Fly Ash Finds Multiple Uses in Highway Construction
Now available from the Federal Highway Administration (FHWA) is the fourth edition of Fly Ash Facts for Highway Engineers (Publication No. FHWA-IF-03-019). Produced in cooperation with the American Coal Ash Association and the U.S. Environmental Protection Agency, the publication provides basic technical information about the many ways that fly ash can be used in highway construction.
Fly ash is the finely divided residue that results from the combustion of pulverized coal. It is a by-product produced by coal-fired electric and steam generating plants. More than 61 million metric tons (68 million tons) of the waste product were generated in 2001. Currently, more than 20 million metric tons (22 million tons) of fly ash are being used annually for various engineering applications. Among the highway engineering uses are for portland cement concrete (PCC), soil and road
base stabilization, flowable fills, grouts, structural fill, and asphalt filler. To be used in concrete, fly ash must meet the requirements of American Association of State Highway and Transportation Officials Standard M295, or ASTM International Standard C618. For use in stabilized bases, fly ash needs to meet the requirements of ASTM C593.
Fly ash can be used in PCC to enhance the performance of the concrete. The fly ash reacts chemically with the lime in PCC to form additional cementitious materials, thus improving many of the properties of the concrete. Some of the resulting benefits are:
- Higher ultimate strength
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Increased durability
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Improved workability
- Reduced bleeding
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Increased resistance to sulfate attack
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Increased resistance to alkali-silica reactivity
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Reduced shrinkage.
Using fly ash in PCC can also lower costs, as a highway agency can reduce the amount of portland cement used. Typically, 15 to 30 percent of the portland cement is replaced with fly ash. And the use of fly ash reduces the amount of waste deposited in landfills, providing an important environmental benefit.
One area of concern to be aware of when using fly ash in PCC is that fly ash concrete mixes typically have a lower early age strength. This lower age strength gain may require that forms be strengthened to mitigate hydraulic loads. However, the lower early strengths can be overcome by using accelerators in the mix. Construction schedules should also allow time for fly ash concrete mixes to gain adequate strength before winter, as strength gain of the concrete is lower at colder temperatures. To keep the temperature of the concrete higher and increase the strength gain, enclosures, heated curing, or insulation blankets can be used.
Another area of concern is that during construction, concrete loads delivered to the project site should be checked for entrained air to ensure that a consistent air content is maintained.
Fly ash can also be used in stabilized base courses. The typical fly ash content for a base course ranges from 12 to 14 percent. Advantages include increased strength and durability and lower costs. Another application is to use fly ash in fills and embankments, ranging from small fills for road shoulders to large fills for Interstate highway embankments. Employing fly ash offers several advantages over using soil and rock, including being more cost-effective and easier to handle and compact. Potential cautions include that dust control and erosion prevention measures must be used and environmental impacts on any nearby groundwater must be considered.
To learn more about the many uses for fly ash in highway engineering, consult Fly Ash Facts for Highway Engineers, which can be found online at www.fhwa.dot.gov/pavement/fatoc.htm. Printed copies or a CD version can be ordered from the American Coal Ash Association at 720-870-7897(www.acaa-usa.org/store-books.htm). FHWA will also be sponsoring six 1-day workshops in 2004 and early 2005 on the use of fly ash in PCC and embankments. For more information on Fly Ash Facts or the upcoming workshops, contact Michael Rafalowski at FHWA, 202-366-1571 (email: michael.rafalowski@fhwa.dot.gov).
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