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COAL FLY ASHUser Guideline


Stabilized Base and Subgrade

INTRODUCTION

Fly ash is often used as a component of stabilized base and subbase mixtures. Both bituminous (pozzolanic) and subbituminous or lignite (self-cementing) fly ashes can be used. Bituminous fly ash (Class F) is used with a chemical reagent or activator, aggregate, and water. Activators such as Portland cement or lime must be added to produce a cementitious product. For most coarse graded aggregates, the amount of fly ash used will normally be in the 8 to 20 percent range. For sandy aggregates, the amount of fly ash used may be in the 15 to 30 percent range. Subbituminous or lignite fly ash (Class C), which is usually self-cementing, does not require a chemical reagent or activator. This ash is blended with aggregate and water, but because of the flash setting properties, the amount of fly ash usually is in the 5 to 20 percent range. Self-cementing fly ash can be used alone as a base course material without any aggregate.

The use of fly ash in stabilized base and subbase mixtures dates back to the 1950s, when a patented base course product known as Poz-o-Pac (consisting of a blend of lime, fly ash, and aggregate) was originally developed. Since the Poz-o-Pac patents expired during the early 1970s, numerous variations of the basic lime-fly ash-aggregate formulations have evolved. There have also been stabilized base mixtures containing Portland cement that have evolved from soil-cement. All of these mixtures contain fly ash and can be described under the general heading of pozzolan-stabilized base (PSB).

The major component of most stabilized base mixtures is the aggregate. Early Poz-o-Pac mixes within high traffic volume roadways used locally available high-quality crushed rock (such as limestone, trap rock, or granite), sand and gravel, or blast furnace slag. PSB mixes have been placed within haul roads, residential streets, and local roadways using power plant aggregates (bottom ash or boiler slag), marginal aggregates (including some off-spec materials), coal refuse, and reclaimed paving materials. Such alternative aggregates are often available and economical in areas where high-quality aggregate materials may be in short supply.

A similar base course application for fly ash is stabilized cold in-place recycling (CIR). Typically the stabilized CIR process consists of milling existing pavement and mixing the recycled pavement with an asphalt emulsion for stabilization. CIR provides a high-quality base for new asphalt surface courses while permitting the in-place recycling of existing asphalt pavement materials. Similar benefits of asphalt emulsion stabilized CIR have been realized by replacing the asphalt emulsion with fly ash.(1;2;3;4;5;6)

PERFORMANCE RECORD

The successful performance of PSB mixtures depends on the development of strength within the cementitious matrix formed by the pozzolanic reaction between the fly ash and the activator. This cementitious matrix acts as a binder that holds the aggregate particles together, similar in many respects to a low-strength concrete. However, unlike concrete, PSB mixtures are produced at a compactable consistency at or near optimum moisture content.

According to a 2005 survey, 12 states have some type of specifications for using fly ash in transportation soil stabilization applications.(7) At least 22 states have used fly ash in stabilized base or subbase material.(8) Many of the stabilized base and subbase installations have been placed in low traffic areas such as local streets or parking lots. These installations have not usually been well documented. There are, however, a number of PSB projects that have been well reported and have provided excellent performance. At least seven states installed PSB base courses as part of the Federal Highway Administration Demonstration Project No. 59, Fly Ash Use in Highway Construction.

PSB pavements have provided good to excellent performance over many years in numerous locations. In general, these mixtures have been more economical than alternative base materials. Nonetheless, a concern of highway engineers using stabilized-based materials, including soil-cement, is the development of cracks within the base course that may reflect up to the pavement surface (so called "reflection cracks"). A Kansas DOT study comparing partial-depth CIR stabilized with either Class C fly ash or an emulsion with lime slurry showed that the fly ash test sections were more susceptible to both transverse and longitudinal cracking.(1) A similar comparative study in Wisconsin between a control section, asphalt emulsion stabilized section, and a fly ash section showed no observed surface cracking after six years of use.(3;2)

Fly ash can also be used to improve soft subgrade materials. Soil and fly ash mixtures prepared with Class C fly ash ranging from 10 to 20 percent increases both unconfined compressive strength and CBR values. Minimizing infield compaction delay of soil and fly ash mixtures also improved strength.(9;10)

