PROJECT REPORT ON FLY ASH
B. S. Patel Polytechnic, Ganpat University,
Gozariya Highway, Kherva, Mehsana.
CERTIFICATE This is to certify that Mr. Patel Hardik R. in 5th semester student
of
Diploma
in
Civil
Engineering,
Reg.
No .
S012706040 has satisfactorily completed his seminar work titled Study of “Fly Ash” and submitted within four wall of B.S.Patel Polytechnic, Kherva.
Shri B. S. Patel Polytechnic
Date of Submission:-
Staff in-charge:Prof. G. K. Zala
Head of Department:-
Gozariya Highway, Kherva, Mehsana.
CERTIFICATE This is to certify that Mr. Patel Hardik R. in 5th semester student
of
Diploma
in
Civil
Engineering,
Reg.
No .
S012706040 has satisfactorily completed his seminar work titled Study of “Fly Ash” and submitted within four wall of B.S.Patel Polytechnic, Kherva.
Shri B. S. Patel Polytechnic
Date of Submission:-
Staff in-charge:Prof. G. K. Zala
Head of Department:-
Prof. V. M. Patel
Acknowledgement We express our sincere thanks to all our guided professors as well as all the professors of our department. For thei theirr val valuabl uable e guid guidan ance ce,, const onstru ruc ctive tive su sugg gges esttions ions and affectionate care with which they have directed us in carrying out out a work work of the the natu nature re could ould not not have have been been pos possibl sible e with withou outt the the perp perpet etua uall enco encour urag agem emen entt and and metic eticul ulou ous s attention they paid We give our kind thanks to our civil department and for giving us invaluable guidance. At all we can’t think to forget our parents, friends & all those persons who helped us directly or indirectly for our destiny to complete this seminar successfully and we gave our divine thanks to them.
Prepared By: Registration No: Patel Hardik R. 012706040 Parekh Saurabh B. 012706030 Chaudhary Vipul D. 012706006 Dave Mehul D.
SSSS-012706011
Patel Jaimin P. Chaudhary Alpesh K. Patel Chirag K. Makwana Dhaval S. 012706024 Patel Amit P. 012706031 Goswami Parth B. 012706017 Patel Manoj S. Patel Prakash R. Patel Kiran A. 012706069
S-012706043 S-012706002 S-012706034 SSSS-012706071 S-012706073 S-
Index:-
i.
Introduction to Fly Ash
ii.
What is Fly Ash?
iii.
“Fly Ash” in Concrete
iv.
Objectives of Fly Ash
v. Fly Ash Used in Various Concrete vi.
Indian Scenario Fly Ash Generation & Utilization
vii. Utilization Areas 2004-05 viii.
Fly Ash Used In Recent a.
Roads and Embankments
b.
Fly Ash in Building Components
c.Fly Ash in Cement, Mortar & Concrete d.
Fly Ash in Mining Sector
e.
Fly Ash in Agriculture
f. Fly Ash in Hydro Sector g.
Others Products
ix. Summary of Utilization x.
Separation Technologies
xi. What Is Quality Concrete? xii.
How Fly Ash Contributes to Concrete Durability and Strength?
xiii.
How Fly Ash Contributes to Concrete Workability?
xiv. How Fly Ash Protects Concrete? xv.
How
Fly
Ash
Reduces
Heat
of
Hydration in Concrete? xvi. Fly Ash Checklist: Enhancing Concrete Workability xvii.
Fly Ash Checklist: Increasing Concrete Performance
xviii. Specifications of Fly Ash xix.
Specification of Fly Ash According to Indian Standard
xx.
AAC: Autoclaved Aerated Concrete
Introduction to Fly Ash:Fly ash closely resembles volcanic ashes used in production of the earliest known hydraulic cements about 2,300 years ago. Those cements were made near the small Italian town of Pozzuoli - which later gave its name to the term "pozzolan." A pozzolan is a siliceous or siliceous / aluminous material that, when mixed with lime and water, forms a cementitious compound. Fly ash is the best known, and one of the most commonly used, pozzolans in the world. Instead of volcanoes, today's fly ash comes primarily from coal-fired electricity generating power plants. These power plants grind coal to powder fineness before it is burned. Fly ash - the mineral residue produced by burning coal - is captured from the power plant's exhaust gases and collected for use. Fly ash is a fine, glass powder recovered from the gases of burning coal during the production of electricity. These micron-sized earth elements consist primarily of silica, alumina and iron. The difference between fly ash and Portland cement becomes apparent under a microscope. Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures. That capability is one of the properties making fly ash a desirable admixture for concrete.
