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COAL FLY ASHUser Guideline


Portland Cement Concrete

INTRODUCTION

By 2010, the net cement production in the world is expected to be nearly two billion metric tons (2.2 billion tons), emitting two billion metric tons of carbon dioxide annually.(1) The replacement of fly ash for Portland cement is therefore one way to reduce greenhouse gas emissions while also advancing sustainable development.

Coal fly ash has been used in Portland cement concrete (PCC) as a mineral admixture, and more recently as a component of blended cement, for nearly 60 years. As an admixture, fly ash functions as either a partial replacement for, or an addition to, Portland cement and is added directly into ready-mix concrete at the batch plant. Fly ash can also be interground with cement clinker or blended with Portland cement to produce blended cements. ASTM C595 defines two blended cement products in which fly ash is a component: 1) Portland-pozzolan cement (Type IP), containing 15 to 40 percent pozzolan, and 2) Pozzolan modified Portland cement (Type I-PM), containing less than 15 percent pozzolan.(2)

ASTM C618 defines two classes of fly ash for use in concrete: 1) low-calcium or Class F fly ash, usually derived from the burning of anthracite or bituminous coal, and 2) high-calcium or Class C fly ash, usually derived from the burning of lignite or subbituminous coal.(3;4) ASTM C618 also delineates requirements for the physical, chemical, and mechanical properties for these two classes of fly ash. Class F fly ash is pozzolanic, with little or no cementing value alone. Class C fly ash has self-cementing properties as well as pozzolanic properties.

Fly ash of both classes reacts chemically with lime to form cementitious materials. Since Portland cement contains about 65 percent lime, some of which becomes free and available during hydration, the inclusion of fly ash in Portland cement forms additional cementitous materials and improves many properties of the resulting concrete.(5)

PERFORMANCE RECORD

Virtually all state transportation agencies indicate that they have used fly ash as a mineral admixture in concrete, as a partial replacement for Portland cement, or in blended Portland-pozzolan cement. Fly ash has been used in concrete pavements and shoulders for years, and most states have specifications for the use of fly ash as a partial replacement for Portland cement in concrete.(6;7)

The principal benefits of fly ash in concrete include enhanced workability, reduced bleeding and less water demand, increased ultimate strength, reduced hydraulic conductivity and chloride ion penetration, lower heat of hydration, greater resistance to sulfate attack, greater resistance to alkali-aggregate reactivity, and reduced drying shrinkage.(5)

The main precautions associated with the use of fly ash in concrete can include slower early strength development, extended initial setting time, difficulty in controlling air content, seasonal limitations during winter months, and quality control of fly ash sources.(5) The use of Class F fly ash usually results in slower early strength development, but the use of Class C fly ash does not and may enhance early strength development.

MATERIAL PROCESSING REQUIREMENTS

Source Control

To ensure the quality of fly ash for use in PCC, the following sources of ash should be avoided:

  • Ash from a peaking plant instead of a base loaded plant;
  • Ash from plants burning different coals or blends of coal;
  • Ash from plants burning other fuels (wood chips, tires, trash) blended with coal;
  • Ash from plants using oil as a supplementary fuel;
  • Ash from plants using precipitator additives, such as ammonia;
  • Ash from start-up or shut-down phases of operation;
  • Ash from plants not operating at a "steady state";
  • Ash that is handled and stored using a wet system.

Drying or Conditioning

When fly ash is used in blended cement or as a partial replacement for Portland cement in ready-mix concrete, the ash must be in a dry form and requires no processing. When used as a raw feed material for the production of Portland cement, either dry or conditioned ash can be used.

Quality Control

Fly ash used in concrete should be as consistent and uniform as possible. The fly ash should be monitored by a quality assurance/quality control (QA/QC) program that complies with the recommended procedures in ASTM C311.(8) These procedures establish standards for methods of sampling and frequency of performing tests for fineness, loss on ignition (LOI), specific gravity, and pozzolanic activity such that the consistency of a fly ash source can be certified.

Many state transportation agencies, through their own program of sampling and testing, have been able to prequalify sources of fly ash within their own state (or from nearby states) for acceptance in ready-mixed concrete. Prequalification of fly ashes from different sources provides an agency with a certain level of confidence in the event fly ashes from different sources are to be used in the same project.

ENGINEERING PROPERTIES

Some of the engineering properties of fly ash that are of particular interest when fly ash is used as an admixture or a cement addition to PCC mixes include fineness, LOI, chemical composition, moisture content, and pozzolanic activity. Most specifying agencies refer to ASTM C618(3) when citing acceptance criteria for the use of fly ash in concrete.

