TECHNICAL PAPERS

The Greening of Concrete
Christopher Dixon RA, CSI

Why target concrete?

Cement, one of the main ingredients in concrete, is particularly harmful to the environment because during manufacturing, a huge amount of carbon dioxide (CO2) is released into the atmosphere. CO2 is known to be a major contributor to the greenhouse effect and global warming. Global emissions of CO2 totaled 21.6 billion tons in 1997. Cement production accounts for some 1.4 billion tons of CO2 emissions, about 7 percent of the total human-generated CO2. The U.S. is responsible for about 80-100 tons of this.

For each ton of cement manufactured, one ton of CO2 is put into the atmosphere. Each cubic yard of concrete poured (without replacement pozzolans), contributes 630 pounds of CO2. An average 2,500 square foot residence with a full basement uses about 100 cubic yards of concrete in its construction which contributes about 63,000 pounds of CO2 into the atmosphere. The average automobile produces about 1 pound of CO2 per mile. Our example residence added an amount of CO2 into the atmosphere equivalent to driving a car 63,000 miles. That's what happens when we build a house. Larger buildings and structures with many, many more times the concrete add an extraordinary amount of CO2 into the environment.

Approximately 1.3 billion tons of portland cement are produced each year. Estimates indicate that this number will double in 25 years. It seems that there is no end in sight to this escalation of consumption and not much we can do to slow it. We can, however, diminish the amount of CO2 emitted by substituting a portion of the cement with pozzolans.

Cement Substitutes

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 (GGBF) slag, and silica fume are man-made. These man-made pozzolans do not contribute any CO2 into the environment but in fact are waste by-products from coal burning and steel production.

The most common pozzolanic substitutes for cement in concrete are fly ash and GGBF slag, by-products of coal burning power generation and steel production, respectively. In the Eastern United 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, California and Wyoming. It is produced abundantly.

The most common commercially available cement substitute in the Pacific Northwest is Class F, or low-calcium, flyash, the highest quality of the two types of fly ash generated. Our locally available fly ash comes from coal burning plants in Centralia, WA and Wyoming. Most of the fly ash produced nationally is not used for anything but landfill. About 7% of what is produced finds its way into concrete. We are using a much higher percentage in the Northwest (nearly 90% of everything from the Centralia plant is diverted from landfills for use in concrete, but other Northwest fly ash producing plants send all the fly ash to landfills) but could be using much more, easily by a factor of ten or more, resulting in less waste senselessly filling up landfills.

Substituting cement with fly ash, in addition to being environmentally responsible, also makes economic sense. Fly ash is approximately 40%-60% less than the cost of cement. The more cement replaced, the less costly the concrete. Most batch plant estimators use cement cost as the benchmark figure to establish the cost of ready-mixed concrete, so reducing the amount of cement in a mix translates to cheaper concrete at the batch plant.

Although local batch plants typically use no more than 15%-20% fly ash in concrete as a replacement for cement, many studies, tests and demonstration projects have been done with 40%, 50% and even as much as 60% 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.

The Japanese, Europeans and Canadians have been using high-volume fly ash concrete for many years now. Cement in Europe and Japan is sold pre-mixed with fly ash. Several projects in Canada, from dams to multi-story mixed use office buildings to sidewalks, have been built using 55% fly ash replacement of cement at an average of $8-$10 savings per cubic yard.

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 superplasticizer. Fly ash in concrete acts as a superplasticizer.

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 3 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 which separates the aggregate from cement paste and allows larger, weaker crystals to form and fill the void. Microcracking in concrete occurs in this transition zone, perpendicular to these crystals. Connected microcracks create pathways for water and waterborne salts to wick deep into the concrete. These microcracks get bigger over time due to freeze thaw cycles and water's reactivity with by-products (alkali-silica reaction) of cement hydration. Water transports salts which corrode reinforcing steel. Introducing a large percentage of fly ash into the mix allows for much lower water/cementitious ratio which 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.

A spin-off benefit to all of this is that fly ash in the concrete mix makes beautiful, "architectural" concrete. It's lighter in color and can be poured with virtually no bug holes, rock pockets or honeycombing. That most famous of architecturally exposed concrete buildings, the Jonas Salk Institute, was built with pozzolanic concrete. The more fly ash, the better looking the concrete becomes. Some contractors in the area have recognized that using fly ash in the mix greatly reduces the repair normally required to fill rock pockets and honeycombing after stripping away the formwork. This is of such economic benefit that contractors have begun using fly ash in concrete for all formed concrete, whether or not it was originally specified.

