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Fly Ash Facts for Highway Engineers

Chapter 2 - Highway Applications

Fly Ash in Portland Cement Concrete

Overview. Fly ash is used in concrete admixtures to enhance the performance of concrete. Portland cement contains about 65 percent lime. Some of this lime becomes free and available during the hydration process. When fly ash is present with free lime, it reacts chemically to form additional cementitious materials, improving many of the properties of the concrete.

Benefits. The many benefits of incorporating fly ash into a PCC have been demonstrated through extensive research and countless highway and bridge construction projects. Benefits to concrete vary depending on the type of fly ash, proportion used, other mix ingredients, mixing procedure, field conditions and placement. Some of the benefits of fly ash in concrete:

  • Higher ultimate strength
  • Improved workability
  • Reduced bleeding
  • Reduced heat of hydration
  • Reduced permeability
  • Increased resistance to sulfate attack
  • Increased resistance to alkali-silica reactivity (ASR)
  • Lowered costs
  • Reduced shrinkage
  • Increased durability

Cautions. Care should be taken when using fly ash in concrete due to:

  • Potential for decreased air entraining ability with high carbon fly ash may reduce durability
  • Reduced early strength
  • Reduced heat of hydration in colder climates

These concerns can be accommodated using proper design and construction practices. See Chapter 3 for additional information.

Fly Ash in Stabilized Base Course

Overview. Fly ash and lime can be combined with aggregate to produce a quality stabilized base course. These road bases are referred to as pozzolanic-stabilized mixtures (PSMs). Typical fly ash contents may vary from 12 to 14 percent with corresponding lime contents of three to five percent. Portland cement may also be used in lieu of lime to increase early age strengths. The resulting material is produced, placed, and looks like cement-stabilized aggregate base

Benefits. PSM bases have advantages over other base materials:

  • Use of locally available materials
  • Provides a strong, durable mixture
  • Lower costs
  • Autogenous healing
  • Increased energy efficiency
  • Suitable for using recycled base materials
  • Can be placed with conventional equipment

Cautions. PSM bases require attention to:

  • Seasonal limitations
  • Traffic loading before complete curing
  • Proper sealing and protection with asphalt or other surface treatment is required to improve skid resistance

Chapter 4 provides a more thorough discussion of stabilized base course.

Fly Ash in Flowable Fill

Overview. Flowable fill is a mixture of coal fly ash, water, and portland cement that flows like a liquid, sets up like a solid, is self-leveling, and requires no compaction or vibration to achieve maximum density. In addition to these benefits, a properly designed flowable fill may be excavated later. For some mixes, an optional filler material such as sand, bottom ash, or quarry fines, is added. Flowable fill is also referred to as controlled low-strength material, flowable mortar, or controlled density fill. It is designed to function in the place of conventional backfill materials such as soil, sand, or gravel and to alleviate problems and restrictions generally associated with the placement of these materials.

The benefits of using flowable fill include:

  • Allows placement in any weather, even under freezing conditions
  • Achieves 100 percent density with no compactive effort
  • Fills around/under structures inaccessible to conventional fill placement techniques
  • Increases soil-bearing capacities
  • Prevents post-fill settlement problems
  • Increases the speed and ease of backfilling operations
  • Decreases the variability in the density of the backfilled materials
  • Improves safety at the job site and reduces labor costs
  • Decreases excavation costs
  • Allows easy excavation later when properly designed

Cautions. When using flowable fill, care must be taken to:

  • Anchor lighter weight pipes to prevent floating
  • Provide confinement before initial set of the material
  • Evaluate corrosion of metal pipe at interface of soil

Chapter 5 includes a detailed discussion of flowable fill applications.

Fly Ash in Structural Fills/Embankments

Overview. Fly ash can be used as a borrow material to construct fills and embankments. When fly ash is compacted in lifts, a structural fill is constructed that is capable of supporting highway buildings or other structures. Fly ash has been used in the construction of structural fills/embankments that range from small fills for road shoulders to large fills for interstate highway embankments.

Benefits. When used in structural fills and embankments, fly ash offers several advantages over soil and rock:

  • Cost-effective where available in bulk quantities
  • Eliminates the need to purchase, permit, and operate a borrow pit
  • Can be placed over low bearing strength soils
  • Ease of handling and compaction reduce construction time and equipment costs

Cautions. Be aware that:

  • State or local environmental regulations may require consideration of the potential impacts to ground water at adjoining properties
  • Requires dust control and erosion prevention measures

Chapter 6 provides additional information.

Fly Ash in Soil Improvement

Overview. Fly ash is an effective agent for chemical and/or mechanical stabilization of soils. Soil density, water content, plasticity, and strength performance of soils. Typical applications include: soil stabilization, soil drying, and control of shrink-swell.

Benefits. Fly ash provides the following benefits when used to improve soil conditions:

  • Eliminates need for expensive borrow materials
  • Expedites construction by improving excessively wet or unstable subgrade
  • By improving subgrade conditions, promotes cost savings through reduction in the required pavement thickness
  • Can reduce or eliminate the need for more expensive natural aggregates in the pavement cross-section

Cautions. The most important considerations for soil improvement projects are:

  • The rate of the hydration reaction upon exposure to water
  • Soil moisture content at the time of compaction
  • Fly ash with a sulfate content greater than 10 percent may cause soils to expand more than desired
  • In many cases, leaching tests may be required by local and state agencies

Soil improvement is discussed more thoroughly in Chapter 7.

Fly Ash in Asphalt Pavements

Overview. Fly ash can be used as mineral filler in HMA paving applications. Mineral fillers increase the stiffness of the asphalt mortar matrix, improving the rutting resistance of pavements, and the durability of the mix.

Benefits. Fly ash will typically meet mineral filler specifications for gradation, organic impurities, and plasticity. The benefits of fly ash include:

  • Reduced potential for asphalt stripping due to hydrophobic properties of fly ash
  • Lime in some fly ashes may also reduce stripping
  • May afford a lower cost than other mineral fillers

Chapter 8 has additional information.

Fly Ash in Grouts for Pavement Subsealing

Overview. Grouts are proportioned mixtures of fly ash, water, and other materials used to fill voids under a pavement system without raising the slabs (subsealing), or to raise and support concrete pavements at specified grade tolerances by drilling and injecting the grout under specified areas of the pavement.

Benefits. Fly ash grouts can:

  • Be used to correct undermining without removing overlying pavement
  • Be accomplished quickly with minimum disturbance to traffic
  • Develop high ultimate strength

Cautions. Fly ash grouts:

  • Require curing period before extremely heavy loading because of low early strength
  • Require confinement of the grout mixture under pavement

Chapter 9 presents more information on this topic.

Updated: 06/27/2017
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