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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-97-148

User Guidelines for Waste and Byproduct Materials in Pavement Construction

[ Flowable Fill ] [ Material Description ] [ Asphalt Concrete ] [ Portland Cement Concrete ] [ Embankment or Fill ]



User Guideline

Stabilized Base


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 in this application.

Bituminous fly ash is used with a chemical reagent or activator (usually lime, Portland cement, or kiln dust), aggregate, and water. 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, 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 of most sources, the amount of fly ash used may only be in the 5 to 15 percent range. There are instances where self-cementing fly ash is used by itself as the base course material without any aggregate.

The use of fly ash in stabilized base and subbase mixtures dates back to the 1950’s, 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 1970’s, 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 used locally available high-quality crushed rock (such as limestone, trap rock, or granite), sand and gravel, or blast furnace slag, especially on high traffic volume roadways. However, many well-constructed 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.



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, not a plastic consistency, for placement at or near optimum moisture content and densification by roller compaction

According to a 1992 survey of state transportation agencies, at least 22 states have made some use of fly ash in stabilized base or subbase applications.(1) Table 5-5 is a summary of the use of fly ash in stabilized base mixtures in these states

Table 5-5. Summary of fly ash use in stabilized base and subbase applications throughout the United States.

State Estimated Number of Projects Time Period Types of Projects Remarks**
Alabama Unknown 1955-1965 State roads Used Class F fly ash
Arkansas* At least 1 1982 State road Test section w/100% Class C
Colorado Unknown Mid to late 1980's Local roads Used in Aurora, Denver suburb
Georgia At least 1 1985 State road EPRI fly ash demonstration
Illinois More than 100 1955-1985 State and county roads Class F & C ashes (Cook Cty)
Iowa At least 1 Early to mid 1960's County road Class F ash used in Linn Cty
Kansas At least 3 1987 County roads EPRI fly ash demonstration
Kentucky* At least 1 1984 State road Used kiln dust and Class F ash
Maryland At least 1 Early 1960's Shoulders Used for I-95 shoulders
Michigan* At least 3 1959-1987 State and private roads EPRI fly ash demonstration
Mississippi* Unknown 1983 State roads Used Class C fly ash
Missouri At least 10 1970-1988 State and local roads Class C ash used in Kansas City
Nebraska At least 1 Unknown State road Used Class C ash in subbase
New Jersey* At least 1 1984 State road Used on portion of I-295
North Dakota Several 1971 to mid 1980's State road Used lignite ash at I-94
Ohio At least 10 1970-1985 State and local roads Used mainly in Toledo
Oklahoma At least 1 Early to mid 1980's State road Used Class C fly ash
Pennsylvania Several dozen 1954-1985 State and local roads Used mainly in S.E. PA
Tennessee* At least 1 1982 State road Used Class F fly ash
Texas Unknown 1960-1990 State and local roads Used Class C and Class F ash
Virginia* At least 1 1982 State road Dulles Airport extension road
Wyoming Unknown Early to mid 1980's State roads Used Class C fly ash
*Participated in Federal Highway Administration (FHWA) Demonstration Program No. 59, "Fly Ash in Highway Construction."

**Note: Class F fly ash refers to the fly ash derived from the burning of bituminous coals. Class C fly ash refers to the fly ash derived from the burning of either lignite or subbituminous coals.

The three states in which PSB mixtures have been most frequently used are Illinois, Ohio, and Pennsylvania. It has been conservatively estimated that since the 1970’s at least 25 to 30 million tons of PSB material have been produced and placed in the United States. One-third to one-half of all the PSB material placed prior to 1990 is thought to have been placed in the metropolitan Chicago area.(2)

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 Federal Highway Administration Demonstration Project No. 59, Fly Ash Use in Highway Construction, during the mid 1980’s. Nearly all of these projects have been documented in terms of design and installation, although in many cases, long-term performance data is not available.

During the mid to late 1980’s, the Electric Power Research Institute (EPRI) funded three demonstration projects in Georgia, Kansas, and Michigan that involved the use of fly ash in stabilized road base compositions. For each project, EPRI contracted with the utility company supplying the ash to determine basic material properties, assess construction performance, and evaluate long-term pavement and environmental performance for 3 years.

