quantity of about 2700 cum of ash from Badarpur thermal power plant was used for filling. The flyover was opened to traffic in Jan 1996.
First un-reinforced fly ash embankment in the country was constructed for eastern side approach of second Nizamuddin Bridge Project. A typical cross-section of the embankment shown in fig below. Pond ash produced at nearby Indraprastha Power Station was used for construction. The project is unique of its kind, since pond ash has been used for construction of high embankment in flood zone. A total quantity of about 1.5 lakh cubic metres of pond ash was used in this project. CRRI were the consultants for this project and provided design of the embankment and were associated for quality control supervision during construction. The project was completed and the road section opened to traffic in September 1998. The experiences gained during this project led to formulation of Guidelines on use of fly ash for embankment construction.

Recent experiences of using fly ash for road embankment Brief details of some of the projects executed in the recent past are given below:

Use of Pond Ash for Road Embankment on NH-6

As part of ongoing National Highway Development Programme, four-laning work of NH-6 from Dankuni to Kolaghat near Kolkata in West Bengal was taken up. The height of the embankment on this road section varied from 1.5 m to 4 m and requirement of earth fill was approximately 20 million cubic metres. However, good earth was not available near the proposed embankment site and the lead would be more than 100 km. This was leading to enormous increase in the project cost and also resulting in delays in the completion of the project. Hence the task on evaluating pond ash as alternative construction material was taken up. Required quantity of pond ash was available near the site at Kolaghat Power Plant. The pond ash samples collected from the Kolaghat power plant and bottom ash from Budge-Budge power plant (Kolkata) were tested in the laboratory to determine their engineering properties. The properties of pond ash, local soil and local sand are given in table below:

The proposed road alignment passes through waterlogged area. The water table in the area is very shallow and rises up to or above the ground during the rainy season. The subsoil at site generally consisted of silty clay or clayey soil up to a considerable depth. Such soils settle even under smaller loads imposed due to embankment of low height. However, if a lightweight material like pond ash is used in place of soil, the amount of settlement would certainly reduce.
The results of the stability analysis indicated improvement in factor of safety when fly ash was adopted as fill material. Results of stability analysis are given in table below. Keeping in view the site conditions, availability of materials near the construction site, it was suggested that after dewatering, geotextile wrapped sand or bottom ash layer of 0.5 m thickness be laid as base of the embankment. Pond ash embankment protected with 1.5 m thick soil cover was designed as given in figure below. However due to contractual constraints, the embankment was constructed by mixing pond ash and sand in ratio of 85:15 and subgrade was constructed using pond ash and soil in the ratio of 75:25

Slope failure of fly ash embankment

The Noida-Greater Noida Express-highway near Delhi, which is 23 km long, was constructed in a total span of approximately 3 years. It is a six lane express highway with divided carriageway. The height of the embankment on the total stretch is generally 1 to 2 m. However, at certain locations where the alignment crosses an under pass, the height of approach embankment varies from 6 to 8 m. The entire embankment was constructed using fly ash in its core and soil cover was provided along the slope and top portion of the embankment to prevent erosion. Intermediate soil layers were also provided with in the fly ash core. The highway was opened to traffic in the year 2003. In August 2004, after the heavy rainfall in quick succession, it was observed that the side slopes in high embankment portion had severely eroded and gullies were formed through out the high embankment slope. It was also observed at few spots that due to the piping action, the water had undermined the entire soil cover provided on the side slopes resulting in the exposure of fly ash layers. Detailed investigations were undertaken and causes of failure were identified as follows: * Severe erosion on the superlevated portion had taken place due to heavy run-off from six-lane carriageway, which was discharged on one side of the embankment. * Absence of longitudinal kerb channel and chutes allowed water to drain off along the slope. * Deep pits were made in the embankment slopes to fix utilities like electric poles and crash barriers, which were backfilled with loose soil. * Run off water entered into the embankment side cover and caused deep cavities exposing fly ash at many locations.

Fly Ash In Railway Embankment

Railway used fly ash from Kolaghat thermal Power plant in Tamluk-Digha new rail link project from ch.10.92 to ch. 12.28 in about 1.30 km in length. This location is situated near Haldi bridge where embankment had failed twice during construction. Fly ash was also used in a near-by location on Panskura-Haldia section between ch.(-600) to ch.775. Section adopted for embankment construction is given below:-

Case History Of Fly Ash Embankment In DMRC

For the rail corridor of mass rapid transport system for Delhi, the base car depot was planned at Shastri Park on the east side of river Yamuna near ISBT. This area of 64 hectare is located at the reduced level of 203 to 204 meters above sea level where as 100 years HFL is 208.9 m hence it was decided to fill the base depot area with soil.
On review it was decided that if a potion of earth work can be done by using pond ash being generated at IP and Rajghat power stations of Delhi Vidyut Board, then a lot of saving in terms of time and money can be achieved due to reduced lead of carting of flyash as compared to the soil which is generally not available near Delhi. Mass filling at Shastri Park was to be completed in a short span of 12 months so as to give adequate time for consolidation of newly built embankment before construction of structures or starting the track laying over the filled up soil. A total quantity of about 17 lakhs cum of earth was required for constructing the main line track embankment at 6 m to 9 m height and about 30 ha of depot area at 6 m height.

