Concrete solutions to transit projects' carbon pollution

Posted on November 27, 2013 by Dave Walsh - Also by this author

Big transit projects often cite reduced carbon pollution as a main selling point to the public. But to take environmental stewardship to the next level, we should look past tailpipes and smokestacks and focus our attention on what goes into these civil engineering marvels. Namely, a lot of concrete.

This is no small matter. Concrete is used throughout the world more than twice as much as steel, wood, plastics and aluminum combined. One of its main ingredients, cement, takes a lot of energy to produce. And that fuel – very often used tires which burn steadily and slowly – produces a lot of carbon pollution. In fact, the process produces approximately a pound of carbon dioxide for each pound of cement. Cement manufacturing accounts of 5 to 10 percent of the world’s total greenhouse gas emissions.

When we consider the tunneling, retaining walls, elevated guideways and stations of mass transit projects, the amount of concrete quickly becomes an environmental concern. Or should be.

While unquestionably essential to infrastructure construction, concrete and its environmental impacts are viewed as unavoidable. For decades, it’s been conventional wisdom that the positive environmental and society benefits derived from a well-functioning light rail, commuter train or bus systems outweigh the negative impacts.

While this may be true, this traditional way of thinking overlooks all the ways concrete can be made more environmentally-friendly.

Along with tracking the greenhouse gas emissions of their construction equipment, some transit agencies such as Sound Transit in the Seattle region are conducting some innovative research around concrete use, estimating the quantities of concrete in planned expansions and seeing how this might impact their carbon footprints.

And there are ways transit agencies can make a difference. Agencies can optimize the concrete mix design by writing guide specifications that avoid prescriptive and often unnecessarily excessive cement contents and instead define a performance-based approach for concrete suppliers that reduce environmentally-impactful ingredients.

KVDP image via Wikimedia Commons
KVDP image via Wikimedia Commons
Instead of exclusively using what’s known as Portland cement (so named for its similar color to limestone quarried in England’s Isle of Portland), suppliers can replace a percentage of the cement with slag, a by-product of steel manufacturing, and fly-ash, which is produced by burning coal.  Another big bonus: replacing Portland cement uses less water, and is either cost neutral or less expensive.

One might argue that neither steel nor coal is a clean industry. But both steel and coal are integral parts of the world economy, and if we can make cleaner concrete by tapping into their respective waste streams, it makes sense to do so.

But what are the engineering trade-offs?

In many ways, there are a lot of positives. Slag, for example, lightens the color of concrete helping to increase the solar reflective index which mitigates the impact of the urban heat island effect. Additionally, slag helps concrete workability enabling a more fluid mix that can easily squeeze between tight rebar cages. This is a big advantage when casting large columns that require concrete pumped from the bottom of the column.

There can be complications in adding extra curing time, but even here, some extra planning and foresight can go a long way.

With contractors, efficiency is key, and anything that extends the schedule costs money. But contractors should be asking when is the latest responsible date that a particular concrete application needs to reach full strength. In some cases, where applications will not receive a load for several months and the construction schedule critical path is not affected, the longer curing times that can come with lower cement mixes may be just fine.

There are lots of examples of so-called “green concrete” in publically-funded construction projects across the country. The San Francisco Public Utilities Commission Headquarters was certified LEED Platinum by the U.S. Green Building Council in part by replacing 70 percent of Portland cement in the project with fly ash and slag cement. Refinements were made throughout the project to meet specified strength without compromising finishability of the slabs or delaying set time.

In the future, a national carbon cap-and-trade market or carbon tax might be instituted; it’s already a reality in California and in British Columbia. If and when that legislation becomes reality in your region, transit agencies that have embraced green concrete will have a decided advantage. And even if that day never comes, transit agencies that focus on cleaner ways of doing business will reap the rewards of generating greater public good will, and leaving future generations a cleaner planet.  

About the Author:
David Walsh is a project manager with Sellen Sustainability. A LEED-accredited architect with experience with both design and construction, Walsh is a longtime sustainability advocate who helps clients meet their sustainability goals and develop sustainability plans.

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