In 1985, the Georgia Department of Transportation constructed a 2.6 km (1.6 mi) relocation of State Route 22 west of Crawfordville. A portion of the two-lane road included a 305 m (1,000 ft) section of cement stabilized pond ash base and a 244 m (800 ft) section of cement stabilized fly ash base. Each of these two base course sections was 215 mm (8 1/2 in) thick. One year after installation, the average strength of the pond ash section was 14,570 kPa (2,115 lb/in2) and the average strength of the fly ash section was 5,990 kPa (870 lb/in2).(3)

During 1987, three county gravel roads in Kansas were recycled in place using Class C fly ash. At each site, the existing road surface was pulverized, mixed, and redeposited in its original location. Fly ash was spread over the pulverized road material and mixed by a means of a pulvi-mixer. Following compaction, a seal coat surface was placed on all three of the roadways. Since the reconstruction of these first three locations, more than 44 km (400 mi) of pavement recycling using Class C fly ash have been accomplished in Kansas and Oklahoma.(4)

Also in 1987, the shoulders of a 460 m (1,500 ft) long section of relocated Michigan Route M-54 near Flint were constructed using 620 metric tons (690 tons) of a cement stabilized high carbon Class F fly ash base. A total of 915 m (3,000 ft) of shoulder material was placed at a width of 2.7 m (9 ft) and a compacted thickness of 250 mm (10 in). Core strengths after 270 days were variable, but reached as high as 6,890 kPa (1,000 lb/in2). Although performance has been acceptable, some isolated areas of cracking were observed in both directions of the shoulder pavement after the first year of service.(5)

The largest single reported project in the United States involving the use of fly ash in PSB construction occurred during the early to mid 1970’s with the building of runways, taxiways, and aprons at Newark International Airport, in Newark, New Jersey. A flexible pavement system using a base of lime, Portland cement, fly ash, and sand was designed to withstand the stresses from jumbo jet aircraft. The base course consisted of three layers, each with slight variations in the mix design. The lime and Portland cement combined ranged from 3 to 4 percent and the fly ash ranged from 10 to 12 percent. The PSB sections ranged in thickness from 610 to 914 mm (24 to 36 in). The ultimate strength of the 4 percent lime and Portland cement mix, which contains a blend of crushed stone and sand as the aggregate, was found to range from 13,780 to 17,910 kPa (2,000 to 2,600 lb/in2).(6) After 20 or more years, the PSB pavements continue to perform satisfactorily.

PSB pavements have provided good to excellent performance over many years in numerous locations. In general, these mixtures have also been more economical than alternative base materials in many areas. Nonetheless, the major concern of highway engineers with stabilized-based materials, including soil-cement, is the development of cracks within the base course. These cracks, which are more often caused by shrinkage rather than fatigue, usually reflect up through the overlying asphalt pavement surface, resulting in increased long-term maintenance costs.



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 to be used in producing 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 would also 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.

If Class C fly ash is used, it is likely to be self-cementing. For self-cementing fly ashes, there are two ways to offset the rapid hardening of base materials using such ashes. One is to initially condition the ash with relatively low amounts (in the range of 10 to 15 percent) of water, stockpile the partially hardened material for several weeks or more, 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.(7)

The aggregate(s) used in PSB mixtures should be in a saturated surface-dry condition during stockpiling. The moisture content of the aggregate(s) should be checked prior to mixing to ensure that excess moisture was not acquired while the aggregate(s) was being stockpiled.



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.

Water Solubility: The physical requirements most frequently cited for the use of fly ash (Class F) in PSB mixtures are provided in ASTM C593(8), which specifies a maximum water soluble fraction of 10 percent.

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

Pozzolanic Activity: One of the most important properties of fly ash, related to its use 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 its fineness, silica and alumina content, loss on ignition, 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.(9)

Fineness: Fineness requirements in ASTM C593 specify that 98 percent of the fly ash should be finer than a 0.6 mm (No. 30) sieve and 70 percent finer than a 0.075 mm (No. 200) sieve. Most fly ash is capable of meeting these specifications. Minimum compressive strength requirements when fly ash is blended with lime at 7 and 21 days are also recommended in ASTM C593.(8)

Organic Content: Fly ash used in PSB mixtures does not have to meet the ASTM C618(10) requirements of fly ash that is used in Portland cement concrete. LOI is not a criterion for fly ash use in PSB mixtures.