Class C fly ash was successfully used in soil stabilization tests at two sites in Wisconsin. Both subgrade soils contained more than 90 percent fines, had in situ water contents 6 to 7 percent wet of optimum, and possessed very low CBRs (between 1 and 3). At both sites, stabilization with self-cementing fly ash considerably improved the strength and stiffness of the soil. The increased stiffness was able to support construction equipment and produced small pavement deflections.(11) Laboratory testing demonstrated increased unconfined compressive strength, CBR, and resilient modulus (Mr) with the addition of fly ash. Testing of mix designs demonstrated that fly ash contents greater than 20 percent provided little gain in improvement, while fly ash contents between 10 and 12 percent optimized cost and strength. Field CBRs using the optimum mix design were approximately two-thirds of the CBRs measured in the laboratory, although this increase in strength was more than adequate to support construction equipment.(11)

MATERIAL PROCESSING REQUIREMENTS

Moisture Control

Aside from possible adjustments to moisture content, there is little to no processing required for using fly ash in PSB mixtures. For Class F fly ash, the moisture content is dictated by the type of equipment used to produce the base course material. If a central-mix concrete plant is used, the fly ash will most likely be fed from a silo in dry form. If a pugmill mixing plant is used, the fly ash will probably be fed from a storage bin in conditioned form. If PSB materials are to be mixed in place at the jobsite, Class F fly ash will be placed and mixed in a conditioned form. Conditioned ash contains a minimal amount of water (usually 10 to 15 percent) to prevent dusting.

Activators (e.g., lime, Portland cement, kiln dust) are nearly always added to the mixture in a dry form. This means that the activators require no processing and will be delivered to the job site and stored in silos or tankers.

Class C fly ash is likely to be self-cementing. There are two ways to offset the rapid hardening of base materials using self-cementing ashes. One is to initially condition the ash with relatively low amounts (10 to 15 percent) of water, stockpile the partially hardened material for several weeks, and then run the ash through a crusher to break down any agglomerations prior to use. The second is to use a commercial retarder (such as gypsum or borax) blended at a low percentage with the fly ash as a means of delaying the initial set.(12)

The aggregate used in PSB mixtures should be in a saturated surface-dry condition during stockpiling. The moisture content of the aggregate should be checked prior to mixing to ensure that excess moisture has not been acquired during stockpiling.

ENGINEERING PROPERTIES

Some of the properties of fly ash that are of particular interest when fly ash is used in stabilized base applications include water solubility, moisture content, pozzolanic activity, fineness, and organic content. Consult ASTM D5239 and C593 for specifications on characterizing fly ash for use in soil stabilization.(13;14) Other properties of interest include compressive strength, flexural strength, resilient modulus, bearing strength, autogeneous healing, fatigue, freeze-thaw durability, and hydraulic conductivity.

Water Solubility: The physical requirements most frequently cited for the use of fly ash (Class F) in PSB mixtures are provided in ASTM C593 which specifies a maximum water soluble fraction of 10 percent.(14)

Moisture Content: If conditioned fly ash is used, the moisture content of the conditioned ash should be determined prior to mixing to confirm the moisture content is in the same range as the ash used during mix design.

Pozzolanic Activity: One of the most important properties of fly ash used in PSB mixtures is pozzolanic activity or reactivity. The pozzolanic reactivity is an indicator of the ability of a given source of fly ash to combine with calcium to form cementitious compounds. The pozzolanic reactivity of fly ash is influenced by the fineness, silica and alumina content, LOI, and alkali content. Besides the gradation of the aggregate used, the pozzolanic reactivity of the fly ash is the major contributor to the strength of the base mix. Pozzolanic activity of fly ash with either lime or Portland cement can be determined using the test methods described in ASTM C311.(15)

Fineness: Fineness requirements for stabilization of soil with Class F fly ash, which requires mixing with lime, are given in ASTM C593. (14) ASTM C593 specifies that 98 percent of the fly ash should be finer than 0.6 mm (No. 30 sieve) and 70 percent finer than 0.075 mm (No. 200 sieve). Most fly ash is capable of meeting these specifications.

