Adding Fly Ash to Concrete Mixes for Floor Construction - Fly Ash Concrete, Admixtures, Mix Design, Retail Projects - Concrete

The article we enclose herein appears in: Concrete Construction Magazine; Annual Floors Issue 2007; and, it affirms any number of our prior reports concerning the environmental and performance benefits of using Coal Ash in the making of Portland-type Cement and Portland Cement Concrete.

That affirmation comes in the form of construction design specifications published by a major US company, whose public sense of social and environmental responsibility is well known.

Wal-Mart Stores Incorporated now requires that a certain minimum amount Coal Ash be used in the concrete floors, and where else possible, in the construction of all their new facilities.

Wal-Mart conducted their own tests and studies, as will be seen, and, their conclusions were, that:

"hardened concrete properties of floors containing fly ash are equal to or exceed those constructed with straight cement";

and:

"the production of portland cement releases CO2 gases into the atmosphere" and, thus, using Coal Fly Ash to replace or supplement Portland cement according to Wal-Mart specifications will, on average, result in "reducing the CO2 emissions by 139 tons per store".

Additional comment follows excerpts from the link to:

"Adding Fly Ash to Concrete Mixes for Floor Construction

Wal-Mart institutes a new specification for its steel-troweled floors

by Kim D. Basham, et. al.

Wal-Mart Stores Inc. has changed their construction specifications to require all interior steel-troweled concrete floors placed at Wal-Mart Stores, Supercenters, Neighborhood Markets, Sam's Clubs, and Distribution Centers to contain 15% to 20% fly ash by cement substitution. This change is part of a larger sustainability effort by Wal-Mart to reduce their "greenhouse gas footprint" and is based on results of a concrete research program started in 2006 by Wal-Mart investigating the feasibility of substituting fly ash for portland cement in integral-colored, steel-troweled floors.

Unfortunately, the production of portland cement releases CO2 gases into the atmosphere and contributes to the greenhouse effect and global warming. For that reason, replacing 15% to 20% of the portland cement with fly ash is important for the environment because it reduces cement usage. For example, replacing 20% of the cement in the interior floor of one Wal-Mart Store will reduce cement usage about 104 pounds per cubic yard of concrete or about 278,096 pounds of cement per store. Using the approximate relationship that producing 1 ton of cement releases about 1 ton of CO2 into the atmosphere, this cement reduction is equivalent to reducing the CO2 emissions by 139 tons per store. Actually, the CO2 reduction will be higher per store because all concrete, including the foundation, sidewalks, curbs and gutters, and other features, must contain at least 15% fly ash.

Given the size and scale of Wal-Mart's construction program, this is a considerable reduction of greenhouse gas emissions. If other building owners, architects, and specifiers follow Wal-Mart's mandatory cement replacement with fly ash initiative, significant reductions in CO2 emissions related to concrete or cement production can occur.

In addition to being good for the environment, Wal-Mart's concrete research showed that substituting fly ash for cement also made good business sense. Here the authors cover highlights of the research program, findings, and recommendations for specifying and placing uncolored and integral-colored, steel-troweled concrete floors containing fly ash.

Fly ash is a byproduct from burning pulverized coal in electric power generating plants. During combustion, mineral impurities in the coal (clay, feldspar, quartz, and shale) fuse in suspension and float out of the combustion chamber with the exhaust gases. As the fused material rises, it cools and solidifies into spherical glassy particles called fly ash. Fly ash is collected from the exhaust gases by electrostatic precipitators or bag filters. The fine powder does resemble portland cement but it is chemically different. Fly ash chemically reacts with the byproduct calcium hydroxide released by the chemical reaction between cement and water to form additional cementitious products that improve many desirable properties of concrete.

