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Experts Seek to Decrease Concrete’s Carbon Footprint

Concrete, Buffalo City Court Building
Experts in the design and construction professions are making headway toward accurately determining the carbon footprint of concrete used in buildings—and in devising methods to reduce that footprint. Wikimedia Commons/Fortunate4now

The next frontier in sustainable construction may be a process known as carbon accounting of concrete—the method of developing standards for measuring the carbon footprint of concrete and developing strategies to reduce that footprint.

October 1, 2013—According to a 2011 report published by the Concrete Sustainability Hub at the Massachusetts Institute of Technology, buildings are responsible for 41 percent of the United States’s total primary energy consumption and 39 percent of its total CO2 emissions. So it is not surprising that nearly every discipline involved in designing and constructing buildings has been discussing the notion of how to reduce the carbon footprint of those buildings. Until now, the focus has been primarily on getting those structures to operate with greater energy efficiency—it’s the bread and butter of the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system.

But now the focus is shifting toward addressing the carbon embodied in the construction materials and construction processes themselves—what Don Davies, P.E., S.E., a senior principal of the Seattle-based structural engineering firm Magnusson Klemencic Associates (MKA), described as the “critical and next frontier in the quest for carbon-footprint reduction in the building industry,” in his paper his paper “Climate-Conscious Building Design: New Approaches To Embodied-Carbon Optimization,” published in Trim Tab, the online magazine for the International Living Futures Institute.

“Embodied carbon is a great step in what structural engineers can do to be more relevant” to making buildings more energy efficient, Davies says. “If I reduce the structure carbon footprint by 50 percent, that’s the equivalent of 6 to 7 years of building operations.”

MKA’s research suggests that structures account for the largest share of embodied carbon in any project, typically between 28 and 33 percent. Davies says the firm conducted a study on a 24-story hotel in Seattle, and determined that optimizing the concrete could reduce the building’s embodied carbon footprint by 50 percent—primarily by addressing where the concrete is made and what energy source is utilized at the point of fabrication.

Now a new process called carbon accounting of concrete—the process of developing standards of measuring the carbon footprint of concrete and developing strategies to reduce that footprint —is gaining traction. Four years ago, a group of experts—including architecture and engineering firms, a general contractor, a material manufacturer, and a concrete supplier—formed the Carbon Leadership Forum (CLF), to determine best practices for measuring carbon in concrete and to push for the adoption of low-emitting concrete

CLF is operated from the College of Built Environment at the University of Washington, and one of its founding members is Phil Williams, P.E., LEED-AP, a vice president of Webcor Builders of San Francisco. In written responses to questions posed by Civil Engineering, Williams said, “We all recognized the GHG [greenhouse gas] accounting was just the start of a process to better define, mitigate, and report the environmental impacts and properties for building materials,” Williams said. “Concrete just happened to be the first one that we elected to address.”

Andrew Deitz, the vice president of business development for Climate Earth, a Berkeley-based firm that, among other services, verifies environmental product declarations (EPDs) from product producers based on life cycle assessment data, says there is now “significant momentum” around providing EPDs for concrete. Deitz cites three reasons: a growing demand for green construction projects, a growing demand for actual proof of environmental claims, and advancements in the technologies used to verify EPDs, including those used for concrete. The shift, he says, is from tens of products with EPD labels to thousands.

As Deitz points out, concrete has a relatively small number of ingredients—around two dozen—which can yield as many as 1,000 different mixes. So it is relatively easy to design a concrete mix specifically to reduce its carbon footprint.

Kathrina Simonen, R.A., S.E., LEED-AP, M.ASCE, an assistant professor at the University of Washington who oversees the work of the CLF, agrees. “Concrete has the most potential to use this data to change and reduce environmental impacts, because we can make new concrete mixes just by developing them,” she says. “You don’t have to develop a new manufacturing facility,” she explains. “You just have to mix a different mix.”

