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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Coal fly ash has been successfully 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(1) defines two blended cement products in which fly ash has been added: 1) Portland-pozzolan cement (Type IP), containing 15 to 40 percent pozzolan, or 2) Pozzolan modified Portland cement (Type I-PM), containing less than 15 percent pozzolan.
ASTM C618 defines two classes of fly ash for use in concrete: 1) Class F, usually derived from the burning of anthracite or bituminous coal, and 2) Class C, usually derived from the burning of lignite or subbituminous coal.(2) 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.
A 1992 survey of all 50 state transportation agencies indicated that 40 states have had experience in the use of fly ash as a mineral admixture in concrete, usually as a partial replacement for Portland cement, although a number of states have also used blended Portland-pozzolan cement. Virtually all 40 of these states have used fly ash in concrete pavements and shoulders. This same survey indicated that 44 states had specifications for the use of fly ash as a partial replacement for Portland cement in concrete.(3)
At the time of this survey, at least eight states did not permit the use of fly ash in either bridge deck or structural concrete. A number of states also did not permit the use of fly ash in white concrete items, such as curbs, sidewalks, and median barriers, and two states (Arkansas and New Mexico) reported questionable performance experience: Arkansas had temporarily discontinued the use of fly ash in bridge deck concrete, and New Mexico had a temporary moratorium on the use of Class C fly ash in concrete, pending further investigation.(4)
The principal benefits ascribed to the use of fly ash in concrete include enhanced workability due to spherical fly ash particles, reduced bleeding and less water demand, increased ultimate strength, reduced permeability 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 usually associated with the use of fly ash in concrete include somewhat slower early strength development, extended initial setting time, possible 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 even enhance early strength development.
MATERIAL PROCESSING REQUIREMENTS
To ensure the quality of fly ash for use in PCC, the following sources of ash should be avoided:
The net result of all these restrictions is that only a relatively low percentage (25 to 30 percent, at most) of all the coal fly ash produced annually is even potentially suitable for use in PCC.
Drying or Conditioning
When used in blended cement or as a partial replacement for Portland cement in ready-mix concrete, fly ash must be in a dry form and as such requires no processing. When used as a raw feed material for the production of Portland cement, either dry or conditioned ash can be used.
Fly ash used in concrete should be as consistent and uniform as possible. Fly ash to be used in concrete should be monitored by a quality assurance/quality control (QA/QC) program that complies with the recommended procedures in ASTM C311.(6) 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.
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(2) 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. Fineness can also be assessed by methods that estimate specific surface area, such as the Blaine air permeability test(7) commonly used for Portland cement.
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,(6) 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, even though the ASTM criteria is a maximum LOI content of 6 percent.(2) 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, who 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, who are most concerned with consistent and predictable quality.
Moisture Content: ASTM C618 specifies a maximum allowable moisture content of 3.0 percent.
Some of the properties of fly ash-concrete mixes that are of particular interest include mix workability, time of setting, bleeding, pumpability, strength development, heat of hydration, permeability, resistance to freeze-thaw, sulfate resistance, and alkali-silica reactivity.
Workability: At a given water-cement ratio, the spherical shape of most fly ash particles permits greater workability than with conventional concrete mixes. 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.(8)
Time of Setting: When replacing up to 25 percent of the Portland cement in concrete, all Class F fly ashes and most Class C fly ashes increase the time of setting. However, some Class C fly ashes may have little effect on, or possibly even decrease, the time of setting. Delays in setting time will probably be more pronounced, compared with conventional concrete mixes, during the cooler or colder months.(8)
Bleeding: Bleeding is usually reduced because of the greater volume of fines and lower required water content for a given degree of workability.(8)
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.(8)
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 up to 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.(9) 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.(10)
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.(8) Class F fly ashes are generally more effective than Class C fly ashes in reducing the heat of hydration.
Permeability: Fly ash reacting with available lime and alkalies generates additional cementitious compounds that act to block bleed channels, filling pore space and reducing the permeability of the hardened concrete.(5) The pozzolanic reaction consumes calcium hydroxide (Ca(OH)2), which is leachable, replacing it with insoluble calcium silicate hydrates (CSH).(8) The increased volume of fines and reduced water content also play a role.
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. Once fly ash concrete has developed adequate strength, no significant differences in concrete durability have usually been observed.(8) 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 will generally improve the sulfate resistance of any concrete mixture in which it is included.(11) Some Class C fly ashes may improve sulfate resistance, while others may actually reduce sulfate resistance(12) and accelerate deterioration.(13) 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.(12)
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, preventing them from reacting with silica in aggregates, which can result in expansive reactions. However, because some fly ashes (including some Class C fly ashes) may have appreciable amounts of soluble alkalis, it is necessary to test materials to be used in the field to ensure that expansion due to alkali-silica reactivity will be reduced to safe levels.(8)
Fly ash, especially Class F fly ash, is effective in three ways in substantially reducing alkali-silica expansion: 1) it produces a denser, less permeable concrete; 2) when used as a cement replacement it reduces total alkali content by reducing the Portland cement; and 3) alkalis react with fly ash instead of reactive silica aggregates.(14) Class F fly ashes are probably more effective than Class C fly ashes because of their higher silica content, which can react with alkalis. Users of Class C fly ash are cautioned to carefully evaluate the long-term volume stability of concrete mixes in the laboratory prior to field use, with ASTM C441(15) as a suggested method of test.