Organic Content: Fly ash used in PSB mixtures does not have to meet the ASTM C618(16) requirements of fly ash that is used in Portland cement concrete. Although LOI is not a criterion for the use of fly ash in PSB mixtures, organic soils have traditionally been more difficult to stabilize chemically due to lower solids content, higher water content, lower pH, and chemical interference with cementing reactions.(17)

Compressive Strength: Compressive strength is the most widely used criterion for the acceptability of PSB materials. Compressive strength testing of PSB mixtures is usually performed on Proctor-size specimens 10.2 cm (4 in) in diameter by 11.7 cm (4.6 in) in height, molded at or very close to the optimum moisture content of the mixture. In general, higher compressive strength indicates higher quality of the stabilized material. For cement-stabilized base mixtures, the Portland Cement Association recommends a minimum 7-day compressive strength after curing at 23°C (73°F) of 3,100 kPa (450 lb/in2).(18) Where lime or kiln dust is used as the activator, ASTM C593 specifies a minimum compressive strength after 7 days of curing at 38°C (100°F) as 2760 kPa (400 lb/in2). The ultimate strength of PSB mixtures containing Class F fly ash may be two to three times higher than the 7-day strength. The rate of strength increase for Class F mixtures diminish rapidly after 56 days.(19)

Actual compressive strength development of PSB mixtures in the field is time- and temperature- dependent. As the temperature increases, the rate of strength gain also increases. At or below 4°C (40°F), the pozzolanic reaction virtually ceases and the mixture no longer gains strength. However, once temperatures exceed 4°C (40°F), the pozzolanic reaction resumes and further strength gains occur. For this reason, PSB mixtures can continue to show incremental gains in strength over many years.(19)

Flexural Strength: Because hardened PSB material is a semi-rigid pavement layer, the flexural strength of PSB mixtures may be a better indicator of the material's effective strength. Although flexural strength can be determined directly by testing, most transportation agencies estimate the flexural strength as a fraction of compressive strength. An average value of 20 percent of the unconfined compressive strength is considered to be a fairly accurate estimate of the flexural strength of PSB mixtures.(20)

Bearing Strength: The California bearing ratio (CBR) test(21) is often used as a way of measuring the bearing strength of soils used in subgrades for highway and airfield pavements. Due to the relatively high strength of compacted PSB mixtures, high CBR values (in excess of 100 percent) are not unusual. Use of the CBR test is more applicable to subgrade soil stabilization with fly ash than in evaluating PSB mixtures.

In addition to reducing the swell potential of soft soils,(22) an increase in CBR has been observed with the addition of fly ash to fine-grained soils with plasticity indices in the range of 15 and 40. At an in situ water content of 7 percent wet of optimum, these soils generally had CBRs between 1 and 5, indicating their poor value as subgrades. Addition of 10 percent fly ash caused the CBRs to increase by a factor of 4; while the addition of 18 percent fly ash increased the CBR by a factor of 8. Soil type also affected the observed CBR increase. The largest CBR gain was found with soft, highly plastic clays, and the smallest CBR gain was with more well-graded, silty clay.(17) Field CBRs can be two-thirds the value measured during laboratory design. This is likely due to clumping of clay particles in the field, reducing the uniformity of cementing.(11)

Resilient Modulus: Resilient modulus is a measure of the modulus of elasticity during rapidly applied loadings. Resilient modulus is related to the long-term performance of materials under service loads. Soft soils treated with fly ash can experience relatively large increases in resilient modulus measured by AASHTO T 292.(23) Poor subgrade material can have appreciable gains in Mr (near 100 MPa) with the addition of 10 to 12 percent fly ash.(11) The relationship, Mr = 3 × CBR (MPa), corresponds well to laboratory results on fly ash stabilized soil.(24;17)

Autogenous Healing: One of the unique characteristics of PSB compositions is the ability to heal or re-cement cracks within the material by means of a self-activating mechanism. This mechanism is referred to as autogenous healing and results from the continuation of the pozzolanic reaction between the activator and the fly ash in the PSB mixture. The extent to which autogenous healing occurs depends on the age of the pavement when cracking develops, the degree of contact of the fractured surfaces, curing conditions, the strength of the pozzolanic reaction, and available moisture.(20)

Fatigue Properties: All engineering materials are subject to potential failure caused by progressive fracture under the action of repeated wheel loadings. In pavement design analysis, the flexural fatigue properties of PSB materials are a very important consideration. The flexural strength of PSB mixtures, like the compressive strength, increases with time, while the stress level (the ratio of applied stress to the modulus of rupture) gradually decreases. Because of autogenous healing, PSB mixtures are even less susceptible to fatigue failure than other conventional paving materials.(25)