Advantages considered
Due to following advantages, it was decided to use flyash to the extent of two- thirds requirement of the earthwork: * Utilization of soil in place of flyash would have resulted in erosion of topsoil from a large area of agricultural land and resultant degradation of land. * Disposal of flyash is a big problem for thermal power stations, hence scientific disposal of flyash by DMRC will pave the way for large scale utilization of flyash in the construction of earthen embankment for roads and rails. * Due to quick draining characteristics, work can be continued in monsoon. * Being a friction material, (no cohesion) proper compacted flyash shows very small long-term settlement. * Construction speed is faster as compaction vs moisture content curve is more even resulting in wider range of moisture content for compaction. * Lower density than soil, hence low overburden pressures for the same height of embankment, hence less chances of toe failure. * Assured availability free of cost. * The cost of transportation of flyash was less as compared to soil. Flyash being lighter material, required less haulage and hence was economical. * Availability of good quality soil in such huge quantities was difficult.

Embankment Design

Embankment was designed on the same line as for soil embankment. A base layer of soil varying from 1.2 m to 1.6 m was provided over which 2 m thick layer of flyash was provided with intermediate soil layer of 0.4 m. On both sides of embankment, soil shoulder of 4.47 m width was provided for ensuring minimum soil cover of 2 m at all the locations. On the top, a minimum soil cover of 400 mm with 300 mm cover of blanket material has been provided for ensuring no erosion of flyash. A typical curve of embankment is shown in figure below -

The side slopes of the embankment have been kept to two horizontal to one vertical. The average height of embankment varies from 6 m to 9 m. Wherever height is more than 6 m, a 3 m berm has been provided at a depth of 6 m below the top. Soil and flyash are fixed as 98% and 95% of modified proctor test ( IS-2720) respectively. Figure below shows typical compaction vs moisture content curve for soil and flyash.

Description of work
The project site is situated in national capital of Delhi near Kashmere Gate, ISBT in the flood planes of river Yamuna and within the eastern marginal bund or Shahdara marginal bund. The work was started in Oct.1998 and full site was under standing water of a few cm to 1.0 m. Large scale dewatering by high discharging pumps and a network of drains was planned to make the ground dry. The top layer of soil having vegetation or poor soil was removed and ground was compacted by sheep foot roller of 10 to 12 tons with 8 to 10 passes so as to achieve a minimum dry density 95% of modified dry density. After the base layer of soil was laid and compacted in layer thickness of 15 cm, 98% of modified dry density was achieved. Base soil layer of 1.2 m to 1.6 m was laid before starting laying of flyash layer of 15 cm thickness each. The embankment designed as shown in figure above was completed by laying layers of soil and flyash as per the requirement of design.

Problems faced and solutions found: * During the winter season, the moisture content in the pond ash was very high and due to non availability of sun-shine consecutively for many weeks, moisture content could not be kept near OMC. Hence, stacking and rehandling of material was done to achieve the desired compaction level of 95% of modified dry density. * During transportation, there were chances of spillage of fly ash, if it is dry even after keeping the dumpers fully coverd by tarpulin and in case of higher moisture content, fly ash used to flow from the opening in the dumper body as it liquefies very quickly. Hence extensive dewatering was done in ash ponds at loading point itself. * On the onset of summer, the peculiar problem of flying of flyash was encountered at ash pond as well as site. A very elaborate arrangement of net work of nine bore wells was developed for sprinkling water continuously over the exposed flyash slopes and top and in addition to that,12 tankers of 6000 to 10000 litres capacity were deployed during peak requirement to sprinkle water at all the roads on which machinery was moving. * During summer, it was observed that rate of loss of water from the compacted layer was very high resulting into loss of compaction, which used to get aggregated by movement of dumpers which came for dumping of flyash. This problem was solved by increasing thickness of compacted layer of fly ash from 150 mm to 300 mm. This helped in speeding the construction, reducing the open area of flyash and reduced loss of moisture and compaction. * Spillage of flyash on roads during transportation is an area of great concern and carelessness can create a very serious pollution problem within the city. Hence preventive and corrective measures were taken to control this menace. On daily basis, it was ensured that all dumpers carrying flyash were fully covered by tarpulin overloading of dumpers was strictly not allowed.

Quality control
A self contained fully equipped soil-testing laboratory was established before starting the work. Some of the tests were Sieve analysis, Moisture content determination, Modified proctor test as per IS 2720, Liquid Limit and Plastic Limit. Following were the main considerations as part of the quality assurance programme - * Each source of soil flyash was decided and approved before bringing the material at site. * Field control on compaction was achieved by ensuring moisture contents near OMC, adequate plain and vibro passes of compactor and compaction level by core cutter method were ensured.

