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Feature Story - May 2008

Change of Perception

Fly Ash Has Come a Long Ways in 30 Years

Once considered nothing more than a waste product from coal-fired power plants, fly ash has gained respect and popularity as a replacement of cement in concrete mix designs.

by Brad Fullmer

Hanging on the wall of Chris Bedford’s office at Headwaters Resources in Salt Lake City is a board he made a few years ago that has several cut out newspaper clippings from the late 1970’s in Utah that decries the use of fly ash as a suitable material for cement replacement in concrete mix designs. It’s there to remind Bedford just how far fly ash has come in 30 years in the design and construction industry in the Intermountain region, even though negative stereotypes still exist in certain markets.

“When fly ash first came out, there was a cement shortage, and fly ash was blamed for a lot of problems,” says Bedford, technical sales representative for Headwaters, a national supplier of fly ash with headquarters in South Jordan, Utah. “The perception back then was that fly ash was cheapening up ready-mix. In fact, Jack B. Parson Companies had a plant (in West Valley City) with a billboard that said, ‘We do not use fly ash.’ In 30 years, there have been some changes. There is still resistance, but it’s a lot less than it used to be.”

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Those changes in perception have at times come about slowly, as owners, contractors and ready-mix suppliers struggled for years to accept a material they believed was little more than a waste by-product of coal combustion.

“The acceptance of fly ash has come 180 degrees,” says Jim Johnson, general manager for Headwaters. “We used to talk to state agencies that would flat out say no to fly ash. They’re now saying its good, not only from an environmental standpoint, but from a durability standpoint.”

“It is common knowledge these days that (fly ash) is a good thing and beneficial in many regards,” says John Butterfield, a materials engineer for Region 2 of the Utah Department of Transportation with 20 years experience. “It’s actually making a better product – it’s not like we’re just burying it in the road section.”

Historical Use

Fly ash is one of the by-products generated in the combustion of coal, mainly captured from the chimneys of coal-fired power generation facilities. In the past, fly ash was generally released into the atmosphere via the smoke stack, but pollution control equipment mandated in recent decades requires that it be captured prior to release. Depending upon the source and makeup of the coal being burned, the components of the fly ash produced vary considerably, but all fly ash includes substantial amounts of silica (silicon dioxide) and lime (calcium oxide). Fly ash is used primarily as a supplement to Portland cement in concrete production, bringing technological, economical, and even environmental benefits.

Historically, the use of fly ash as a pozzolanic ingredient was recognized as far back as 1914, although the earliest noteworthy study of it was in 1937, according to the report Use of Fly Ash in Concrete by W. Halstead in 1986 for the National Cooperative Highway Research Project.

Prior to that, the ancient Romans utilized volcanic ash – which has similar pozzolanic properties as fly ash – in concrete structures such as aqueducts and even the Pantheon, according to the book The Roman Pantheon: The Triumph of Concrete by David Moore.

In the Intermountain region, Butterfield says the earliest use of fly ash was around 1978.

“It was in the late 70s when we first started hearing about it,” says Butterfield. “The use of fly ash coincided with the first use of slip form curb and gutter systems. Curb and gutter used to be all hand formed. (Contractors) started using fly ash and the concrete scaled. The first culprit was thought to be fly ash – it took a bad rap.”

When Bedford moved to Utah in 1980, he went to work as a quality control manager for Monroc, a precast concrete company in North Salt Lake. He recalls perceptions of fly ash as being rather negative in the marketplace.

“I had experimented with various mixes and I remember going to my boss and asking why we weren’t using more fly ash,” Bedford says. “He said there was an image problem with fly ash, and that the company policy stated that you cannot use more than 15% fly ash in any concrete mix.”

Bedford says the Intermountain chapter of the American Concrete Institute performed a petrographic analysis on those early failed curb and gutter projects, and concluded that the problem was a lack of air entrainment in the concrete mix, not the use of fly ash.

It wasn’t until the mid-90s until government agencies, including UDOT, started realizing the increased benefits of using fly ash as a partial replacement of Portland cement. Some of these benefits include:

  • Reducing the heat of hydration, which reduces thermal cracks in concrete
  • Improving the workability of concrete
  • Making concrete mixes homogeneous and reducing segregation and bleeding
  • An improved concrete finish, due to perfectly spherical fly ash particles
  • A reduction in the permeability of concrete, which enhances the life of the structure
  • An increase in long term strength in concrete

Butterfield says UDOT currently mandates replacing 20% of the Portland cement content with fly ash in its highway concrete pavement mixes. And while 20% may seem like a large number, Bedford and Johnson say their firm has done substantial testing through the years with concrete mixes that utilize up to 70% fly ash replacement.

“We’ve done extensive work promoting the use of fly ash among specifiers and government agencies,” Johnson says. “Originally, the attitude was ‘You want me to replace 15% of my cement with this stuff?’ It was unheard of. Now we’re talking about 40% replacements and above.”