Some of the properties of stabilized base mixes that are of particular interest when fly ash is incorporated include compressive strength, flexural strength, modulus of elasticity, bearing strength, autogeneous healing, fatigue, freeze-thaw durability, and permeability.

Compressive Strength: This 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. Within limits, the higher the compressive strength, the better the 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).(11) 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 2,760 kPa (400 lb/in2). The ultimate strength of PSB mixtures containing Class F fly ash is considerably higher than the 7-day strength. In many cases, long term compressive strength development of Class F fly ash mixes may be two to three times higher than the 7-day strength.

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. In this way, PSB mixtures continue to show incremental gains in strength over many years.

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 effective strength of this material. Although flexural strength can be determined directly by testing, most transportation agencies estimate the flexural strength of these materials as a fraction of the material’s 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.(12)

Modulus of Elasticity: The modulus of elasticity is a measure of the stiffness or bending resistance of a material. For semi-rigid materials such as PSB mixtures, the relationship between stress and strain is not linear and, therefore, the modulus of elasticity is not a constant value, but increases as the compressive strength of the material increases. The modulus of elasticity, as determined from flexural strength testing instead of compressive strength testing, is recommended for use in pavement design calculations. For most PSB mixtures, the modulus of elasticity is in the range of 9.6 x 106 kPa (1.4´ 106 lb/in2) to 17.2 x 106 kPa (2.5´ 106 lb/in2).(13)

Bearing Strength: The California Bearing Ratio (CBR) test(14) is often used as a way of measuring the bearing strength of soils used as subgrade materials 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 that unusual. Use of the CBR test is more applicable to subgrade soil stabilization with fly ash than in evaluating PSB mixtures.

Autogenous Healing: One of the unique characteristics of PSB compositions is their inherent 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 it 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.(12)

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.(15)

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 400 psi unconfined compressive strength following vacuum saturation testing. For cement-based activators, the durability test procedure specified in ASTM D560(16) is used. The acceptance criterion is a maximum 14 percent weight loss after 12 freeze-thaw cycles.(16)

The minimum strength required prior to the first freezing cycle, in order to provide sufficient durability against freeze-thaw damage, is dependent on the severity of the climate. The American Coal Ash Association (ACAA) recommends the minimum compressive strengths of 6,900, 5,500 and 4,100 kPa (1000, 800, and 600 lb/in2), respectively, for severe, moderate, and mild freeze-thaw conditions.(7)

Permeability: The permeability of hardened PSB materials is very low, depending on aggregate gradation, particularly when compared with that of crushed stone or granular bases and subbases, or even with the permeability of asphaltic base courses. In most cases, as the compressive strength of the PSB material increases, the permeability decreases. Initial permeability readings for hardened PSB mixtures can be expected to range between 10-5 and 10-6 cm/sec, as measured by the falling head permeability test. As the pozzolanic reaction proceeds, PSB materials may have permeability values between 10-6 and 10-7 cm/sec.(15)



Mix Design

A wide range of aggregate sizes can be accommodated in stabilized base and subbase mixtures. After determining the particle size distribution of the aggregate to be used in the 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. An estimated optimum moisture content is selected "by eye" 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.

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

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.(8)

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 likely to be in the 1:1 to 1:2 range.

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, it is recommended that the PSB mixture used in the field 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.(7)

Structural Design

The design of pavements using PSB mixtures can be undertaken using AASHTO structural equivalency design methods.(17) The main factors influencing the variation in the structural layer coefficient for thickness design using the AASHTO method are the compressive strength and modulus of elasticity of the PSB material. The value of compressive strength most often used to determine the structural layer coefficient for PSB mixtures is the field design compressive strength. The field design compressive strength is simulated by the compressive strength determined in the laboratory after 56 days of moist curing at 23°C (73°F).(7) Other curing conditions may be required by various specifying agencies.

Based on the comparative performance of pozzolanic and crushed stone base materials from studies performed at the University of Illinois(15), and using a value for a1 of 0.44 (representing a bituminous concrete wearing surface) and a value for a3 of 0.15 (representing a crushed stone base), structural layer coefficients for PSB mixtures were recommended. Table 5-6 provides a listing of recommended coefficients based on the early strength development of the mixtures.