Freeze-Thaw Durability: Durability testing of PSB materials is performed using one of two established test procedures. For lime and lime-based activators (including kiln dusts), the durability test procedure specified in ASTM C593 is used. This is a vacuum saturation procedure that has been correlated to weight loss after 12 freeze-thaw cycles. The acceptance criterion for ASTM C593 durability testing is that test specimens must have at least 2750 kPa (400 psi) unconfined compressive strength following vacuum saturation testing. For cement-based activators, the durability test procedure specified in ASTM D560(26) is used. The acceptance criterion is a maximum of 14 percent weight loss after 12 freeze-thaw cycles.(26)

The minimum strength required prior to the first freezing cycle to provide sufficient durability against freeze-thaw damage depends on the severity of the climate. The American Coal Ash Association (ACAA) recommends minimum compressive strengths of 6900, 5500, and 4100 kPa (1000, 800, and 600 lb/in2), respectively, for severe, moderate, and mild freeze-thaw conditions.(12)

Hydraulic conductivity: Initial hydraulic conductivity for hardened PSB mixtures can be expected to range between 10-5 and 10-6 cm/sec. As the pozzolanic reaction proceeds, PSB materials may have hydraulic conductivity values between 10-6 and 10-7 cm/sec.(25)

DESIGN CONSIDERATIONS

Mix Design for PSB

A wide range of aggregate sizes can be accommodated in stabilized base and subbase mixtures. After determining the particle size distribution of the aggregate in a PSB mixture, the initial step in determining the mix proportions is to find the optimum fines content. This is done by progressively increasing the quantity of fines (consisting of fly ash plus activator) and making density determinations for the blends of aggregate and fines. Estimated optimum moisture content is selected and held constant for each blend. Each blend of aggregate and fines is compacted into a Proctor mold using standard compaction procedures. At least three such blends are required and five blends are recommended. Dry density versus fines content is plotted and this procedure is used to identify the percentage of fines (expressed as a percentage by dry weight of the total mixture) that results in the highest compacted dry density.

The optimum fines content selected by this procedure should be 2 percent higher than the fines content at the maximum dry density. The optimum moisture content must then be determined for this mix design proportions.

Once the fines content and optimum moisture have been determined, the ratio of activator to fly ash must also be determined. Using a series of trial mixtures, final mix proportions are selected on the basis of the results of both strength and durability testing according to ASTM C593 procedures.(14)

To determine the most suitable proportion of activator to fly ash, five different mix combinations should be evaluated at the optimum moisture content. The typical range of activator to fly ash ratios is 1:3 to 1:5 when using lime or Portland cement. The typical range of kiln dust to fly ash ratios is in the range of 1:1 to 1:2.

The ratio of fines (activator plus fly ash) to aggregate determines the amount of matrix available to fill the void spaces between aggregate particles. Normally, activator plus fly ash contents range from 12 to 30 percent by dry weight of the total mix, although fine-graded aggregates require a higher percentage for satisfactory strength development than well-graded aggregates.

In general, the trial mixture with the lowest ratio of activator to fly ash that satisfies both the strength and durability criteria is considered the most economical mixture. To ensure an adequate factor of safety for field placement, the PSB mixture used in the field should have an activator content that is at least 0.5 percent higher (1.0 percent higher if kiln dust) than that of the most economical mixture identified in the laboratory tests.(12)

Laboratory tests conducted on Class F fly ash-soil mixtures prepared with cement and lime as activators showed that the cement mixes performed better than the mixes using lime as an activator. The CBR, unconfined compressive strength, and resilient modulus increased with increasing cement content; up to 5 percent cement. Conversely, lime treatment had a detrimental effect and an increase in lime content decreased the unconfined compressive strength of both 7 and 28 day specimens.(19)

Case studies indicate that Class C fly ash is typically used in the range of 7 to 10 percent dry weight for stabilized CIR.(6;27;2;1;5)

Structural Design

The design method for pavements including PSB mixtures can follow AASHTO flexiable pavement methods provided in Guide for Design of Pavement Structures.(28) This method accounts for the predicted loading (the predicted number of 80 kN equivalent single axle loads that the pavement will experience), required reliability (degree of certainty that a design will function properly during the design life), serviceable life (ability to lose quality during the pavement life), the pavement structure (characterized by the structural number), and subgrade support (related to the resilient modulus of the subgrade).(28)

The structural number of a pavement design accounts for the relative strength of the constructed materials. The total structural support from the surface course, base course, and any subbase course equals the required structural number. Layer thicknesses are calculated using layer coefficients that define the structural support. The layer coefficients can be obtained from the relationship provided by AASHTO based on CBR or Mr.(28) When available, assigning layer coefficients for fly ash stabilized soils based on correlations for granular subbase materials is reasonable.(11)

