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Fly Ash Facts for Highway Engineers

Chapter 4 - Fly Ash in Stabilized Base Course


Fly ash stabilized base courses are proportioned mixtures of fly ash, aggregate, and an activator (cement or lime) that, when properly placed and compacted, produce a strong and durable pavement base course. Fly ash stabilized base courses are cost-effective substitutes for properly engineered full-depth asphalt, cement-treated, and crushed stone base courses. Fly ash stabilized base course is suitable for both flexible and rigid pavements.

Mix Design and Specification Requirements

Mix design. The stabilization of aggregate road bases with fly ash has a long and successful history. This application, termed pozzolanic-stabilized mixture (PSM) uses several materials and material combinations to construct stabilized aggregate bases. Class C fly ash can be used as a stand-alone material. Class F fly ash can be used when blended with lime, portland cement or cement kiln dust (CKD). Typical proportions for the Class F fly ash lime blends are two to eight percent lime blended with 10 to 15 percent Class F fly ash. Also, 0.5 to 1.5 percent Type I portland cement can be blended with Class F fly ash to produce the stabilizing agent. The stabilization of aggregate bases provides several advantages:

  • Adds significant strength and durability
  • Allows the use of marginal or low quality aggregates
  • Permits better use of open graded base courses
  • Reduces project cost

Strength. Closely controlled curing conditions are important as both time and temperature significantly affect strength. Use standard proctor-sized specimens; normal curing for lime/fly ash/aggregate mixtures is at plus 38 degrees C (100 degrees F) for 7 days. Some states use different curing times and temperatures.

Durability. It is important to ensure that adequate resistance to freeze-thaw cycling is achieved before the onset of colder months. The vacuum saturation test is normally used per ASTM C 593.

Testing to Determine Mix Proportions

The following steps summarize the procedures for a laboratory determination of mix proportions:

  • Obtain a representative sample of aggregate. Determine the particle size distribution of the aggregate. Screen the aggregate through a three-quarter-inch sieve, and use the portion passing the three-quarter-inch sieve for testing.

  • Use proctor-size molds for all test samples. Add fly ash to the aggregate in five different proportions, starting at the lower limit (10 percent for coarse aggregate) and proceed in convenient increments to the upper limit (20 percent for coarse aggregate). Mold one test specimen at each fly ash content in accordance with ASTM C 593 compaction procedures at an estimated optimum moisture content.

  • Determine the molded dry density of each aggregate-fly ash blend. Plot the test results to identify a peak value or maximum dry density.

  • Select an optimum matrix content at least two percent above the matrix content found at the maximum dry density. Then determine the optimum moisture content and maximum dry density for that blend.

  • Determine the most suitable proportions of activator to fly ash. Use five different activator-to-fly ash combinations at the optimum matrix content. The five combinations should span the recommended range of ratios for each activator. The typical range of activator to fly ash ratio is 1:3 to 1:4 using lime or portland cement; with either lime kiln dust or cement kiln dust as an activator, the typical range is 1:1 to 1:2.

  • Prepare six proctor-size specimens for each combination in accordance with the compaction procedures in ASTM C 593. Cure all six test specimens for seven days in sealed containers. For lime or kiln dust activators, cure at 37.8 degrees C (100 degrees F). For portland cement as the activator, cure in a moist room at ambient conditions of 22.8 degrees C (73 degrees F) and 100 percent relative humidity.

  • Test three specimens for compressive strength and test the other three specimens for durability at the end of the seven-day curing period, as described in ASTM C 593. Some agencies utilize the ASTM D 560 freezing and thawing test, which incorporates a brushing procedure and related performance criteria developed by the Portland Cement Association for soil-cement mixtures. In areas with virtually no freezing and thawing, durability testing may be waived in accordance with local practices.

  • Plot a curve of compressive strength as a function of activator percentage for each of the five activator-to-fly ash combinations. Only test mixtures with a seven-day compressive strength exceeding 2,760 kPa (400 psi) and acceptable durability should be considered as a potential PSM for field use.

  • Select the most economical (lowest percentage activator) mixture that exceeds the compressive strength and durability requirements. The PSM actually used in the field should contain a higher percentage of activator (a 0.5 percent increase for lime or portland cement; and a one percent increase for lime kiln dust or cement kiln dust) than the most economical mixture identified in the laboratory. This assures an adequate factor of safety for placement techniques available in the field

Control of Materials

Lime. Hydrated lime is the most popular form used, although quick lime and other products containing lime (kiln dust, etc.) can be used successfully with appropriate precautions. Type 1 portland cement has also been used successfully as a reactant when higher early strength requirements or reactant market conditions dictate. Determine actual lime content from samples using approved titration methods (ASTM D 2901, AASHTO T 232).

