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.

Table 1: Test Slab Summary
Table 1: Test Slab Summary

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 tonsper 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 (learn more about fly ash). 

Research

What is 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. All fly ashes exhibit cementitious properties to varying degrees depending on the chemical and physical properties of both the fly ash and cement. Compared to cement and water, the chemical reaction between fly ash and calcium hydroxide typically is slower resulting in delayed hardening of the concrete. Delayed concrete hardening coupled with the variability of fly ash properties can create significant challenges for the concrete producer and finisher when placing steel-troweled floors.

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.

— Kim Basham

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. Table 1 summarizes the test slabs placed, finished, and evaluated.

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 placedand 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 proceeded to strike off by hand the concrete instead of a laser screed and finished the concrete by hand in accordance with standard industry practices. Finishers used ride-on machineswith 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. See Table 2.

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.

The initial set time of the control mix (observed with slabs 1 and 2) was first determined in accordance with ASTM C403. Then the concrete supplier determined the accelerator dosages for each of the fly ash mixes using a set time target equal to the initial set time of the control mix. Initial set times for the fly ash mixes fell within a 73-minute window of the set time for the control mix. 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. See Table 3.