Structural layer coefficient values (a2) of 0.30 to 0.35 have been recommended for bituminous or Portland cement stabilized bases.(17)



Material Handling and Storage

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

Mixing, Placing, and Compacting

The blending or mixing of PSB materials can be accomplished either in a mixing plant or in place. Plant mixing is recommended because it provides greater control over the quantities of materials batched, and it also results in the production of a more uniform PSB mixture.

Table 5-6. Recommended structural coefficients for PSB mixtures.

Quality Compressive Strength,
kPa (lb/in2)
(7 days @ 38°C (100°F)
Recommended Structural Coefficient
High Greater than 6,900 (1,000) a2 = 0.34
Average 4,500 to 6,900
(650 to 1,000)
a2 = 0.28
Low 2,800 to 4,500
(400 to 650)
a2 = 0.20

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 also be used with properly calibrated field conveyors from bins or silos for a continuous mixing operation. Rotating drum mixers have also been successfully used for blending PSB materials in batches.(13)

An alternative to plant mixing is in-place mixing. Although the mix-in-place approach does not usually result in as accurate a proportioning of the mix components as plant mixing, it is still possible to produce a high-quality PSB material using this approach. The various components of the PSB mixture are delivered and spread on the road site, then mixed in place using a pulvi-mixer, a self-propelled rotating device with mixing pads capable of mixing to a depth of 300 to 450 mm (12 to 18 in).

Plant-mixed materials should not be stockpiled, but should be delivered to the job site as soon as possible after mixing. Compaction of PSB materials should be completed as quickly as possible after placement, especially with mixtures containing Class C (self-cementing) fly ash.

The delivery of PSB materials to the job site has been most frequently handled by covered end-dump hauling vehicles. The same equipment is basically used for spreading plant-mixed PSB material, as well as material to be mixed in place. 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. The material must 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 has also been applied in a slurry form. The materials are then mixed together by means of a rotary mixer. Water is added as needed using a water truck with spray nozzles.

Equipment used for compaction is the same, regardless of whether PSB material is plant-mixed or mixed 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.(7)

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.


After placing and compacting the PSB material, it must be properly cured to protect against drying and to assist in the development of in-place strength. 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 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 is dependent on the development of in-place strength following placement, compaction, and curing. Depending on the anticipated traffic loadings, it is necessary to analyze when traffic can be permitted to travel on the base material, while also avoiding potential fatigue damage due to early overloading.

Based on laboratory testing for strength development, it is usually possible to determine when the PSB material is likely to achieve an in-place compressive strength of 2,410 kPa (350 lb/in2). Unless an asphalt surface or binder course has been placed over the PSB material, vehicles should not be permitted to use the PSB layer until it has achieved at least 2,410 kPa (350 lb/in2) in place. Ordinarily, placement of asphalt paving over the PSB material is recommended within 7 days after the PSB material has been placed.(7) If a Portland cement concrete pavement is to be constructed over the PSB layer, a waiting period of 7 days is also recommended.

Special Considerations

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. Normally, very little strength development occurs when the temperature within the PSB material is 5°C (40°F) or lower.

For northern states, state transportation agencies have previously established construction cutoff dates for PSB materials that range from September 15 to October 15, depending on location within state and/or ability to develop compressive strength in the laboratory.(18)

Self-Cementing 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.(19)

Low percentages of water in the range of 10 to 25 percent by weight of ash, sufficient to retard dusting, can be added at the mixing plant, with added water that is required for proper compaction applied to the PSB material 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. It has been found 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.(19,20)

Crack Control

Pavement base layers constructed with PSB materials are subject to shrinkage cracking. The development of cracks is related to the hydration reaction between the activator and fly ash and is more evident when Portland cement is used in the mix. Some cracks do reflect up through the overlying asphalt pavement, but are less likely to do so if a concrete pavement is used. If surface cracks are sealed to prevent the intrusion of water and subsequent damage due to freezing and thawing, the overall durability of the PSB layer should not be adversely affected.

Approaches for controlling or minimizing the potential effects of reflective cracking associated with PSB layers have been recommended by the ACAA.(7)



Crack control has long been considered by many state transportation agencies as a prime concern associated with the use of PSB mixtures. Although there have been a number of experimental projects related to joint placement for pozzolanic bases, there is still no universally accepted procedure for minimizing, or even possibly eliminating, shrinkage cracking in such mixtures. Addressing this issue may make possible greater acceptance and use of PSB mixtures by state transportation agencies.