What Is Fly Ash? Fly ash closely resembles volcanic ashes used in production of the earliest known hydraulic cements about 2,300 years ago. Those cements were made near the small Italian town of Pozzuoli - which later gave its name to the term "pozzolan." A pozzolan is a siliceous or siliceous / aluminous material that, when mixed with lime and water, forms a cementitious compound. Fly ash is the best known, and one of the most commonly used, pozzolans in the world. Instead of volcanoes, today's fly ash comes primarily from coalfired electricity generating power plants. These power plants grind coal to powder fineness before it is burned. Fly ash - the mineral residue produced by burning coal - is captured from the power plant's exhaust gases and collected for use. Fly ash is a fine, glass powder recovered from the gases of burning coal during the production of electricity. These micron-sized earth elements consist primarily of silica, alumina and iron. The difference between fly ash and Portland cement becomes apparent under a microscope. Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures. That capability is one of the properties making fly ash a desirable admixture for concrete.
“Fly Ash” in Concrete:Fly ash is a byproduct of coal burning: 600 million tons are produced per year and over 80% goes to the landfill.
Up to 50% of cement (by volume) can be replaced with fly ash (1535% is typical).
Today only about 10% of available fly ash is used in concrete.
Objectives of Fly Ash:
In India 65% of the total installed power generation is coalbased. We use 250 million MT coal every year for power generation. India has a huge coal reserves. Current non coking coal reserve is 76 billion MT. Indian coal has high ash content (30%–50%) and thus it contributes to large volumes of ash after combustion. Around 100 million MT of ash generated every year. This figure likely will go up to 170 million MT by 2010. The World Bank has cautioned India that by 2015, disposal of coal ash would require one square meter of land per capita. And therefore Ash Management would remain an important area of national concern. Reduce environmental impact.
Improve workability (better finish).
Increase corrosion resistance.
Improve long term concrete strength.
Fly Ash Used In Various Countries Sr. No.
Country
Annual Ash Production (Million Tons.)
Ash Utilization % of Ash Production
1.
USA
75
65
2.
China
100
45
3.
Germany
40
85
4.
UK
15
50
5.
Japan
8
60
6.
Australia
10
85
7.
Canada
6
75
8. 9. 10. 11. 12.
France Denmark Italy Netherlands India
3 2 2 2 112
85 100 100 100 38
Major Area of Utilization Cement, concrete, bricks, fill material Concrete, bricks, fill materials, cement Cement, concrete, mine fill Cement, fill material Cement, concrete agriculture, fill Material Blended cement, fill material Cement, fill materials, building materials . . . . . . . do . . . . . . . . . . . . . . do . . . . . . . . . . . . . . do . . . . . . . . . . . . . . do . . . . . . . . . . . . . . do . . . . . . .
Indian Scenario Fly Ash Generation & Utilization:-
1994:FA Generation --- 40 Million Tone FA Utilization --- 3% (1.2 Million Tone)
March 2005:FA Generation --- 112 Million Tone FA Utilization --- 38% (42 Million Tone)
2012(Assumed):FA Generation --- 170 Million Tone FA Utilization --- 100% (Target)
Utilization Areas 2004-05:-
Fly Ash Used In Recent:-
Roads and Embankments:-
Technical
• • • • • •
Advantages:-
Good compaction High range of OMC High internal angle of friction Free draining (less interruption due to rain) No large lumps to be broken (easy to spread) Light in weight (can be used on weak sub-grades)
Economics
Savings:-
No royalty to be paid as excavation of soil is eliminated. Reduces excavation cost of borrow material. Normally reduces transportation cost. Easy and faster construction leads to reduction in construction cost. • Saving in ash management expenditure of thermal power plants. • Additional agricultural produce from the land which would otherwise have been. Excavated for getting soil. Used for fly ash disposal. • • • •
Fly Ash in Building Components:-
Technical • • • • •
Advantages:-
Better finish High strength Less water absorption No efflorescence Lower unit weight, less load on foundation
Economic
Savings:-
Reduced Energy Consumption Reduces excavation of clay Lower cost of brick as compared to clay brick of same quality. • Number of bricks required per unit volume of construction is less. • Less consumption of mortar. • Less number of joints in case of blocks. • Plastering may be avoided or if it is to done, the thickness of plaster required is less. • • •
Fly Ash in Concrete:-
Cement,
Mortar
&
Fly • • • • • •
Ash:-
Ordinary Concrete : 20-40% fly ash Roller Compacted Concrete : 60-70% fly ash Cellular Light Weight Concrete : 20-40% fly ash High Performance Concrete : 20-40% fly ash High Volume Fly Ash Concrete : 50-70% fly ash Fly Ash in Mortar : 20-40%
Technical • • • • • • •
High long term strength Better workability Higher impermeability Less heat of hydration Corrosion resistance High resistance to aggressive environment More durability
Economic • • • •
Advantages:-
Savings:-
Saves on clinker cost Reduce energy consumption Reduces raw material (lime stone, iron ore etc.) Cost Reduces overall consumption of cement
Fly Ash in Mining Sector:Satisfied with the experience of demonstrations of Pond Ash stowing in underground mine fills the project for large scale adaptation of the technology at SCCL-Manuguru has started. A complete mine panel of 1.5 lack m³ void capacity has been taken up for pond ash stowing at SCCL-Manuguru. A similar large-scale project is being planned at WCLChandrapur also.