By stabilizing the subgrade, the stabilized layer effectively acts as a subbase course that is directly between the base course and the subgrade. A stabilized layer replaces the conventional subbase layer and should be included in the structural number. In this manner, a pavement incorporating a stabilized subbase can be designed with a structural number just as a conventional cut-and-fill pavement.(11)

CONSTRUCTION PROCEDURES

Material Handling and Storage

If fly ash used in a PSB mixture is mixed in a dry form, the fly ash should be stored in a silo or pneumatic tanker. If conditioned fly ash (usually Class F fly ash) is used, then the conditioned fly ash can be stockpiled. When fly ash is stockpiled for an extended period of time in dry or windy weather, the stockpile may need to be periodically moistened to prevent unwanted dusting.

Mixing, Placing, and Compacting of PSB

The primary concerns related to construction and placement of self-cementing fly ash include:

  • Uniform distribution of the fly ash
  • Proper pulverization and thorough mixing of the fly ash with subgrade material
  • Control of moisture content to achieve maximum density and strength
  • Final compaction within the prescribed time frame.(29)

The blending or mixing of PSB materials can be accomplished either in a mixing plant or in-place.

Plant mixing provides greater control over the quantities of materials batched, which results in a uniform PSB mixture. Blending of PSB ingredients in a mixing plant can occur in discrete batches or by continuous mixing. Pugmill mixing plants blend accurately controlled amounts of aggregate, fly ash, activator, and water in batches in a mixing chamber, usually for periods of 30 to 45 seconds. Pugmill mixing plants can be used with properly calibrated field conveyors from bins or silos for a continuous mixing operation. Rotating drum mixers have been used for blending PSB materials in batches.(30) Plant-mixed materials should be delivered to the job site as soon as possible after mixing.

Alternatively, in-place mixing can be used for cold in-place recycling (CIR) of asphalt pavement. On site mixing does not require the establishment of a mixing plant and also takes advantage of the rapid set time of self-cementing fly ash.(31) Although mix-in-place typically does not result in an accurately proportioned mix, mix-in-place still produces high-quality PSB material. The various components of the PSB mixture are delivered, spread on the road site, and mixed in place using a pulvamixer or construction disc.

Delivery of PSB material is typically handled by covered end-dump vehicles. The same equipment used for spreading plant-mixed PSB material can be used for mix-in-place material. Once the PSB material is dumped, spreading is usually accomplished by a bulldozer or a motor grader. However, plant-mixed material can also be spread to a more uniform and accurate loose thickness by a spreader box or a paving machine. PSB material should be as close as possible to optimum moisture content when placed.

During the in-place mixing operation, fly ash should be placed on the roadway first, either directly on a prepared subgrade, or above a layer of aggregate, if the PSB mixture contains aggregate. Fly ash is usually applied in a conditioned form to minimize dusting. The activator is then placed on top of the fly ash, usually in a dry condition, although lime can also be applied in a slurry form. The materials are then mixed together by means of a rotary mixer.

Controlling the water content of the fly ash treated materials is one of the most important steps in the construction procedure. Moisture contents should be between 0 and 4 percent above optimum moisture content.(29) If water is added after blending with the stabilized material, hydration can occur before compaction. Adding water to the pulverized material may make the untreated material unstable for construction equipment. Furthermore, applying water to the fly ash directly distributed on the surface of the subgrade is ill-advised due to premature hydration. Introducing water in the drum of the rotary mixer is the recommended option and proven to be most efficient means of uniformly distributing water.(29)

Compaction of PSB materials should be completed as quickly as possible after placement, especially with mixtures containing Class C (self-cementing) fly ash. The stabilized material can lose strength capacity if the fly ash hydrates in an uncompacted state. The pozzolanic reactions between Class F fly ash and lime is a relatively slow reaction, and a maximum delay of 4-hours should be followed whereas a maximum delay of 2-hours has been recommended for Class C fly ash.(31;23)

Equipment used for compaction is the same, regardless of whether PSB material is plant-mixed or mix-in-place. For granular or more coarsely graded PSB materials, compaction requires the use of steel-wheeled, vibratory, or pneumatic rollers. For more fine-grained PSB materials, initial compaction often requires the use of a sheepsfoot roller, followed by a pneumatic roller.(12)

PSB materials should not be placed in layers that are less than 100 mm (4 in) or greater than 200 to 225 mm (8 to 9 in) in compacted thickness. The material should be spread in loose layers that are approximately 50 mm (2 in) greater in thickness prior to compaction than the desired compacted thickness. The top surface of an underlying layer should be scarified prior to placing the next layer.