Two types of fly ash are commonly used in concrete: Class C and Class F. Class C are often high-calcium fly ashes with carbon content less than 2%; whereas, Class F are generally low-calcium fly ashes with carbon contents less than 5% but sometimes as high as 10%. In general, Class C ashes are produced from burning sub-bituminous or lignite coals and Class F ashes bituminous or anthracite coals. Performance properties between Class C and F ashes vary depending on the chemical and physical properties of the ash and how the ash interacts with cement in the concrete. Many Class C ashes when exposed to water will react and become hard just like cement, but not Class F ashes. Most, if not all, Class F ashes will only react with the byproducts formed when cement reacts with water. Class C and F fly ashes were used in this research project.

Currently, more than 50% of the concrete placed in the U.S. contains fly ash. Dosage rates vary depending on the type of fly ash and its reactivity level. Typically, Class F fly ash is used at dosages of 15% to 25% by mass of cementitious material and Class C fly ash at 15% to 40%. However, fly ash has not been used in interior, steel-troweled slabs because of the inherent problems or challenges associated with fly ash variability and delayed concrete hardening. Rate and uniformity of concrete hardening are critical parameters in establishing the window-of-finishability and can influence directly the quality of final floor finish. Delayed or nonuniform concrete hardening significantly increases the risk of premature or improper finishing resulting in poor quality steel-troweled finishes. Until now, building owners, concrete suppliers, and finishers have been reluctant to replace cement with fly ash in steel-troweled floors because of the increased risks associated with the fly ash. These risks include surface stickiness, delayed concrete hardening, and early volume shrinkage cracking caused by delayed setting.

In August 2006, eight 16x18-foot test slabs were placed, finished, cured, densified, and polished in accordance with Wal-Mart's specifications in a pole barn located in Wellington, Colo. Each slab contained a total cementitious material content of 520 pounds per cubic yard of concrete, however, the amount and type of fly ash varied for each test slab.

On the morning of the placement, the ambient air temperature was near 60F and at the end of the placement the air temperature was near 80F. The humidity was estimated to be about 20%. The subbase and base were placed and compacted in accordance with Wal-Mart's specifications and standard industry practice. Instead of discharging the fresh concrete directly on the ground, a truck-mounted concrete pump was used because of the low door openings of the barn. After discharge, workers ... finished the concrete by hand in accordance with standard industry practices. Finishers used ride-on machines with pans, floats, and blades to finish the floor and obtain the mottled appearance required for Wal-Mart's integral-colored floors. Finishers were certified American Concrete Institute (ACI) Flatwork Finishers. After finishing, contraction joints were sawed and the slabs were wet cured for seven days. After curing and three days of air drying, portions of each test slab was densified with a potassium silicate based floor hardener and polished according to Wal-Mart's specified procedures.

Prior to placing concrete, the concrete producer performed ASTM C403-05, "Standard Test Method for Time of Setting of Grout Mixtures by Penetration Resistance," tests to establish and adjust hardening rates for each of the concrete mixtures to ensure acceptable finishing times.

Slabs 1 and 2 with straight cement were the control slabs and did not contain any chemical accelerators, whereas a nonchloride accelerator was added to each of the fly ash mixes with various calcium chloride (CaCl) equivalent dosages.

Because CaCl is less expensive and much more efficient than nonchloride accelerators, tests also were conducted at a later date using colored concrete to evaluate the use of CaCl versus nonchloride accelerators. In winter conditions, the added cost of nonchloride accelerators exceeds the savings found in fly ash replacement. Results show that CaCl dosage rates up to 2% by weight of portland cement, with chloride ion contents below the 1% code limit for reinforced concrete that is dry in service (Wal-Mart slabs typically are not reinforced), are acceptable and Wal-Mart's revised floor specifications allow either nonchloride or CaCl to be used.

Controlling the time of initial set or rate of concrete hardening is the key to achieving an acceptable window-of-finishability for steel-troweled slabs with straight cement or fly ash. There is not a direct correlation between concrete setting times and the ASTM C403 mortar testing, however, Wal-Mart's research team recognized the importance of early detection of potentially slower setting mixes as a critical step to ensure success when steel-troweling floors with fly ash. After densifying and polishing, the test slabs were evaluated for a number of properties.