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 (16) and high-strength concrete are provided by ACI.(10)
One mix design approach commonly used in proportioning fly ash concrete mixes is to use a mix design with all Portland cement, remove some of the Portland cement, and then add fly ash to compensate for the cement that is removed. Class C fly ash is usually substituted at a 1:1 ratio. Class F fly ash may also be substituted at a 1:1 ratio, but is sometimes specified at a 1.25:1 ratio, and in some cases may even be substituted at a 1.5:1 ratio.(5) There are some states that require that fly ash be added in certain mixes with no 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 when Class C fly ash is used.(10)
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 mixture that is being investigated in the laboratory. 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 is to replace Portland cement on an equal weight (1:1) basis, the total weight of cementitious material in each trial mix will remain the same. However, because of differences in the specific gravity values of Portland cement and fly ash, the volume of cementitious material will vary with each trial mixture.(10)
When a Type IP (Portland-pozzolan) or Type I-PM blended cement is used in a concrete mix, fly ash is already a part of the cementing material. There is no need to add more fly ash to a concrete mix in which blended cement is being used, and it is recommended that no fly ash be added in such cases. The blended cement can be used in the mix design process in essentially 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, with a typical range being from 0.37 to 0.50. When using a blended cement, the water demand will probably be somewhat reduced because of the presence of the fly ash in the blended cement. 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, instead of the conventional w/c ratio.(16)
The design of any concrete mix, including fly ash concrete mixes, 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 detailed, step-by-step directions regarding trial mix proportioning of the water, cement (or cement plus fly ash), and aggregate materials. Fly ash has a lower specific gravity than Portland cement, which must be taken into consideration in the mix proportioning process.
Structural design procedures for concrete pavements containing fly ash are no different than design procedures for conventional concrete pavements. The procedures are based to a great extent on the design strength of the concrete mix, usually determined by testing after moist curing for 28 days.
For structural concrete, the design strength is usually the unconfined compressive strength as determined by ASTM C39.(17) For pavement concrete, the design strength may be either the tensile or flexural strength, or possibly the unconfined compressive strength.
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 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 of the concrete is within specified limits. The air content of the concrete must be carefully checked and adjusted during production to ensure that it remains within those limits. As with any concrete, excessive vibration should be avoided because it may reduce the air content of the in-place concrete.(5)
Placement and handling of fly ash concrete is in most respects 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 benefit workability and finishing.(5) Normal procedures for screeding, finishing, edging, and jointing of conventional PCC are also applicable to fly ash concrete.
The slower strength development of concrete containing Class F fly ash may require that the moisture be retained in the concrete for a longer period of time than what is normally 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. Normal curing practices should be adequate for concrete containing Class F fly ash.
Scheduling of pavement construction should allow adequate time for the desired or specified strength gain prior to the placement of traffic loads, the onset of freeze-thaw cycles, and the application of deicing salts because of the detrimental effect of cold weather on strength gain. Some states, such as Wisconsin, 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.
Rather than relying on a cut-off date, the percentage of fly ash could be reduced during colder weather, or other measures (such as additional Portland cement, or the possible use of high-early strength cement, or a chemical accelerator) could be taken to maintain or improve strength development under low temperature conditions. 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 some fly ash.(18)
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 and does not fluctuate to any greater extent than a normal PCC mix. To ensure that such is the case, air content testing of fly ash concrete mixes may initially need to be done at a greater frequency than with normal PCC mixes. Another important quality control consideration in freshly mixed PCC is its workability, as determined by performing slump tests. Slump testing of fly ash concrete can be done at the same frequency as for normal PCC mixes.
An improved means of classifying and specifying fly ash sources for use as a mineral admixture in PCC is needed.
There are considerable laboratory and limited field data that indicate that high percentage (50 to 70 percent) Class F or Class C fly ash, in combination with a high range water reducing admixture, produces concrete with exceptional compressive strength.(19) Trial usage of high percentage fly ash concrete mixes is needed in order to be able to evaluate the field performance of these mixes.
Class F fly ash may have cementitious ability when blended with other by-products such as cement kiln dust prior to being introduced into a concrete mix. Additional data are needed on the characteristics and long-term performance of concrete mixes in which a blend of fly ash and other cementitious (or pozzolanic) by-products is used.
As a consequence of the Clean Air Act, many coal-fired power plants are being equipped with low NOx burners. The short-term effect of burning coal in a low NOx burner appears to be an increase in the LOI of the fly ash. The coal ash industry is developing comparative information on the characteristics and engineering properties of ASTM C618 sources of fly ash before and after installation of low NOx burners. Some fly ash sources do not have acceptable LOI values once low NOx burners have been installed and put into operation.