Curing

After placement and compaction of the PSB material, the material should be properly protected against drying to assist in the development of in-place strength. Water can be periodically applied between lifts or before application of a wearing surface. If an asphalt concrete pavement is to be placed as an overlay, an asphalt emulsion seal coat should be applied to the top surface of the base or subbase within 24 hours of pavement placement. The exact type of emulsion, rate of application, and temperature of the asphalt must be in compliance with applicable specifications.

The performance of pavement systems incorporating PSB material depends on the development of in-place strength following placement, compaction, and curing. Depending on the anticipated traffic loadings, an analysis of when traffic can be permitted to travel on the base material may be necessary to avoid potential fatigue damage due to early overloading.

Unless an asphalt surface or binder course has been placed over the PSB material, vehicles should not be permitted on the PSB layer until achieving an in place compressive strength of at least 2410 kPa (350 lb/in2). Based on laboratory testing for strength development, the time to achieve this strength can be determined. Ordinarily, placement of asphalt paving over the PSB material is recommended within 7 days after the PSB material has been placed.(12) If a Portland cement concrete pavement is constructed over the PSB layer, a waiting period of 7 days is also recommended.

Late Season Construction

Unless pozzolan stabilized materials are able to develop a certain level of strength prior to the first freeze-thaw cycle, these materials may be unable to withstand repeated freezing and thawing. Since strength development is time- and temperature-dependent, PSB material placed when the air temperature is too cold may not be able to develop the strength and durability needed for adequate freeze-thaw resistance. Minimum temperatures of 4.5°C (40°F) to 10°C (50°F) are commonly used for lime and Portland cement stabilization.(29)

Another concern with late season construction includes mixing operations in clays of high plasticity. Experience has shown that high plasticity clays may require more than one pass at lower temperatures.(29)

Snow has been shown to introduce extra water during the compaction of fly ash without causing the workability problems encountered when attempting to compact fly ash at wetter than optimum states. Addition of snow can cause a 30 percent increase in void ratio, 14 percent decrease in unit weight, and a 70 percent increase in long-term shear strength. The higher strength is believed to be due to the availability of more water for cementation reactions.(32)

Self-Cementing (Class C) Fly Ash

Self-cementing fly ash mixed with water alone usually results in a very rapid time of set. Delays between placement and compacting of PSB material containing self-cementing fly ash are accompanied by a significant decrease in the strength of the compacted base material, unless a retarder is used. Accordingly, PSB mixtures containing self-cementing fly ash should be compacted as soon as possible after mixing, with a recommended maximum elapsed time of no more than 2 hours between mixing and completion of compaction.(33)

A low percentage of water, in the range of 10 to 25 percent by weight of ash, is sufficient to reduce dusting and can be added at the mixing plant. The additional water that is required for proper compaction of the PSB material can be applied in place at the construction site before compaction.

A commercial retarder (such as gypsum, borax, or concrete retarding admixture) may be added in low percentages to the PSB material at the mixing plant. Tests have shown that the addition of 1 percent gypsum did not adversely affect the overall strength development of PSB material, but was effective in retarding rapid setting.(22;33)

Crack Control

Stabilized base layers constructed with fly ash are less likely to produce reflection cracking in overlying pavement as is sometimes the case with Portland cement stabilized base layers. This is most likely due to a less stiff bond within the fly ash stabilized base. Approaches for controlling or minimizing the potential effects of reflective cracking associated with PSB layers have been recommended by the ACAA.(12) A field and laboratory study showed that lime and fly ash stabilized soil is less prone to shrinkage and cracking than cement-stabilized soil base course.(34)

A cement-stabilized Class F fly ash mixture that was used beneath a highway shoulder demonstrated localized cracking due to low density and strength. Severe heave and cracking also developed adjacent to grooves and joints that were cut in the asphalt pavement between the shoulder and traveled way. These joints intercepted and diverted runoff into the underlying fly ash which caused cracking.(35)