Fly ash. Unconditioned (dry) or conditioned (water added) fly ash can be used successfully. Check the reactivity of fly ash with cement in accordance with ASTM C 593 and for comparison and mix design results. Reactivity and fineness are the fly ash characteristics that most directly affect PSM quality.

Aggregates. Aggregates must be sound and resist deterioration from environmental elements. They may include sands, gravels, or crushed stones. Gradation should be such that the final mixture is mechanically stable and highly compactable.

Construction Practices

Blending of materials. Central plant mixing provides the best quality, although in-place mixing has also been successful. Most plants use a continuous pugmill, but central mix concrete plants also work well. When unconditioned (dry) fly ash is used, a silo and surge bin are needed for lime or cement and fly ash. When belt feeding, drop dry fly ash on top of the aggregate to keep it from rolling down the belt during pugmill loading. Conditioned Class F fly ash can be routinely added through an aggregate.

In-place mixing. In-place mixing involves the use of portable pulverizing and mixing equipment to blend granular soil or aggregate materials with PSM reagent material and water in pre-determined proportions at the project site. Class F fly ash is usually added in conditioned form, although it may also be added dry. The reagent materials (lime and/or portland cement) are usually added after the fly ash and are most often introduced in a dry form, although they may be added in a slurry form in order to minimize dusting. Water is usually sprayed on the mixture as needed just prior to in-place mixing.

Full depth reclamation. When deteriorated asphalt pavements are recycled in place, a technique known as full-depth reclamation can be used. The flexible pavement and a pre-determined portion of the underlying base material are milled and pulverized to a depth that can range from 150 to 300 mm (6 to 12 inches) or more. The pulverized material is mixed within the reclaiming machine while stabilization reagents (such as lime or portland cement and fly ash) and water are introduced and blended with the pulverized recycled paving aggregate. The reclaimer is then followed by grading, spreading, and compaction equipment working in the same manner and sequence as if plant-mixed PSM material were delivered and placed at the project site.

Spreading. A PSM can be placed with spreader boxes or asphalt laydown machines. Equipment with automated grade control is highly recommended. Layers are normally spread to a thickness of 15 to 30 percent greater than the desired compacted thickness. Maximum lift thickness is 200 to 250 mm (8 to 10 in). Place the second lift on the same day or take appropriate measures to ensure adequate sealing and subsequent bonding of additional lifts.

Compaction. Achieving a high degree of compaction is crucial to the successful performance of PSM roadbases. Final density should be reached as quickly as possible to achieve the highest ultimate strengths. This is especially the case when using Class C fly ash as the stabilization reagent, since nearly all Class C fly ashes are rapid setting and will not achieve the desired density unless compacted immediately following spreading. Compacting this noncohesive material with steel-wheel pneumatic, and vibratory rollers has been successful. The PSM surface should be kept moist throughout compaction. PSM moisture should be on the low side of optimum to achieve the best field compaction. If placed with a spreader box, the final surface should be fine graded with a motor grader before final rolling with a steel-wheeled roller. When fine grading, take care not to fill in the low spots because the feathering-in will tend to reduce bonding at that location, thus creating a potential trouble spot. If equipment with graded control is used, fine grading is not required.

Curing. Compacted layers should be quickly sealed to prevent drying. Apply a prime coat of 0.45 to 0.90 liters per square meter (0.1 to 0.2 gal/sy) of cut back or emulsified asphalt to the moist surface within 24 hours of final compaction. Multiple applications of lighter coats tend to produce better penetration and improve adhesion.


Ash quality. Fly ashes, which contain sulfur in excess of 5.0 percent as SO3 or contain scrubber residues, should be carefully evaluated with specific project soils to evaluate the expansion potential of the materials combination.

Seasonal limitations. PSMs often require several weeks of warmer weather to develop adequate strength to resist freeze-thaw cycling of the first winter. If late season placements are necessary, add portland cement in lieu of some of the lime to increase early strength.

Design and Construction References

See Appendix C.

Figure 4-1: Full-depth reclamation of a bituminous road.

Figure 4-1: Full-depth reclamation of a bituminous road.

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Updated: 09/09/2015
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