During construction, all test slabs were placed and finished without problems. Finishers reported the concrete surfaces of the 22.5% and 30% fly ash slabs were a little sticky but not so sticky as to cause finishing problems. Also, the 30% fly ash slabs showed more bleed water but again the bleed water did not create any finishing issues or problems. Also the finishers were aware of the increased bleeding and were careful not to finish the bleed water into the top surface of the concrete. Set times or the rate of concrete hardening was acceptable.

It became apparent that the finishing window for the 22.5%, and especially the 30% fly ash slabs, were shorter than the control and 15% fly ash slabs. The rate of concrete hardening for the 22.5% and 30% slabs was delayed but the window was actually shorter than the other test slabs. In other words, the 22.5% and 30% fly ash slabs remained dormant longer but when setting or hardening began, the rate of hardening was faster than either the control or 15% fly ash slabs.

The hardening behavior of the 22.5% and 30% fly ash slabs indicates the slopes of the ASTM C403 setting curves are important. The window-of-finishability opens when the surface is firm enough to start finishing and closes when the surface is too hard to finish. Increasing the amounts of fly ash delays the window opening but may reduce the time period the window is open compared to the finishing window for a concrete containing only cement.

Field superintendent Gary Marrou, Marrou Concrete, Fort Collins, Colo., worried about this project beforehand because he expected problems with poor setting times, excessive bleeding, and stickiness, and that his crew would not be able to achieve acceptable finishes. But after finishing was completed and the equipment was loaded onto the trucks, Marrou said, "The mixes used finished like a dream, like one continuous slab. I am no longer afraid of fly ash, though I remain cautious about fly ash mixes."

Workers sawcut contraction joints by 4 p.m. and the joint edges looked sharp and clean. The saw operator reported there were no discernable differences on how the slabs sawed. Early volume shrinkage cracking did not occur. Water was applied and curing blankets for the seven-day wet curing were in place by 6 p.m.

Overall appearances of the test slabs were good and any differences between the appearance of the slabs with and without fly ash were indistinguishable. All slabs had an acceptable glossy and mottled appearance. Floor flatness was measured and (the) values exceeded the minimum requirements. After the slabs were densified with a liquid floor-hardening chemical and polished, appearances were good with no discernable differences between the slabs.

Test slabs were next acid and stain tested. An eye dropper applied drops of vinegar, mustard, grape juice, and olive oil to the test slabs for 1, 5, 15, and 30-minute intervals. After the time period had elapsed, test materials were removed with an absorptive cloth and the resulting floor stains inspected and compared. As the amounts of the fly ash increased, the resistance to staining improved. Most likely, additional cementitious products resulting from the fly ash reacting with the calcium hydroxide formed in the surface pores making the surface less porous and susceptible to staining. Results also indicated that densified floors are more resistant to staining than nondensified floors.

Next, surface abrasion tests were conducted according to the British Standard BS EN 13892-4. This test consists of abrading a circular path in the concrete surface by rotating three hardened steel wheels attached to a circular plate. The plate is rotated by an electric motor for 2850 revolutions under a standard 65-kilogram (143-pound) load. Depth measurements are made along the wear path after running the machine. Measured wear depth along the wheel path is an indication of the abrasion resistance of the floor surface. As wear depth increases, the floor surface is considered less resistant to abrasion.

Test results showed that hard-troweled, polished concrete floors with 15% to 30% Types C or F fly ashes, by cement substitution, had less abrasion than floors constructed from straight portland cement concrete (the results also were the same for the control mixes). In addition, applying silicate-based floor-hardening chemicals did not increase the abrasion resistance of the floors constructed with concrete containing fly ash or straight portland cement.

(Fly Ash concrete, in other words, proved to be more resistant to abrasion than straight Portland cement concrete.)

As part of this research, a cost analysis model was constructed using past Wal-Mart projects. Fly ash replacement savings and chemical accelerators costs were modeled and compared. Results indicate that most likely the annual savings of using fly ash will be compensated by increased costs associated with adding chemical accelerators to offset delayed hardening. The specified allowance of CaCl as an accelerating admixture is a key element in achieving balanced installation costs while maintaining the quality of the trowel finish slab surface in colder climates.

