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Fly Ash Facts for Highway Engineers
Fly Ash in Portland Cement Concrete TOP
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:
Cautions. Care should be taken when using fly ash in concrete due to:
These concerns can be accommodated using proper design and construction practices. See Chapter 3 for additional information.
Fly Ash in Stabilized Base Course TOP
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:
Cautions. PSM bases require attention to:
Fly Ash in Flowable Fill TOP
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:
Cautions. When using flowable fill, care must be taken to:
Fly Ash in Structural Fills/Embankments TOP
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:
Cautions. Be aware that:
Fly Ash in Soil Improvement TOP
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:
Cautions. The most important considerations for soil improvement projects are:
Fly Ash in Asphalt Pavements TOP
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:
Fly Ash in Grouts for Pavement Subsealing TOP
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:
Cautions. Fly ash grouts: