Results of strength tests for a 50 percent high volume fly ash concrete mix design.
It is generally accepted that for each ton of cement manufactured, one ton of carbon dioxide is discharged into the atmosphere. An average cubic yard of concrete contains 500 to 700 pounds of cement, which equals 500 to 700 pounds of CO². New construction of an average U.S. residence with a full basement will require about 100 cubic yards of concrete, a contribution of about 60,000 pounds of CO². The average U.S. automobile produces about 1 pound of CO² per mile. Our example residence contributes an amount of CO² into the atmosphere roughly equivalent to driving a car 60,000 miles. Commercial buildings, roads, bridges, and other civil infrastructure contribute many more times the added amount of CO² into the environment just in the use of concrete. Cement is used in many other building materials including sewer pipe, architectural precast concrete, plaster, mortars and grouts.
By some estimates, worldwide demand for cement will reach 3.5 billion metric tons by the year 2012. Projections indicate that this number will continue to grow for at least another 20 years.
Because cement production is such a large CO² contributor, it has received a lot of attention as the world looks for ways to reduce our carbon footprint. Indeed, states are beginning to address reduction of CO² with “cap-and-trade” legislation-and cement is being specifically called out as a material of particular interest. In 2006, the state of California passed Assembly Bill 32, formally called the California Global Warming Solutions Act. This bill requires a reduction of the state’s total CO² output to 1990 levels by the year 2020. The state’s largest contributor of greenhouse gases is CO², accounting for nearly 85 percent of all in-state emissions. Of this, 96 percent comes from fossil fuel combustion. The top producer of CO² of the remaining 4 percent is cement production. After automobiles and electricity generation, cement production is next on the list.
Following closely in California’s footsteps are the states of Washington and Oregon, and not far behind the federal government (which is waiting to see how the states fare in this CO² reduction effort before they jump in). Washington State is in the process of drafting its CO² reduction legislation in House Bill 1819. The Western Climate Initiative released the latest draft of its proposed program design for Oregon’s greenhouse gas cap-and-trade program in July 2008.
GREENER CEMENT-BASED MATERIALS
Arguably, the simplest ways that cement-based building products can reduce their GHG contribution is cement reduction and replacement. A combination of the two is often employed. Reduction of cement can be as simple as reducing the cement content of a 700 pound per cubic yard concrete mix to 600 pounds per cubic yard. This can be done without sacrificing much in the way of strength or workability. Replacement of cement is a more sophisticated method, resulting in a far greater cement reduction potential.
Pozzolans are natural and man-made substances which, when mixed with concrete, react to make the same “glue” that cement makes when added with water. Natural pozzolans are essentially volcanic ash. The gray stuff you see from I-5 lining the banks of the Toutle River is pozzolanic and could be used to make concrete but not very suitable for structural use. Better, purer pozzolans such as fly ash, ground granulated blast furnace slag, and silica fume are man-made. These man-made pozzolans are waste byproducts from coal burning and steel production.
The most common pozzolanic substitutes for cement in concrete are fly ash and GGBF slag, byproducts of coal burning power generation and steel production, respectively. In the eastern states, both of these waste products are readily available and commonly used as cement substitutes. GGBF slag is produced in steel mills located in the eastern U.S. The demand for GGBF slag in the east is greater than the supply, making it difficult and expensive to acquire for use on the West Coast. The sources of fly ash in the West Coast come from the states of Washington, Oregon and Wyoming.
Substituting cement with fly ash, in addition to being environmentally responsible, also makes economic sense. Fly ash is approximately 80 percent the cost of cement. The more cement replaced, the less costly the end product.
Although local batch plants typically use no more than 15 to 20 percent fly ash in concrete as a replacement for cement, many studies, tests and demonstration projects have been done with 40 percent, 50 percent and even as much as 60 percent of the cement replaced with fly ash, with superior results. Concrete with this much fly ash is referred to as high-volume fly ash concrete.
HOW FLY ASH WORKS
The physical shape of fly ash particles can be best described as fine glassy beads. When added to a concrete mix, these smooth round objects act as a sort of lubrication by attaching themselves to the cement particles and keeping them from globbing together during hydration. This increases workability and responsiveness of the mix during placing and vibration. The small size of the particles also fills voids between cement particles that would normally be filled with excess water. This allows a for lower water/cement ratio without sacrificing workability. High-volume fly ash mixes have water/cement ratios much lower than conventional mixes, without having to add a super-plasticizer. Fly ash in concrete acts as a super-plasticizer.