Projects
Undertaken:-
1. Durgapur Raitwari Colliery, WCLChandrapur through Normal Surface Bunker & Katora Bunker. More than 12000 m³ pond ash stowed in panel Nos. 16, 17, 18, 28 & 30 (6000 m³ pond ash & 6500 m³ pond ash mixed with sand) 2. PK-1 mine, SCCL- Manuguru. More than 10000 m³ pond ash stowed in panel No.SP-1
Technical:• • • • • •
Excellent flow characteristics Good load bearing capacity of ash fill No subsequent settlement Good water percolation rate Load on barricade is very low Fines escaping through barricade <1%
Environmental:• • • • • •
No excavation and transportation of scarce river bed sand Enormous reduction in water requirement Less water to be pumped out Discharged water is absolutely safe Large scale utilization of ash Release of coal stuck up in pillars
Fly Ash in Agriculture: The produce as well as the biomass have been evaluated for toxicology, nutritional value as well as radioactivity and have been certified as good material for human and animal consumption. In some cases significant increase in minerals (iron and calcium) have been noticed which is beneficial.
Agriculture • • • • • • • •
Improves soil texture Reduces bulk density of soil Improves water holding capacity Optimizes pH value Improves soil aeration Reduces crust formation Provides micro nutrients like Fe, Zn, Cu, Mo, B etc. Provides macro nutrients like K, P, Ca, etc
Agriculture • • • •
Applications:-
Related Application of Fly Ash:-
Field crops Forestry Floriculture Reclamation of Wasteland
Fly Ash in Hydro Sector:Fly ash not only provides solution to heat of hydration in mass concreting but also makes the concrete stronger, durable and more impervious. In Roller Compacted Concrete fly ash substitutes around 65 per cent of cement. Thus, in addition to faster construction it makes construction more durable, economical and environment friendly.
Avenues of Fly Ash Utilization in Hydro Sector:•
•
• • •
Mass Concrete o Conventional concrete with low ash content (10-15%) o
Conventional concrete with medium ash content (30-35%)
o
Roller Compacted Concrete with high ash content (65-70%)
Tunnel Lining Reinforced cement concrete works o o
Foundations
o
Super-structures
o
Pre-stressed concrete structure
Shotcrete / Gunite Diaphragm walls for under seepage control Canals Canal lining : Insist concrete, fly ash pre-cast tiles, fly ash bricks, CLC tiles Pre-stressed concrete, masonry, RCC o o
• • • • • • •
o
Super-passages, aqueducts
o
Canal structures, etc.
Grouting Gravity Dams River banks Concrete and Masonry Barrages Office / Residential Complexes Roads & Embankments Development of Land and Horticulture, etc.
Others Products:Granite Substitute:• • • • •
Fly ash content 60% Good finish, properties comparable to natural granite Developed by BHEL Pilot production started Used at BHEL Corporate Office, New Delhi
Paint & Enamels:Fly ash exhibits better extending properties (less absorption) • Fly ash percentage 30-40% (in paints), 18-22% (in enamels) • Corrosion & abrasion resistant • Durable • Developed by RRL-Bhopal. •
Summary of Utilization Area:Fly Ash Utilization Potential Sr. No.