Why Isn't Everyone Using High-Volume Fly Ash Concrete?

The short answer to this question is, simply, lack of knowledge and outdated codes and standards. Local area engineers and ready-mixed batch plants typically design mixes with up to a maximum of about 25% fly ash replacement, a sort of low-volume fly ash approach. Much more common is for fly ash to be added to, not used as replacement for, cement for locally produced medium-volume (as much as 33%, mostly for roadway overlays and architectural concrete) fly ash mixes. 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. The local batch plants seem uninterested or unwilling to forge the way to less costly, high performance, high-volume fly ash concrete as the standard instead of the exception. Structural engineers seem uninterested or unwilling to design concrete mixes any differently than they have been for years, or to increase their knowledge about high-volume fly ash concrete.

Engineers and architects specify what essentially has existed for many years a default of no more than 15%-25% fly ash as a total of cementitious content. ACI 318, until recently, allowed up to 25% fly ash only for all concrete. This is a standard that most designers are familiar with and are sticking to. The latest and greatest ACI 318, hot off the presses, qualifies that the limit of 25% fly ash applies only to concrete that will be subject to de-icing salts and sulfates. Even this limit is contrary to the benefits that we know fly ash imparts to concrete. Canadian studies have shown that to optimize the benefits from adding fly ash to concrete, between 40%-60% needs to be substituted for cement. Anything less is uneconomical from a dollars-to-psi perspective. We can expect to see standards such as ACI 318 change further, as more and more information about high-volume fly ash concrete is produced and presented.

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% 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 91 days. This would allow high-volume fly ash mixes to be used in almost all concrete in buildings, enjoying the benefits without sacrificing anything. Structural engineers that are at first skeptical of the claims about high-volume fly ash concrete often become the biggest advocates upon studying the research and developing a greater understanding of it.

Remember, high-volume fly ash concrete is successfully being used in mass concrete project pours today. There can be no reasonable argument that high-volume fly ash will cause insurmountable structural problems, early strength gain deficiencies, handling, placing or setting problems after seeing what has been accomplished. The Parklane Development in Halifax, Nova Scotia, Canada is a seven story structure and was built with 55% high-volume fly ash concrete (high strength mix in the table below) entirely. 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%-40% on average.

The table below comes from a paper presented by Wilbert Langley and Gordon Leaman at the Sixth CANMET / ACI / JCI International Conference, held May 31 - June 5, 1998. These are actual mixes used in demonstration projects throughout Canada to prove the practicality of using high-volume fly ash concrete for a variety of projects. All mixes contained air entraining admixtures and superplasticizers.

High-Volume Fly Ash Concrete

  Conventional Mix Low Strength 55% Replacement Medium Strength 55% Replacement  High Strength 55% Replacement
Total Cementitious Content (c+fa) (lb/cu.yd) 483 374 566 660
Cement (lb) 483 166 250 300
Class F Fly Ash (lb) 0 208 316 360
Sand (lb) 1334 1467 1250 1266
Stone (lb) 1700 1834 1834 1850
Water (lb) 220 185 198 185
Water to Cement Ratio .46 .49 .35 .28
Compressive Strength (psi)        
3 day 4,600 1,250 2,320 3,190
7 day 5,000 1,750 3,040 4,900
28 day 6,500 3,350 5,500 8,300
91 day 7,100 4,050 7,700 10,900
365 day 7,550 7,400 10,000 -
Set Time (hours:minutes)        
Initial 6:25 8:25 5:35 -
Final 7:50 11:15 7:40 -

There are many other projects that have been designed and built with high-volume fly ash concrete in Canada, Europe, Japan and the United States. In the US, the state of Wisconsin has been using a 60% Class F fly ash concrete mix since 1989. Washington State DOT has begun experimenting with high-volume fly ash mixes for state civil work. There are a few built and demonstration projects in California taking advantage of high-volume fly ash concrete as well.

Conclusion

What more do we need to convince ourselves that this is the way we should be designing and building with concrete? Will we be content to continue as we have for decades, providing average quality concrete and needlessly loading up the atmosphere with millions of pounds of CO2 each year?

We are in a unique position to be the leaders in efforts to produce high performance, high-volume fly ash concrete. We have extremely high quality aggregates, a plentiful central source of the best fly ash known, and a unified desire to build sustainably as residents of the beautiful Pacific Northwest. What are we waiting for?