Fly ash is reactive but it reacts more slowly than cement and therefore takes longer to turn into the glue that holds concrete together. The early strength developed in concrete with fly ash is slightly less than that of a cement-only design mix but the later strengths developed are much greater. A high-volume fly ash concrete that develops 3,000 psi in three days will be in the neighborhood of 7,000 psi at 56 days and will continue to increase until about 365 days. Conventional concrete will stop gaining strength at around 100 days.
The weakest part of cured concrete is what is called the transition zone, a layer of water that separates the aggregate from cement paste and allows larger, weaker crystals to form and fill the void. Micro-cracking in concrete occurs in this transition zone, perpendicular to these crystals. Connected micro-cracks create pathways for water and waterborne salts to wick deep into the concrete. These micro-cracks get bigger over time due to freeze-thaw cycles and water’s reactivity with byproducts (alkali-silica reaction) of cement hydration. Water transports salts that corrode reinforcing steel. Introducing a large percentage of fly ash into the mix allows for much lower water/cementitious ratio that makes for smaller transition zones around the aggregate. Fine fly ash particles fill the voids and react over a long period of time to change the transition zones into cement paste, the glue that holds concrete together.
High-volume fly ash concrete is denser, stronger, more durable, less permeable, seawater and sulfate resistant, protects reinforcing steel better, and creates much less heat during hydration (another factor contributing to microcracking). Higher quality concrete means longer lasting concrete. One of the most important tenets of sustainable design is longevity.
WHY ISN’T EVERYONE USING HIGH-VOLUME FLY ASH CONCRETE?
The short answer to this question is simply a lack of knowledge, and outdated codes and standards. Local area engineers and ready-mixed batch plants typically design mixes with a maximum of about 25 percent fly ash, and this is not a 1:1 cement replacement. Much more common is for fly ash to be added to the mix. This makes the mix more workable and reduces cracking but costs more. This has caused most people to equate fly ash in the mix with more expensive concrete.
Engineers and architects specify what essentially has existed for many years-a default of no more than 15 to 25 percent fly ash as a total of cementitious content. ACI 318, until recently, allowed a maximum of 25 percent fly ash for concrete. However, the current version of ACI 318 qualifies that the limit of 25 percent fly ash applies only to concrete that will be subject to de-icing salts and sulfates. That leaves most concrete used in buildings open to a much higher percentage of fly ash use.
Structural engineers have traditionally been schooled to believe that fly ash in the mix reduces the early strength gain to a point where it isn’t prudent to allow it into most structural design mixes. Instead of using 28 days as the time at which strengths are measured, it makes more sense to specify strength gain minimums at 56 days for the majority of concrete. Most of the concrete structural elements in buildings do not need to have achieved 90 percent of ultimate strength in 28 days. This is impractical, expensive and a waste of resources. More reasonable is to require minimum strength gains at 56 or even 90 days. This would allow high-volume fly ash mixes to be used in almost all concrete in buildings, enjoying the benefits without sacrificing anything.
Examples of buildings built using high-volume fly ash mixes stand as evidence against commonly espoused myths about insurmountable structural problems, early strength gain deficiencies, and handling, placing or setting problems. The Parklane Development in Halifax, Nova Scotia, Canada is a seven story structure and was built entirely of 55 percent high-volume fly ash concrete. Cast-in-place columns and beams were poured with concrete specified to meet design strengths of between 4,350 psi at 28 days and 7,250 psi at 120 days. Actual strengths developed exceeded required strengths by 30 to 40 percent. In the U.S., Wisconsin has been using a 60 percent Class F fly ash concrete mix since 1989. Washington State DOT has begun experimenting with high-volume fly ash mixes for state civil work. I have personally been involved with concrete mix designs for buildings using as much as 50 percent replacement of cement with fly ash, with amazing success.
CONCLUSION
States are not waiting for the federal government to show us the way toward reduced green house gas emissions. As legislation is enacted, and action plans developed to meet the new legal requirements, cement is at the top of the list as something that can be improved upon. We know how to reduce cement in concrete by as much as 50 to 60 percent. Other cement-based materials producers are also experimenting with reduction of cement in products such as grouts, mortars, pipe, CMU, and plaster with similar positive results. Newly enacted legislation will likely transform what is now experimental into what will be mainstream. To quote a Bob Dylan lyric, “You better start swimmin’ or you’ll sink like a stone/for the times they are a-changin’.” W&C
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