1.
2. 3. 4. 5. 6.
Utilization Area
Roads & Embankment Building Components Bricks / Blocks Cement Mine Fill Agriculture Reclamation Of Low Lying Areas Wasteland Development Total
Potential / Year Utilization (Million. Tons.)
Benefits (Rs. In Crores)
15-20
100
30 25 15 100
300 2500 50+Addn Coal 3000
15
75
100 >300
Enormous Lands Would Be Recovered 5855 =1.25 billion US$
Separation Technologies:For a material to have reasonable commercial value, this material must be well defined and be able to meet industrial specifications. It must also be available at consistent quality levels and in sufficient quantities to meet market demands. Characterization of fly ash obtained from various sources shows that the mineral components of the ashes are similar, even though the bulk chemistry of these ashes may vary widely. Based on scanning electron microscope studies, the major mineral components in fly ash can be categorized into silicates, iron oxides, low density silicates (cenospheres) and unburned carbons. The silicates are usually present as spherical particles. They are believed to be the melted products of clays, feldspars, quartz, calcite, and other common minerals in coal. The iron oxides are usually spherical magnetite. They are believed to be derived from pyrite, hematite, siderite, and limonite in the coal. Low density silicates are frequently high alkaline silicates which entrapped gas to yield hollow spherical particles. The lower melting point of these high alkaline silicates may facilitate gas entrapment. Unburned carbons are generally chars with irregular shapes and wide range of particle sizes. Variations in fly ash bulk chemistry are usually due to the changes of ratios of these mineral components. Based on these characterization results, a separation process has been designed and tested. A schematic flow sheet of the separation process is shown in Figure. This process consists of a gravitational separation process to separate the cenospheres, a magnetic separation process to separate the iron oxide spheres, and a froth flotation process to separate the unburned carbon [6, 7]. The material left after these separations is designated as clean ash. Depending on user needs, individual process separation circuits can be switched in sequence, eliminated, or new circuits can be added. This offers great flexibility for meeting varying
requirements due to changes in ash (e.g. from a fuel switch in a power plant) and markets. For example, a froth flotation circuit may be all that is needed for ash processing if the material is to be used only for cement replacement. But magnetic separation would have to be included if the cleaned ash is to be used for refractory applications. For plastic filler applications, a hydro cyclone circuit would need to be included in order to separate out the appropriate fine particle fraction. Table 1 shows the results of separation for an AEP low NOx fly ash sample. Fly ash was mixed with water at 20% solids content in a pilot plant operation running at 200 lb/hr. The slurry was fed into a tank where the cenospheres were skimmed off from the top since these cenospheres have a density less than that of water. Then the slurry was fed into a magnetic drum separator to recover the magnetic spheres. After magnetic separation, the slurry was conditioned with an oil collector at a dosage of 2 lb/ton. The oil has an affinity for carbon and is preferentially adsorbed onto the carbon particles. The slurry was then fed into a flotation machine where air was bubbled through it. During flotation, the rising air bubbles collided with the oil coated carbon particles and attached themselves to these particles due to a hydrophobic interaction. This caused the carbon particles to float to the top of the flotation cell, where they were skimmed off. This flotation operation left the clean ash in the cell. This clean ash was then filtered and dried. The carbon fraction was transferred to another flotation cell and re-floated to upgrade the carbon content in the carbon concentrate. The reject from the carbon-refloat operation was then returned to the first flotation cell. A typical operation showed the carbon (LOI) content in the clean ash to be only 0.40%, greatly reduced from the 21.70% carbon content of the as-received fly ash. The carbon concentrate had a carbon content of 67.70%. The magnetic concentrate contained 77.18% iron oxide.
This process has been applied to many different fly ash samples obtained from various power companies including Detroit Edison, Consumers Power, Baltimore Gas and Electric, Virginia Power, American Electric Power, Nevada Power. Table 2 shows typical results obtained on these ashes. Note that clean ash with less than 1% carbon content can always be obtained.