Fineness: Fineness is the primary physical characteristic of fly ash that relates to pozzolanic activity. As the fineness increases, the pozzolanic activity can be expected to increase. Current specifications include a requirement for the maximum allowable percentage retained on a 0.045 mm (No. 325) sieve when wet sieved. ASTM C618 specifies a maximum of 34 percent retained on a 0.045 mm (No. 325) sieve. Methods of specific surface area estimation can also assess fineness, such as the Blaine air permeability test commonly used for Portland cement.(9)

Pozzolanic Activity (Chemical Composition and Mineralogy): Pozzolanic activity refers to the ability of the silica and alumina components of fly ash to react with available calcium and/or magnesium from the hydration products of Portland cement. ASTM C618 requires that the pozzolanic activity index with Portland cement, as determined in accordance with ASTM C311(8), be a minimum of 75 percent of the average 28-day compressive strength of control mixes made with Portland cement.

Loss on Ignition: Many state transportation departments specify a maximum LOI value that does not exceed 3 or 4 percent, although ASTM criteria allow a maximum LOI content of 6 percent.(3) This is because carbon contents (reflected by LOI) higher than 3 to 4 percent have an adverse effect on air entrainment.

Fly ashes must have a low enough LOI (usually less than 3.0 percent) to satisfy ready-mix concrete producers that are concerned about product quality and the control of air-entraining admixtures. Furthermore, consistent LOI values are almost as important as low LOI values to ready-mix producers that strive for consistent and predictable quality.

Concrete mixes containing fly ash with a very high LOI can produce dark-colored surface streaks as carbon particles float to the top during concrete finishing.(5)

Moisture Content: ASTM C618 specifies a maximum allowable moisture content of 3.0 percent.

Workability: At a given water-cement ratio, the spherical shape of most fly ash particles permits greater workability than with conventional concrete mixes. The fly ash particles act as miniature "ball bearings" within the concrete mix, providing the mix with a lubricant effect.(5) When fly ash is used, the absolute volume of cement plus fly ash usually exceeds that of cement in conventional concrete mixes. The increased ratio of solids volume to water volume produces a paste with improved plasticity and more cohesiveness. (10)

Pumpability: Pumpability is increased by the same characteristics affecting workability, specifically, the lubricating effect of the spherical fly ash particles and the increased ratio of solids to liquid that makes the concrete less prone to segregation.(10)

Time of Setting: When replacing up to 25 percent of the Portland cement in concrete, all Class F fly ashes increase the time of setting. Some Class C fly ashes may increase or decrease the time of setting. Delays in setting time are more pronounced, compared with conventional concrete mixes, during the cooler months.(10)

Bleeding: Bleeding is usually reduced when fly ash is used in concrete mix because of the greater volume of fines and lower required water content for a given degree of workability.(10)

Strength Development: Previous studies of fly ash concrete mixes have generally confirmed that most mixes that contain Class F fly ash that replaces Portland cement at a 1:1 (equal weight) ratio gain compressive strength, as well as tensile strength, more slowly than conventional concrete mixes for as long as 60 to 90 days. Beyond 60 to 90 days, Class F fly ash concrete mixes will ultimately exceed the strength of conventional PCC mixes.(5) For mixes with replacement ratios from 1:1 to 1.5:1 by weight of Class F fly ash to the Portland cement that is being replaced, 28-day strength development is approximately equal to that of conventional concrete.

Class C fly ashes often exhibit a higher rate of reaction at early ages than Class F fly ashes. Some Class C fly ashes are as effective as Portland cement in developing 28-day strength.(11) Both Class F and Class C fly ashes are beneficial in the production of high-strength concrete. However, the American Concrete Institute (ACI) recommends that Class F fly ash replace from 15 to 25 percent of the Portland cement and Class C fly ash replace from 20 to 35 percent.(12)

Heat of Hydration: The initial impetus for using fly ash in concrete stemmed from the fact that the more slowly reacting fly ash generates less heat per unit of time than the hydration of the faster reacting Portland cement. Thus, the temperature rise in large masses of concrete (such as dams) can be significantly reduced if fly ash is substituted for cement, since more of the heat can be dissipated as it develops. Not only is the risk of thermal cracking reduced, but greater ultimate strength is attained in concrete with fly ash because of the pozzolanic reaction.(10) Class F fly ashes are generally more effective than Class C fly ashes in reducing the heat of hydration.