Conclusions and Recommendations

Risks associated with using fly ash in steel-troweled floors are:

- Delay setting or hardening of the concrete due to the slower chemical reaction of fly ash compared to portland cement.

- Poor floor finish resulting from premature finishing caused by delayed surface bleed water and concrete hardening.

- Early volume shrink age cracking caused by delayed concrete strength gain.

- Ragged sawed contraction joints caused by delayed concrete hardening.

- Inconsistent concrete performance due to the variable of fly ash materials.

- Potential for increased costs during cold-weather placements caused by the addition of chemical accelerators and labor.

Specification recommendations include:

- Specify 15% to 20% fly ash by cement substitution.

- Require only one source of cement and fly ash be used for the entire project to minimize inconsistent concrete performance resulting from material variations.

- Allow either CaCl or nonchloride accelerators. Calcium chloride accelerators (up to 2% by weight of cement) are much more efficient in reducing set times and can help minimize potential admixture cost increases.

Quality of the floor finish for concrete containing fly ash is comparable to floors with straight cement, as long as the concrete setting characteristics are similar. Overall appearance, gloss, and the sharpness of sawed contraction joints are good for the fly ash test floors. Also concrete floors with fly ash are just as abrasion resistant as floors with only cement.

Results from this test program showed that ... hardened concrete properties of floors containing fly ash are equal to or exceed those constructed with straight cement."

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First, the fact that concrete made with Fly Ash will have "hardened properties" that can "exceed" those of concrete made "with straight cement", which properties include "resistance to staining", or resistance to chemical attack, is one we have documented for you a number of times previously, as, for one example, in:

West Virginia Coal Association | Federal Highway Administration Recommends Fly Ash Concrete | Research & Development; which concerned the Federal Highways Administration report, from the FHWA's "Infrastructure Group": "Fly Ash"; wherein we're told, that: "the use of fly ash in concrete improves its workability, reduces ... permeability, inhibits alkali-aggregate reaction, and enhances sulfate resistance".

Further, concerning Wal-Mart's statement, that; "producing 1 ton of cement releases about 1 ton of CO2 into the atmosphere", we have previously documented that the basic compound of "cement" is Calcium Oxide, CaO, which is produced by heating limestone, i.e, Calcium Carbonate, in accordance with the formula: CaCO3 + Heat = CaCO + CO2. And, that, "producing 1 ton of cement" out of limestone "releases about 1 ton of CO2 into the atmosphere".

And, the CO2-emission advantage of using Coal Ash as a substitute for cement, and for cement raw materials, will be highlighted and further explained in some reports to follow.

However, we have herein clear evidence that a large corporation, headquartered in the state of Arkansas, which, as of 2010, the most current information available to us, has exactly two Coal mines, one underground and one surface, both on the smaller side, that produce, combined, a little over 30,000 tons of Coal per year. God bless their little hearts.

But, the, by far, largest company headquartered in Arkansas thought enough about Coal, and enough about the environment, to prove that it makes economic and environmental sense to utilize Coal Ash as a substitute for, or additive to, in as much as is possible, Portland-type cement and Portland Cement Concrete.

And, because it makes so much sense, they have begun to require such use of Coal Ash.

There are state governments in US Coal Country that oversee and manage a whole lot more pouring of Concrete in one year, in their roads and other pubic-administered structures, than we could imagine that Wal-Mart would use, or could have used, in the building of all of their stores everywhere.

Do any of those Coal Country state governments specify and require, as does Wal-Mart, the use of at least a minimum amount of Coal Ash in any of the Cement and Concrete utilized in their road building and other public works construction activities?

If not, why not?

As Wal-Mart themselves state it:

"If other building owners, architects, and specifiers follow Wal-Mart's mandatory cement replacement with fly ash initiative, significant reductions in CO2 emissions ... can occur."


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