Figure of Fly Ash Separation Process
Table No. 01: Separation Products From An AEP Low NOx Sample Clean Cenospher Magnetic As-Rec’d Carbon Ash e s 44.00 58.6 19.26 57.58 14.34 SiO2 22.40 29.2 9.92 29.57 8.2 Al2O3 5.30 5.2 0.04 3.71 77.18 Fe2O3 0.86 1.11 0.5 1.38 0.5 MgO 0.76 0.85 0.5 0.35 0.45 CaO 0.32 0.42 0.05 0.38 0.04 Na2O 2.35 3.16 0.8 4.23 0.43 K 2O 1.11 1.33 0.7 0.91 0.31 TiO2 0.03 0.09 0.22 0.03 0.01 P2O5 0.01 0.02 0.01 0.02 0.06 MnO 21.70 0.40 67.7 2.4 -1.4 LOI 98.84 100.38 99.89 100.83 100.28 Total
Table No. 02: Carbon Removal by Froth Flotation Ash Type LOI, AsReceived LOI, Clean Ash
F (#1)
F (#2)
F+C
C
21.70
7.25
4.35
4.00
0.40
0.61
0.90
0.96
What Is Quality Concrete? To fully appreciate the benefits of fly ash in concrete, the basics of producing exceptional concrete must be understood. Concrete is a composite material, which essentially consists of two components: aggregates and cementitious paste. To produce exceptional concrete, it is extremely important to have a smooth gradation of material from rock down to the finest particles (in other words, a good mix of particle sizes, so that the largest practicable rock fills the majority of the volume, while the progressively smaller rock and sand fill the voids left between the larger particles). Ideally, it is best to have as much volume as possible filled with strong, durable aggregate particles, with enough paste (comprised of as much CSH and as little lime as possible) to coat every particle. Also, voids should not be present in the paste unless they are specifically provided as microscopic entrained air bubbles to provide durability in freeze-thaw environments. In real life, though, economics and local aggregate sources dictate the quality of materials used. The result is that excess voids often exist between the aggregate particles that must now be filled by paste and air. The challenge becomes producing an appropriate amount of the best possible quality paste, so that the resulting hardened paste will fill the excess voids with durability and strength approaching that of the aggregates.
How Fly Ash Contributes to Concrete Durability and Strength? Most people don’t realize that durability and strength are not synonymous when talking about concrete. Durability is the ability to maintain integrity and strength over time. Strength is only a measure of the ability to sustain loads at a given point in time. Two concrete mixes with equal cylinder breaks of 4,000 psi at 28 days can vary widely in their permeability, resistance to chemical attack, resistance to cracking and general deterioration over time — all of which are important to durability. Cement normally gains the great majority of its strength within 28 days, thus the reasoning behind specifications normally requiring determination of 28-day strengths as a standard. As lime from cement hydration becomes available (cements tend to vary widely in their reactivity), it reacts with fly ash. Typically, concrete made with fly ash will be slightly lower in strength than straight cement concrete up to 28 days, equal strength at 28 days, and substantially higher strength within a year’s time. Conversely, in straight cement concrete, this lime would remain intact and over time it would be susceptible to the effects of weathering and loss of strength and durability.
As previously described, the paste is the key to durable and strong concrete, assuming average quality aggregates are used. At full hydration, concrete made with typical cements produces approximately 1/4 pound of non-durable lime per pound of cement in the mix. Most people have seen concrete or masonry walls or slabs with the white, chalky surface coating or streaks called efflorescence. Efflorescence is caused by the face of the concrete being wetted and dried repeatedly or by the movement of water vapor from the damp side of the concrete to the dry side through the capillaries (voids), drawing out the water soluble lime from the concrete, block or mortar. A typical 5 sack concrete mix having 470 pounds of cement per cubic yard has the potential of producing 118 pounds of lime. Fly ash chemically reacts with this lime to create more CSH, the same “glue” produced by the hydration of cement and water, thereby closing off the capillaries that allow the movement of moisture through the concrete. The result is concrete that is less permeable, as witnessed by the reduction in efflorescence.
Other evidence of the contribution fly ash makes to strength and durability includes:
Cement has an upper limit of roughly 7.5 sacks (7.5 x 94# sack = 705#) when using 1" maximum size aggregate, above which the psi per pound of cement strength contribution in a concrete mix diminishes rapidly. The tallest concrete structures in the world are made with concrete where fly ash is a necessary component. Its ability to contribute to additional CSH, lower water demand, reduced heat of hydration and its fine particle size are crucial to making high-strength concrete (8,000 psi to over 20,000 psi).