Guidelines For Embankment (As per BIS 10153-1982)

Studies have shown the suitability of fly ash as a fill material for the construction of embankments. The properties to be kept in view are grain size, density; shear strength, compaction characteristics & permeability. The fly ash has to be compacted at OMC, which is normally in range of 15-30 percent. Because of low density the material is suitable for location where clayey soils get consolidated under overburden material. The permeability of compacted fly ash is low so in cases where the water table is very high or surface water likely to percolate down the embankment, it is advisable to provide for drainage a layer of coarse material 300-450 mm thick below the fly ash.

Controlled Low Strength Material
Controlled Low-Strength Material (CLSM) is a cementitious fill that is in a flowable state at the time of placement and has limited compressive strength to facilitate subsequent excavation. CLSM is also known as flow-crete and controlled density fill as well as by various proprietary names. The material has gained appreciable recognition in the USA and Canada as a fill material because of its inherent advantages. These include flowing placement without segregation, self-consolidation,controlled density, controlled strength, ease of excavation, and economy.Development of CLSM has centred around mixtures using a sand filler. The inter-particle voids are slightly over-filled with a fluid paste composed of cement and water, with the possible addition of fly ash. Components of the paste are varied in quantity to achieve the required performance in terms of strength development, self-consolidation, flow behavior characteristics, durability, economy and ease of removal. Overseas experience has shown the sand filler mixtures to have performed well, giving all the properties desired. It may be difficult to achieve satisfactory flowability and there may be severe bleeding in cement-sand slurries – although fly ash can be of assistance in obtaining properties of flowability with reduced bleeding
From the above description, it can be seen that CLSM is a cementitious material which can be mixed, transported and delivered using normal ready-mix operation techniques and processes. The material does not look like concrete but it performs in a similar manner since it has cementitious properties and, with time, will become quite hard. The latter must be controlled so that it does not become too hard. As with any concrete, strength is influenced by the quantity of water and cement. Very high water content (300 to 450 kg/m3) and very low cement content (25 to 100 kg/m3) are quite normal for CLSM. Fly ash, sand and, in some cases, coarse aggregates, are selected for their ability to flow rather than for their contribution to strength properties. This implies that sand not suitable for regular concrete may perform quite well in CLSM. Indeed, sands with as much as 30% fines have proven satisfactory.
CLSM is ideal for use in any backfilling operation where it is important to minimise settlement or where there is restricted access for compaction equipment. It is also advantageous for backfilling excavations in soils that are prone to collapse when normal compaction equipment is used.Typical applications are: * backfilling utility trenches * filling abandoned underground structures eg tanks, sewers, tunnels, etc * backfilling bridge abutments and retaining walls * most situations where soil backfill is required. where backfill may have to be excavated at a later date, the strength of CLSM must be limited. Strengths up to 2 MPa at 28 days can generally be excavated using normal construction equipment.As CLSM is a highly fluid material, consideration must be given to the lateral pressure exerted during placement; lightweight pipes, etc may need to be anchored to prevent flotation.
Mixes
CLSM mixes should be designed for the particular flowing characteristics required as well as the compressive strength necessary. As indicated previously aggregates are chosen more for their compatibility with the CLSM’s flowing characteristics than their contribution to strength.
A wide range of fine aggregates including sands, gravels and quarry waste material can be used to produce satisfactory CLSM mixes. It is not necessary for aggregate to comply with AS 2758.1–1985. Aggregates should, however, be free of reactive or expansive materials. Coarse aggregates are not normally included in CLSM.Types gB or gP cement in accordance with AS 3972 can be used in the production of CLSM.
Where fly ash is used to improve flowability it should be noted that early strengths will be reduced. A maximum 28-day strength of 0.5 MPa is suggested where future hand excavation is likely and 1.5–2 MPa for mechanical excavation. Slumps that will suit most applications of CLSM are around 200 mm.

Fly Ash in Sub Base and Base Course

Fly ash can be usefully employed for construction of sub base/base course. Mixing of soil and fly ash in suitable proportions improves the gradation as well as plasticity characteristics in the mix, thereby improving the compacted strength. Fly ash (preferable) / Pond ash can be used for sub base and base course construction and stabilization. The fly ash is usually used in combination with lime to form the matrix that cements the aggregate particles together. Generally clay soils are stabilized with fly ash alone whereas silty soils respond well to stabilization with fly ash and lime or cement.

References * Fly Ash for Cement Concrete- Resource For High Strength and Durability of Structures at Lower Cost
NTPC Fly Ash division, NTPC Limited,Noida * Study Report On Use Of Coal Ash In Railway Embankments
Report No GE:0-S005, Feb 2003
Geo-technical Engineering Directorate,
Research Designs and Standards Organisation
Manak Nagar, Lucknow – 11 * Feasibility of mixing Fly ash in Manufacture of burnt clay bricks – Report by Aligarh Muslim University * TERI report No. 2006RD25 Policy, Institutional and legal barriers to economic utilisation of fly ash * Fly Ash bricks, Technology Information, Forecasting and Assessment Council (TIFAC), DST, New Delhi * http://www.wikipedia.com/fly_ash * http://www.wikipedia.com/concrete * http://www.civil-resources.blogspot.in/2010/06/fly-ash.html * www.bis.org.in