Oops... (corrected graphic below)

Well, all you can do when you screw up is try to make it into a learning opportunity, I guess. The image we featured most prominently with our "Counting Carbon" article in July had a blatant error. In our defense, the image we asked for was OK — we just failed to make sure that the one we got was the same as the one we thought we were getting... The graphic had cubes representing one metric ton of steel, concrete, and wood, and much larger cubes representing the associated carbon emissions. The carbon quantity shown for concrete, however, actually represented the carbon associated with one metric ton of cement. A ton of concrete is responsible for much less carbon, because cement only represents about 12% of a typical concrete mix, and the other ingredients are much less carbon intensive. In addition to the fully justified outcry we got from the concrete folks about this graphic, we also got a complaint from the steel industry. They quibbled with the numbers, but they also had a more interesting point: that it is somewhat misleading to compare these three materials in this way, because their mass does not represent their utility. A structure made of concrete will weigh much more than a structure made of steel or wood, for example. (Here's a bonus graphic coming at it from this angle.) Here's the full text of both letters, plus a corrected graphic:

Corrected graphic

Dear Mr. Wilson:

For years I have admired Environment Building News' skill in providing balance on the many areas of sustainable development and design, clearly researching and documenting the details, in a manner that is easy to read and comprehend. So, it was somewhat surprising to see on the front cover of your latest issue (Vol. 17, Issue 7, July 2008) a volumetric depiction of a ton of three basic building materials and the carbon dioxide generated to manufacture them. If I had a nickel for every time someone confused cement and concrete, I'd be a wealthy man. It's easy to do, it happens all the time. Sometimes it is less important, like when someone speaks of installing a new "cement" patio. On other occasions, it matters a great deal. The design community is being asked to evaluate how we design, construct, operate, and deconstruct buildings in a carbon-constrained world. Carbon taxation and cap-and-trade issues are being discussed in the states; globally, nations vie for position on international climate change standards. Cement, as an ingredient in concrete, is energy intensive, but accounts for a small percentage of concrete's overall mix design (around 8 to 14%). The remaining ingredients of sand, gravel, and water generally require very little energy to obtain, process, and ship. Furthermore, today's concrete frequently contains supplemental cementitious materials (SCM) derived from industrial by-products. These further reduce the embodied energy and CO2 for a unit of concrete. Instead of the 1.2 metric tons depicted in the graphic, the Portland Cement Association has calculated the following CO2 equivalent per metric ton of concrete:

• .11 metric tons for 3000-psi with no SCMs
• .09 metric tons for 3000-psi with 20% fly ash
• .065 metric tons for 3000-psi with 50% slag cement

Note: these numbers do not include the CO2 that would be absorbed from the air through carbonation over the life of the concrete. Also note that we get the same results using two other methods: The EDIP (Environmental Design of Industrial Products) method (Danish) and IMPACT 2000+ method (Dutch). We calculated the carbon equivalent footprint of three typical concretes using (1) the climate change factors from the Intergovernmental Panel on Climate Change (IPCC) with a timeframe of 100 years (this is one of the methods in life cycle assessment software SimaPro) and (2) the life cycle inventory data in from research (PCA SN3011). We chose a 3000 psi strength; however, specifications can range widely. I am not denying that the concrete industry does have a large footprint. However, the sole reason is not its energy intensive component, but because of concrete's multitude of applications. It's everywhere: from houses to high-rises, roads and runways, stormwater systems and stadiums. What was once a material for roads and building foundations has evolved to create high-performance insulated wall systems, water piping, siding, roof tiles, decorative flooring and countertops, and cultured stone. It's even a solution for in-situ soil remediation. The industry, however, is not merely dedicated to promoting the uses of its product. We recognize our responsibility to continue manufacturing and usage improvements. We have reduced the amount of energy to make a ton of cement by more than 37% since 1972 and pledge progress toward future reductions. Recycled ingredients make up an ever larger portion of our business. And our industry has invested a great deal of resources into better educating our customers about how to use concrete for superior sustainable solutions. The most significant environmental impacts over the building's lifetime are not from construction products but from the production and household-use of electricity and natural gas. Today, and in the future as we strive to improve our products, concrete's versatility and use in many green building applications makes it an excellent material for sustainable designs. Sincerely,

David Shepherd, AIA

Director, Sustainable Development

Portland Cement Association

Skokie, IL

Mr. Nadav Malin

Editor

Environmental Building News122 Birge Street, Suite 30

Brattleboro, VT 05301

Dear Nadav, Your article, "Counting Carbon" in the July 2008 Environmental Building News is thoughtful throughout, as always. Unfortunately, however, the simplistic rendering on the cover page of this newsletter does not lend itself to "Understanding Carbon Footprints of Buildings" accurately. Specifically, the small and large cubes in the rendering very seriously misrepresent the carbon footprint of steel relative to concrete and wood. Additionally, the values provided for each material are believed in error. Steel, for example, is 1.7 metric tons rather than 2.0. We think the values for concrete and wood are out of date, too, but they are not our domain. Of course, the careful reader realizes for any given building application, one ton of steel does not equal one ton of concrete or one ton of wood. Steel has a very high strength to weight ratio and is strong in both tension and compression. Concrete is strong in compression but relies upon embedded steel reinforcing bar for tensile strength. Naturally, no building is made with all steel or all concrete. (The same is usually true of wood.) Case studies are available that show the quantities of steel vs. concrete in alternative building designs. The resulting carbon footprint for steel is smaller than for concrete. Another consideration that makes this rendering misleading is its failure to address end of life recycling for steel, as its embodied energy is amortized over many future generations of new steel. A growing case is also being made for an alternative end of life for steel, namely, re-use, as part of one or more iterations before its ultimate recycling. Steel is well known for durability. We see that its longer service life, with less replacement, is a major point not incorporated into the rendering. Therefore, the rendering in question offers no meaningful comparison of these three materials in a building application or in general. We recommend that EBN's on-line downloadable archive newsletter for July 2008 be revised by removing the rendering on page 1 and replacing it with the other rendering from page 11. We appreciate your consideration in making this important correction, as LCA and other approaches for studying and effecting environmental improvement go forward responsibly. Sincerely,

Gregory L. Crawford

Vice President, Operations

Steel Recycling Institute

Pittsburgh, PA