Hydraulic conductivity: Fly ash reacting with available lime and alkalies generates additional cementitious compounds that act to block bleed channels, filling pore space, and reducing the hydraulic conductivity of hardened concrete.(13) The pozzolanic reaction consumes calcium hydroxide (Ca(OH)2), which is leachable, replacing it with insoluble calcium silicate hydrates (CSH).(10) The increased volume of fines and reduced water content also play a role in reducing hydraulic conductivity.

Resistance to Freeze-Thaw: As with all concretes, the resistance of fly ash concrete to damage from freezing and thawing depends on the adequacy of the air void system, as well as other factors, such as strength development, climate, and the use of deicer salts. Special attention must be given to attaining the proper amount of entrained air and air void distribution because fly ash may reduce the effectiveness of air entraining agents.(14) Once fly ash concrete has developed adequate strength, no significant differences in concrete durability have usually been observed.(10) There should be no more tendency for fly ash concrete to scale in freezing and thawing exposures than conventional concrete, provided the fly ash concrete has achieved its design strength and has the proper air void system.

Sulfate Resistance: Class F fly ash in concrete generally improves the sulfate resistance. However, replacement of low-calcium fly ash has reduced the resistance of Portland cement to acid rain attack.(15;16) Some Class C fly ashes may improve sulfate resistance, while others may actually reduce sulfate resistance and accelerate deterioration.(17;18) Class C fly ashes should be individually tested before use in a sulfate environment. The relative resistance of fly ash to sulfate deterioration is reportedly a function of the ratio of calcium oxide to iron oxide.(17)

Alkali-Silica Reactivity: Class F fly ash has been effective in inhibiting or reducing expansive reactions resulting from the alkali-silica reaction. In theory, the reaction between the very small particles of amorphous silica glass in the fly ash and the alkalis in the Portland cement, as well as the fly ash, ties up the alkalis in a nonexpansive calcium-alkali-silica gel. This prevents the alkali from reacting with silica within aggregates that would have resulted in expansive reactions. However, because some fly ashes (including some Class C fly ashes) have appreciable amounts of soluble alkalis, testing of mixes is recommended to ensure that expansion due to alkali-silica reactivity will be at acceptable levels.(10)

A modified ASTM C1260 accelerated mortar-bar test can be employed to identify potential alkali-silica reactivity as well as assess the effectiveness of supplementary cementitous materials in decreasing alkali-silica reactivity effects. The original ASTM C1260 that is generally performed quickly (within 14 days) and under harsh conditions (high temperature and highly alkaline solution). This test can produce high alkali-silica reactivity in fly ash mixes even when field performance is adequate. A modified ASTM C1260 for fly ash extends testing times to 28 days and considers various test solutions resulting in more representative levels of alkali-silica reactivity. Further research into other factors such as temperature and constituents of sample mixtures need to be performed.(19)

Fly ash, especially Class F fly ash, is effective in three ways in substantially reducing alkali-silica expansion: 1) fly ash produces a denser, less permeable concrete, 2) when used as a cement replacement fly ash reduces total alkali content by reducing the Portland cement; and 3) alkalis react with fly ash instead of reactive silica aggregates. Class F fly ashes are probably more effective than Class C fly ashes because of higher silica content, which can react with alkalis. Users of Class C fly ash are encouraged to evaluate the long-term volume stability of concrete mixes in the laboratory prior to field use, with ASTM C441(20) as a suggested method of test.

DESIGN CONSIDERATIONS

Mix Design

Concrete mixes are designed by selecting the proportions of the mix components that will develop the required strength, produce a workable consistency concrete that can be handled and placed easily, attain sufficient durability under exposure to in-service environmental conditions, and be economical. Procedures for proportioning fly ash concrete mixes differ slightly from those for conventional concrete mixes. Basic mix design guidelines for normal concrete (21) and high-strength concrete are provided by ACI.(12)

One mix design approach used in proportioning fly ash concrete is to design a mix with all Portland cement, remove a portion of the Portland cement, and then add fly ash to compensate for the cement removed. Class C fly ash is typically substituted at a 1:1 ratio, while Class F fly ash may also be substituted at a 1:1 ratio, but is sometimes specified at a 1.25:1 to 1.5:1 ratio.(5) Some states require that for certain mixes, fly ash be added to a mix without a reduction in cement content.