Cement was invented in 1824, over 170 years ago. There are examples on the west coast of Italy, in a town named Cosa, where a mixture of natural pozzolans (volcanic) were combined with lime to produce concrete that has withstood waves and attack from seawater for over 2,000 years and is still intact. – The Pantheon in Rome is a pozzolan and lime concrete structure built around 300 B.C. and still stands today. It features a cast concrete dome 124 feet in diameter and was the world’s largest domed structure until modern times.
How Fly Ash Contributes to Concrete Workability?
First, fly ash produces more cementitious paste. It has a lower unit
weight, which means that on a pound for pound basis, fly ash contributes roughly 30% more volume of cementitious material per pound versus cement. The greater the percentage of fly ash “ball bearings” in the past, the better lubricated the aggregates are and the better concrete flows. Second, fly ash reduces the amount of water needed to produce a given slump. The spherical shape of fly ash particles and its dispersive ability provide water-reducing characteristics similar to a water reducing admixture. Typically, water demand of a concrete mix with fly ash is reduced by 2% to 10%, depending on a number of factors including the amount used and class of fly ash. Third, fly ash reduces the amount of sand needed in the mix to produce workability. Because fly ash creates more paste, and by its shape and dispersive action makes the paste more “slippery”, the amount of sand proportioned into the mix can be reduced. Since sand has a much greater surface area than larger aggregates and therefore requires more paste, reducing the sand means the paste available can more efficiently coat the surface area of the aggregates that remain? Evidence of the contribution fly ash makes to workability includes: •
•
Lightweight concrete including fly ash is much easier to pump. Finishers notice the “creamier” texture when working. They also see reduced “bug holes” and segregation when stripping forms. Slip form pavers eliminate rock pockets and voids in an otherwise harsh, no-slump paving mix
How Fly Ash Protects Concrete? An extremely important aspect of the durability of concrete is its permeability. Fly ash concrete is less permeable because fly ash reduces the amount of water needed to produce a given slump, and through pozzolanic activity, creates more durable CSH as it fills capillaries and bleeds water channels occupied by watersoluble lime (calcium hydroxide). Fly ash improves corrosion protection. By decreasing concrete permeability, fly ash can reduce the rate of ingress of water, corrosive chemicals and oxygen — thus protecting steel reinforcement from corrosion and its subsequent expansive result. Fly ash also increases sulfate resistance and reduces alkali-silica reactivity. At this point a distinction between Class C and Class F fly ashes needs to be made. While both improve the permeability and general durability of concrete, the chemistry of Class F ashes has proven to be more effective in mitigating sulfate and alkalisilica expansion and deterioration in concrete. Some Class C fly ashes have been used to mitigate these reactions, but must be used at higher rates of cement replacement. Fly Ash in concrete can reduce sulfate attack in two additional ways: •
•
Fly ash reduces calcium hydroxide, which combines with sulfates to produce gypsum. Gypsum is a material that has greater volume than the calcium hydroxide and sulfates that combine to form it, causing damaging expansion. Aluminates in the cement also combine with sulfates to form expansive compounds. By replacing cement, the amount of
available aluminates is reduced, thereby lowering the potential for this type of expansive reaction.
How Fly Ash Reduces Hydration in Concrete?
Heat
of
The hydration of cement is an exothermic reaction. Heat is generated very quickly, causing the concrete temperature to rise and accelerating the setting time and strength gain of the concrete. For most concrete installations, the heat generation is not detrimental to its long-term strength and durability. However, many applications exist where the rapid heat gain of cement increases the chances of thermal cracking, leading to reduced concrete strength and durability. In these applications, replacing large percentages of cement with fly ash (fly ash generates only 15 to 35 percent as much heat as compared to cement at early ages) can reduce the damaging effects of thermal cracking. While the first structures to apply this concept in earnest were hydroelectric dams built in the 1930s and 1940s with 40% to 50% cement replacement, warm weather concreting and the risk of thermal cracking is a problem that exists today for all concrete. Warm weather will naturally raise the temperature of concrete aggregates, which make up the majority of the mass in concrete. This natural heating of the aggregates, coupled with solar heating at the construction site, can cause even thin concrete slabs to suffer the damaging effects of thermal cracking, along with finishing difficulties caused by rapid uncontrolled setting. Replacing 20% to 35% of the cement for “everyday” concrete in warm conditions will help reduce thermal cracking and provide the time needed to obtain the desired finish.