A Kansas DOT study comparing partial-depth CIR stabilized with either Class C fly ash or an emulsion with lime slurry showed that the fly ash test sections were more susceptible to both transverse and longitudinal cracking.(1) A similar comparative study in Wisconsin between a control section, asphalt emulsion stabilized section, and a fly ash section showed no observed surface cracking after six years of use.(3;2)

ENVIRONMENTAL CONSIDERATIONS

As described in the Coal Fly Ash Material Description, the use of fly ash as a stabilized base is an unencapsulated use and therefore has the potential for contaminant leaching. Use of fly ash in base material requires good management and care to ensure that the fly ash does not result in a negative impact on the environment. In particular, areas with sandy soils possessing high hydraulic conductivities and areas near shallow groundwater or drinking aquifers should be given careful consideration. An evaluation of groundwater conditions, applicable state test procedures, water quality standards, and proper construction are all necessary considerations in ensuring a safe final product.(36)

WiscLeach is a modeling program specifically developed for coal combustion by-product reuse in highway applications. WiscLeach is available in the public domain and uses analytic methods to simulate two-dimensional flow and transport.(37) Factors found to have the greatest influence on concentrations in groundwater are depth to the groundwater table, thickness of the fly ash layer, hydraulic conductivity of the least conductive layer in the vadose (unsaturated) zone, hydraulic conductivity of the aquifer, and initial trace element concentrations in the fly ash layer.(37)

Current leaching tests have the limitation of not considering the hydrogeologic setting in which coal combustion by-products such as fly ash will be used. These tests consider the use of by-products in bulk form, but not in mixtures.(38)

Lysimeters installed at field sites below a fly ash stabilized base showed an average annual water flux through the stabilized layer of 4 to 6 percent of average annual precipitation, which was comparable with a control section constructed without fly ash. Concentrations of trace elements in the fly ash stabilized leachate were higher than those from the control section. Additionally, concentrations of field leachate agreed well with concentrations in the effluent of laboratory column leaching tests. Further research, however, is needed in developing standards and testing governing fly ash amended soils.(38)

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. Thomas T, Kadrmas A, Huffman J. Cold in-place recycling on US-283 in Kansas. Transportation Research Record 2000;1723(1):53-6.
  2. Crovetti JA. Construction and performance of fly ash-stabilized cold in-place recycled asphalt pavement in Wisconsin. Transportation Research Record 2000;1730:161-6.
  3. Ramme BW, Tharaniyil M. Coal combustion products utilization handbook. We Energies, Milwaukee, WI; 2004.
  4. Wen H, Tharaniyil M, Ramme BW. Investigation of performance of asphalt pavement with fly-ash stabilized cold in-place recycled base course. Transportation Research Record 2003;1819(1):27-31.
  5. Wen H, Tharaniyil MP, Ramme BW, Krebs S. Field performance evaluation of Class C fly ash in full-depth reclamation: Case history study. Transp Res Rec 2004(186):41-6.
  6. Li L, Benson CH, Edil TB, Hatipoglu B. Sustainable construction case history: Fly ash stabilization of recycled asphalt pavement material. Geotechnical and Geological Engineering, An International Journal, 2007.
  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. 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.
  9. Trzebiatowski BD, Edil TB, Benson CH. Case study of subgrade stabilization using fly ash: State highway 32, Port Washington, Wisconsin. In: Beneficial reuse of waste materials in geotechnical and transportation applications. A. Aydileck, J. Wartman, editors, ASCE Reston, VA: ; 2004.
  10. Senol A, Edil TB, Bin-Shafique MS, Acosta HA, Benson CH. Soft subgrades' stabilization by using various fly ashes, Resources, Conservation and Recycling 2006;46:365-76.
  11. Bin-Shafique MS, Edil TB, Benson CH, Senol A. Incorporating a fly-ash stabilized layer into pavement design. Proceedings of the Institution of Civil Engineers: Geotechnical Engineering 2004;157:239-49.
  12. American Coal Ash Association (ACAA). Flexible pavement manual: Recommended practice - coal fly ash in pozzolanic stabilized mixtures for flexible pavement systems. 1991:128 p.
  13. ASTM D5239-04 standard practice for characterizing fly ash for use in soil stabilization. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2004.
  14. ASTM C593-06 standard specification for fly ash and other pozzolans for use with lime for soil stabilization. In: Annual book of ASTM standards, ASTM; West Conshohocken, Pennsylvania: 2006.
  15. 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.
  16. 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.
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Last Update 7/28/08