The percentage of Class F fly ash used as a percent of total cementitious material in typical highway pavement or structural concrete mixes usually ranges from 15 to 25 percent by weight.(5) This percentage usually ranges from 20 to 35 percent for Class C fly ash.(12)

Mix design procedures for normal, as well as high-strength, concrete involve a determination of the total weight of cementitious materials (cement plus fly ash) for each trial mix. The ACI mix proportioning guidelines recommend a separate trial mix for each 5 percent increment in the replacement of Portland cement by fly ash. If fly ash replaces Portland cement on an equal weight basis (1:1), then the total weight of cementitious material in each trial mix remains the same. However, because of the differences in the specific gravity values of Portland cement and fly ash, the volume of cementitious material will vary with each trial mixture.(12)

When Type IP (Portland-pozzolan) or Type I-PM blended (Pozzolan modified Portland) cement is used in a concrete mix, fly ash is already part of the cementing material; therefore, there it is recommended not to add additional fly ash. The blended cement can be used in the mix design process in the same way as a Type I Portland cement.

To select a mix proportion that satisfies the design requirements for a particular project, trial mixes must be made. In a concrete mix design, the water-cement (w/c) ratio is a key design parameter, typically ranging from 0.37 to 0.50. When using a blended cement, the water demand typically is reduced because of the presence of fly ash. When fly ash is used as a separately batched material, trial mixes should be made using a water-cement plus fly ash (w/c+f) ratio, sometimes referred to as the water-cementitious ratio.

The design of any fly ash concrete mix is based on proportioning the mix at varying water-cementitious ratios to meet or exceed requirements for compressive strength (at various ages), entrained air content, and slump or workability needs. The mix design procedures stipulated in ACI 211.1 provide a step-by-step process regarding trial mix proportioning of the water, cement (or cement plus fly ash), and aggregate materials. However, fly ash has a lower specific gravity than Portland cement, which should be taken into consideration in the mix proportioning process.

Structural Design

Structural design procedures for concrete pavements containing fly ash are no different than design procedures for conventional concrete pavements. The procedures are based on the design strength of the concrete mix, usually determined by testing after moist curing for 28 days. Design strength for pavement concrete may be either the tensile or flexural strength, or possibly the unconfined compressive strength. Design strength of structural concrete is usually the unconfined compressive strength as determined by ASTM C39.(22)

CONSTRUCTION PROCEDURES

Material Handling and Storage

When fly ash is used as a mineral admixture, the ready-mix producer typically handles fly ash in the same manner as Portland cement, except that fly ash must be stored in a separate silo from the Portland cement.

Mixing, Placing, and Compacting

Certain fly ashes will reduce the effectiveness of air entraining agents, requiring a higher dosage of air entraining agents to meet specifications. Therefore, the concrete producer must ensure that the proper amount of air entraining admixture is added during mixing, so that the air content meets specifications. The air content of concrete should be carefully checked and adjusted during production to ensure that the air content remains within specified limits. As with any concrete, excessive vibration should be avoided to maintain the air content of the in-place concrete.(5)

Placement and handling of fly ash concrete is similar to that of normal concrete. Fly ash concrete using Class F fly ash has a slower setting time than normal concrete. As a result, finishing operations may have to be delayed, possibly by 1 to 2 hours, depending on the temperature. Also, fly ash concrete surfaces may tend to be more sticky than normal concrete during placement and finishing, although properly proportioned concrete mixes containing fly ash should improve workability and finishing.(5) Normal procedures for screeding, finishing, edging, and jointing of conventional PCC are also applicable to fly ash concrete.

Curing

The slower strength development of concrete containing Class F fly ash may require moisture be retained in the concrete for a longer period of time than is required for conventional concrete. The proper application of a curing compound should retain moisture in the concrete for a sufficient period of time to permit strength development. Beyond the application of a curing compound, typical curing practices should be adequate for concrete containing Class F fly ash. Moist curing should be carried out for a minimum duration of 14 days to ensure good strength and durability.(23)

Construction should be scheduling to allow adequate time for strength gain prior to: traffic loads, freeze-thaw cycles, or the application of deicing salts. Some states have a construction cut-off date beyond which fly ash is not permitted to be used in concrete until the following spring. There is less of a concern with the use of Class C fly ash in cold weather than Class F fly ash.