Fly Ash Checklist: Concrete Workability
Enhancing
Workability: Concrete is easier to place with less effort, responding better to vibration to fill forms more completely.
Ease of Pumping: Pumping requires less energy and longer pumping distances are possible.
Improved Finishing: Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.
Reduced Bleeding: Fewer bleed channels decrease permeability and chemical attack. Bleed streaking is reduced for architectural finishes.
Reduced Segregation: Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and blemishes.
Fly Ash Checklist: Concrete Performance
Increasing
Higher Strength:Fly ash continues to combine with free lime, increasing compressive strength over time.
Decreased Permeability:Increased density and long term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability.
Increased Durability:Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, soft (lime hungry) water, and seawater.
Reduced Sulfate Attack:Fly ash ties up free lime that can combine with sulfates to create destructive expansion.
Reduced Efflorescence:Fly ash chemically binds free lime and salts that can create efflorescence, and dense concrete holds efflorescence producing compounds on the inside.
Reduced Shrinkage: The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying shrinkage.
Reduced Heat of Hydration: The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to reduce Portland cement.
Reduced Alkali Silica Reactivity:Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, causing destructive expansion.
Specifications:ASTM (West Conshohocken, PA USA):•
•
•
•
•
•
•
ASTM C 618: Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete. ASTM C 311: Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use as a Mineral Admixture in Portland-Cement Concrete. ASTM D 5239: Standard Practice for Characterizing Fly Ash for Use in Soil Stabilization. ASTM E 850: Standard Practice for Use of Inorganic Process Wastes as Structural Fill. ASTM E 1861: Standard Guide for Use of Coal Combustion By-products in Structural Fills. ASTM D 5370: Standard Specification for Pozzolanic Blended Materials in Construction Applications. ASTM C 1240: Standard Specification for Silica Fume for Use in Hydraulic-Cement Concrete and Mortar.
IDOT (Illinois Dept. of Trans. Springfield, IL USA):•
•
•
•
306.01: Special Provision for Fly Ash Modified Soils 308.01: Special Provision for Fly Ash Stabilized Soil Mixture Sub base Special Provision for Cement-Fly Ash-Aggregate Mixture (CFAM) Base Course Special Provision for Pozzolanic Base Course, Type A
•
Special Provision for Use of Fly Ash in P.C.C. Pavement, Base Course, Base Course Widening.
AASHTO:•
AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing.
European Standards:•
•
•
BS 3892 Part 1 Fly Ash standard; Part 2 Fly Ash for Use as a Type II Addition. BS EN 450 European Standard for Fly Ash. BS EN 197 European Standard for Multiple Binders ( fly ash, cement, silica fume) Allowed in Concrete.
Australia:•
•
•
•
Portland Cement -AS 3972-1991. Fly ash -AS 3582.1-1991 and Ground Granulated Blast Furnace Slag -AS 3582.2. A.S. 1129 Fly Ash Specification.
Canada:•
CAN/CSA A23.5-97 Canadian Specification for Supplementary Cementing Materials (includes fly ash).
Germany (Deutsches Normung-Berlin):•
•
•
•
•
•
•
•
•
•
for
DIN 1164-1 German Cement standard DIN 1045 Reinforced Concrete Structures; Design and Construction DIN EN 450 Fly Ash In Concrete-Definition, Demands and Quality control ENV 206:1990 (CEN/TC 104) Beton - Eigenschaften, Herstellung, Verarbeitung und Gütenachweis EN 445:1996 (CEN/TC 104) Einpreßmörtel für Spannglieder - Prüfverfahren EN 446:1996 (CEN/TC 104) Einpreßmörtel für Spannglieder - Einpreßverfahren EN 447:1996 (CEN/TC 104) Einpreßmörtel für Spannglieder Anforderungen für üblicheSn Einpreßmörtel EN 450:1994 (CEN/TC 104) Flugasche für Beton Definitionen, Anforderungen und Güteüberwachung EN 451-1:1994 (CEN/TC 104) Prüfverfahren für Flugasche Teil 1: Bestimmung des freien Calciumgehalts EN 451-2:1994 (CEN/TC 104) Prüfverfahren für Flugasche - Teil 2: Bestimmung der Feinheit durch Naßsiebung
Netherlands:•
Institute
NEN 3550 Dutch cement standard.