In fact, a special concrete mix utilizing 55% fly ash was designed for the rotunda footing base isolation pads of the Utah State Capitol Renovation and Seismic Base Isolation project.

Rod Higley, technical sales representative for Jack B. Parson Companies, says he was skeptical at first that a mix design using that much fly ash could achieve the required strengths.

“That was somewhat abnormal for us,” says Higley. “We were approached by the structural engineer out of California (San Francisco-based Forell Elsesser) who had used this mix before. We didn’t think it was possible to meet the strengths they were looking for.”

Higley says during initial testing, the mix design wouldn’t work. “We had to modify it,” he says. “We kept 55% fly ash, and couldn’t believe how good our trial batches were. The mix design didn’t generate a lot of heat, plus had low shrinkage. We try to live in the real world. You can write a lot of things on paper but that doesn’t mean it’s true.”

Higley says the 55% fly ash mix on the Capitol needed to meet code requirement of 4,000 psi after 28 days, and 5,000 psi after 56 days. It reached 5,000 psi in just 28 days.

Bedford also mentions a residence in Pocatello, Idaho, that is using up to 70% fly ash in its mix designs. Johnson says that up to 65% fly ash replacement has been used on roller-compacted concrete (RCC) projects like the Stillwater Dam in Duchesne County, Utah.

“You’re going to see a generation coming in which is going to have an entirely different view of fly ash,” says Johnson. “It’s no longer Hamburger Helper. You’re taking a healthy product, Portland cement, and improving it. The perception has changed from smoke and mirrors, to science.”

Bedford says the green building movement has also helped promote the benefits of fly ash, since it’s a recycled material that adds valuable points to projects aiming for LEED certification. In addition, he says using less Portland cement is beneficial to the environment.

“With fly ash, you’re already using a recycled product,” says Bedford. “For every ton of cement that’s made, there is about a ton of carbon dioxide emitted into the air. So when we replace up to 20, 30, 40% of cement with fly ash, we’re reducing the carbon footprint of the building.”

According to Headwaters, environmental experts estimate that cement production contributes to about seven percent of carbon dioxide emissions from human sources. If all the fly ash generated each year were used in producing concrete, the reduction of carbon dioxide released because of decreased cement production would be equivalent to eliminating 25 percent of the world’s vehicles.

Unique Mix Designs Utilized on Utah Capitol

Part of the success of the recently completed Utah State Capitol Renovation and Seismic Base Isolation project in Salt Lake City was the utilization of unique concrete mix designs.

Experts from Jack B. Parson Companies, a division of Staker Parson Companies, were brought in to work with various concrete mixes used for the new foundation and base isolation system.

“We considered the opportunity to work on the State Capitol as a serious stewardship to help preserve one of the most significant and important buildings in our state” says Scott Parson, president of Staker Parson. “We dedicated our resources to provide technical expertise, innovation, and the service necessary to help meet the construction schedules.”

Parson says the special concrete mix for the base isolation system had to bond to the existing structure and support it during the restoration. A low heat of hydration was necessary to protect the concrete from cracking during the curing and to safeguard the existing structure from undue stresses.

JBP used maturity meters embedded within the concrete to monitor temperatures throughout the curing process. The concrete was placed around the rotunda footings to support the Capitol dome and between all the supporting columns in the structure. After the concrete was placed, contractors jacked the entire building up off the original footings so the new foundation could be installed. For this portion of the project, a high fly-ash mix (55%) that included high range water reducers was designed.

Another special mix that is classified as a self consolidating concrete (SCC) was used to support the base isolation piers. The building literally rests upon this mix. The design strength required was only 4,000 psi, but the strength went much higher, in some cases above 9,000 psi. The concrete was designed to be placed with plasticity near the consistency of water. This allowed the concrete to flow and consolidate in and around any reinforcing steel without the use of vibrators. General contractor Jacobsen-Hunt also used this mix when it was necessary to quickly achieve strength.

Another challenge that demanded an innovative solution occurred when contractors needed to place concrete next to fragile areas without creating any damage, particularly where sheer walls were placed inside the existing granite façade. JBP supplied a 4,000 psi low-slump shot-crete mix that was pumped and then sprayed into place. This mix was placed on all the walls that came in contact with the exterior facade.

In upper sections of the building where the structure needed reinforcing without adding undue weight, JBP supplied a 3,000 psi lightweight mix that cured with a density of 110 +/- 3 lbs. per cubic foot.

Some areas of the building did not allow for proper access and forming practices in order to place standard concrete. That’s when JBP used a mix design of 4,000 PSI 60/40 pea gravel mix with a high-range water reducer to promote flowability.

Certain isolated sections of the building required special admixtures such as DCI for high early-strength enhancement and Xypex for waterproofing.

 

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