Alternative approaches to a cutoff date include: reducing the percentage of fly ash used during colder weather, increasing the amount of Portland cement, using high-early strength cement, or including a chemical accelerator. Normal construction practices for cold weather concreting (such as heated aggregates and mixing water, reducing the slump of the concrete, covering the poured concrete with insulation material, and using space heaters for inside pours) are also applicable for concrete containing fly ash.(24)

Quality Control

The most important quality control consideration concerning the use of fly ash in PCC mixes is to ensure that the air content of the freshly mixed concrete is within specified limits. Air content testing of fly ash concrete mixes may need to be performed at a greater frequency than with normal PCC mixes. Another quality control consideration in freshly mixed PCC is workability, as determined by slump tests. Slump testing of fly ash concrete should be done at the same frequency as for normal PCC mixes.

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of coal fly ash literature is available here.

  1. Bilodeau A, Malhotra VM. High-volume fly ash system: Concrete solution for sustainable development. ACI Mater J 2000;97:41-8.
  2. ASTM C595-07 standard specification for blended hydraulic. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania; 2005.
  3. ASTM C618-05 standard specification for fly ash and raw or calcined natural pozzolan for use as mineral admixture in Portland cement concrete. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2005.
  4. Papadakis VG. Effect of fly ash on Portland cement systems: Part I. low-calcium fly ash. Cem Concr Res 1999;29(1):1727-36.
  5. Federal Highway Administration (FHWA), American Coal Ash Association (ACAA). Fly ash facts for highway engineers. Federal Highway Administration (FHWA); 2003 Report nr FHWA-IF-03-019.
  6. Collins RJ, Ciesielski SK. Recycling and use of waste materials and by-products in highway construction. National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board; Washington, DC: 1994.
  7. Dockter BA, Jagiella DM. Engineering and environmental specifications of state agencies for utilization and disposal of coal combustion products. In: 2005 world of coal ash conference, Lexington, KY. ; 2005.
  8. ASTM C311-05 standard test methods for sampling and testing fly ash or natural pozzolans for use in portland-cement concrete. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2005.
  9. ASTM C204-07 standard test methods for fineness of hydraulic cement by air-permeability apparatus. In: Annual book of ASTM standards, ASTM, West Conshohocken, Pennsylvania: 2007.
  10. Halstead WJ. Use of fly ash in concrete. Transportation Research Board; Washington, D.C.: 1986.
  11. Cook JE. A ready-mixed concrete company's experience with class C ash. National Ready-Mix Concrete Association; Silver Spring, Maryland: 1981. Report nr 163.
  12. ACI 211.4R. ACI 211.4R guide for selecting proportions for high-strength concrete with Portland cement and fly ash. ; 1993. Report nr ACI 211.4R.
  13. Meyers JF, Pichumani R, Kapples BS. Fly ash: A highway construction material. Washington, DC: Federal Highway Administration (FHWA); 1976. Report nr FHWA-IP-76-16.
  14. Freeman E, Gao Y, Hurt R, Suuberg E. Interactions of carbon-containing fly ash with commercial air-entraining admixtures for concrete. Fuel 1997 6;76(8):761-5.
  15. Hester JA. Fly ash in roadway construction. First ash utilization symposium, U.S. Bureau of Mines, Washington, D.C.:; 1967.
  16. Jia X, Zhou S. Effect of low-calcium fly ash on the resistance of cement mortar to sulfate attack in the form of acid rain. Key Engineering Materials 2006;302-303:84-90.
  17. Dunstan ER,Jr. A possible method for identifying fly ashes that will improve sulfate resistance of concrete. Cement, Concrete and Aggregates 1980;2(1).
  18. Helmuth R. Fly ash in cement and concrete. Skokie, Illinois: Portland Cement Association; 1987. Report nr SP040.01T.
  19. Chang-Seon S, Shondeep LS, Zollinger DG. Application of modified ASTM C1260 test for fly ash-cement mixtures, Transportation Research Record [Internet]. [revised 2003;1834.
  20. ASTM C441-05 standard test method for effectiveness of pozzolans or ground blast-furnace slag in preventing excessive expansion of concrete due to the alkali-silica reaction. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2005.
  21. ACI 211.1. ACI 211.1 standard practice for selecting proportions for normal, heavyweight, and mass concrete. American Concrete Institute; Detroit, Michigan: 2002.
  22. ASTM C39/C39M-05e1 standard test method for compressive strength of cylindrical concrete specimens. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania; 2005.
  23. Hani N, Nakin S. Effect of curing methods on durability of high-performance concrete. Transportation Research Record 2002;1798.
  24. Portland Cement Association (PCA). Cold-weather concreting. Portland Cement Association Skokie, Illinois: 1980. Report nr IS154.06T.

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Last Update 7/28/08