Although the overwhelming majority of the PSB placed over the years has been with Class F fly ash, there has been an increasing usage of Class C fly ash in PSB mixtures. Because the handling characteristics of Class C fly ash are so different from Class F fly ash, more specific direction is needed on how to best handle Class C fly ash when used as an activator in PSB mixtures.

Information needed includes how to evaluate the extent of flash setting, how to select the proper conditioning technique for different degrees of reactivity, and when and how much retarder should be used.



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  2. Hunt, Robert D., Larry E. Seiter, Robert J. Collins, Richard H. Miller, and Benjamin S. Brindley. Data Collection and Analysis Pertinent to EPA's Development of Guidelines for Procurement of Highway Construction Products Containing Recovered Materials. Final Report, EPA Contract No. 68-01-6014, Washington, DC, 1981.

  3. Larrimore, L. and C.W. Pike. Use of Coal Ash in Highway Construction: Georgia Highway Demonstration Project. Electric Power Research Institute, Report No. GS-6175, Palo Alto, California, February, 1989.

  4. Ferguson, G. Use of Coal Ash in Highway Construction: Kansas Highway Demonstration Project. Electric Power Research Institute, Report No. GS-6460, Palo Alto, California, September, 1989.

  5. Berry, W.H., D.H. Gray, and E. Tons. Use of Coal Ash in Highway Construction: Michigan Demonstration Project. Electric Power Research Institute, Report No. GS-6155, Palo Alto, California, January, 1989.

  6. Yang, Nai C., Harry Schmerl, and Myron Waller. "Newark Airport Expansion Pilots Cost-Saving Runway Paving Concept," Civil Engineering, June, 1978.

  7. American Coal Ash Association. Flexible Pavement Manual. ACAA, Alexandria, Virginia, 1991.

  8. ASTM C593-89. "Standard Specification for Fly Ash and other Pozzolans for Use with Lime," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.01, West Conshohocken, Pennsylvania, 1994.

  9. ASTM C311-92a. "Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use as a Mineral Admixture in Portland-Cement Concrete." American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.02, West Conshohocken, Pennsylvania, 1994.

  10. ASTM C618-92a. "Standard Specifications for Fly Ash and Raw or Calcines Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.02, West Conshohocken, Pennsylvania, 1994.

  11. Portland Cement Association. Soil-Cement Laboratory Handbook. PCA, Skokie, Illinois, 1992.

  12. Meyers, James F., Roman Pichumani, and Bernadette S. Kapples. Fly Ash as a Construction Material for Highways. Federal Highway Administration, Report No. FHWA-IP-76-16, Washington, DC, 1976.

  13. Barenberg, Ernest J. and Marshall R. Thompson. Lime-Fly Ash Stabilized Bases and Subbases. National Cooperative Highway Research Program Synthesis of Highway Practice No. 37, Transportation Research Board, Washington, DC, 1976.

  14. ASTM D1883-87. "Standard Test Method for Bearing Ratio of Laboratory-Compacted Soils," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume No. 04-08, West Conshohocken, Pennsylvania, 1994.

  15. Ahlberg, Harold L. and Ernest J. Barenberg. Pozzolanic Pavements. University of Illinois, Engineering Experiment Station, Bulletin 473, Urbana, Illinois, February, 1965.

  16. ASTM D560-89. "Standard Methods for Freezing-and-Thawing Tests of Compacted Soil-Cement Mixtures," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.09, West Conshohocken, Pennsylvania, 1994.

  17. AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, DC, 1986.

  18. Hoffman, Gary L., Gaylord Cumberledge, and Amar C. Bhajandas. Establishing a Construction Cutoff Date for Placement of Aggregate-Lime-Pozzolan. Pennsylvania Department of Transportation, Research Report, 1975.

  19. Thornton, Samuel I. and David G. Parker. Construction Procedures Using Self-Hardening Fly Ash. Federal Highway Administration, Report No. FHWA/AR/80, 004, Washington, DC, 1980.

  20. Ferguson, Glenn. "Use of Self-Cementing Fly Ashes as a Soil Stabilization Agent," Fly Ash for Soil Improvement. American Society for Civil Engineers, Geotechnical Special Publication No. 36, New York, New York, 1993.


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