The key ingredient, of course, is cement, and it is usually the most resource-intensive. Addressing carbon emissions requires either switching to renewable energy like wind or hydroelectric in the cement manufacturing process, or using other cementitious products—fly ash or slag, for example. Davies notes that improving quality control at batch plants can reduce cement content without reducing concrete strength—a win from an environmental perspective. Ryan Henkensiefken, P.E., LEED-AP, M.ASCE, the business development manager for San Jose-based Central Concrete, a U.S. Concrete Company, notes that concrete produced with portland cement has higher emissions than concrete produced with fly ash or slag, even if the replacement materials must be transported across long distances.

And measuring embodied carbon is growing more precise. San Jose-based Central Concrete is the first manufacturer to provide EPDs for specific concrete mixes. The company turned to Climate Earth to help develop its accounting system and allow it to be applied across its entire product line. Central Concrete’s process measures raw material supply, including extraction, handling and processing; transportation; manufacturing; and water use in mixing concrete.

Currently, Central Concrete has 1,400 EPDs that have been validated through a third party, with another 1,500 undergoing validation now. The company’s standard concrete mixes utilize 50 percent cement replacement materials, and the company has delivered mixes with up to 75 percent replacement materials. The 50-percent mixes deliver approximately a 30 to 35 percent carbon reduction over traditional portland cement mixes, Henkensiefken said.

MKA uses a six-step process that begins by utilizing building information modeling (BIM), which helps to accurately establish the material quantity for a building’s structural frame. From there, the source of the material being used is determined, as well as travel times and shipping methods, the energy required to produce the material, and the carbon footprint of the energy source—all of which yield the overall carbon footprint.

Williams points to new product category rules for concrete sponsored by the National Ready Mixed Concrete Association as a sign that there is “significant movement towards a common set of rules for reporting the environmental factors for different concrete mixes, just as we currently report engineering factors such as concrete strength, water-cementitious material ratios, and slump.”

Further, LEED version 4, due out later this fall, will award two points for just for having EPDs, even if they are not particularly flattering. Transparency, Simonen says, is a first step forward. “You have to encourage people to share their information rather than punishing them because it might not be the best,” she explains. “It’s good for everyone to share their information.”

When asked if carbon accounting of concrete is merely a passing trend or a staple of environmental accounting for the future, Williams points to the new LEED standard. (See “USGBC Beta Tests LEED Version 4” on Civil Engineering online.) “The question of whether EDPs will be the future or a fad has been answered, and that unequivocal answer is [that it is] the future for materials—and many would say [it] is actually the present.”

And to move that trend along, the carbon footprint of concrete has to be as readily identifiable a metric as its strength or workability. “There are some clients who might be willing to pay slightly more money to have a lower-impact concrete,” says Simonen. “So you could add an environmental footprint threshold to your specifications and be willing to pay more for a low environmental footprint threshold.” It would be the same, she says, as paying more for higher-strength concrete.

“Theoretically this could help move the market,” she continues. “It could help spur innovation, give people competitive advantages if they can think of less expensive ways to make low-impact concrete.”

But Davies and Simonen suggest that in this case, it’s important not to let perfection become the enemy of the good. “Most of the data is not presented with its expected error [margin], so it sounds like a very precise number—but it’s not that precise,” Simonen says. “I think that there’s a risk that people could be discouraged by that. [But] I think the benefit is more in terms of understanding relative impacts and working to reduce them. We don’t need to take 50 years to get exact numbers,” she adds. “We just need to make reductions.”

“If we’re going to use carbon accounting in a meaningful way,” says Davies, “you need to be able to get the information quickly and easily. I really don’t care about absolute numbers. I care about relative numbers. The absolute number of carbon is not the point. The point is, use less.”

Embodied carbon, ultimately, may prove to be low-hanging fruit along the path of real sustainability. Tougher questions about land use planning and the planned longevity of buildings also need to be asked.

Still, there is a “general interest in the structural engineering community to do the right thing,” says Davies. “If there’s a way to do the right thing and make a difference, people are supportive of it. But it has to be kept practical and simple enough so it’s worth the effort.”



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