BuildingGreen Tells You Everything You Ever Wanted To Know About Concrete
Wikipedia/CC BY 2.0
Since Roman times, some of the world's most important and beautiful buildings have been made from concrete. Like the Pantheon in Rome, they can last for thousands of years. But there is a huge carbon footprint to the stuff, from the chemistry of turning limestone into cement, to the excavation and transport of aggregate. For years I have complained about the concrete companies claims of being "sustainable" and have gone on about insulated concrete forms being a poison sandwich.
Now the BuildingGreen people have produced a new guide, What You Need to Know About Concrete and Green Building; they describe it:
This special report takes a look at how these materials are made, presents the key environmental considerations relating to their prodution, use, and eventual disposal, and describes ways to reduce their environmental impacts.
The report is what they used to call "fair and balanced" before Fox News appropriated the term: they clearly and carefully go through both sides of the story. They acknowledge (as I do, under duress):
Concrete has many environmental advantages including durability, longevity, heat storage capability, and (in general) chemical inertness. For passive solar applications, concrete’s ability to function as a structural element while also providing thermal mass makes it a valuable material. Its ability to act as both structure and finish in interior was well as exterior applications is unique, and with good design can contribute to an overall reduction in use of building materials on a project.
On the other hand, making cement produces half a ton of CO2 for every ton of cement made. It is the largest industrial emitter of CO2 in America. Its production emits mercury and other heavy metals. But it is getting better; the the amount of portland cement in the mix is being reduced, replaced by recycling fly ash. While there is some concern about putting fly ash or slag into concrete, the consensus appears to be that it is bound in and is safe.
The authors make a lot of suggestions for using concrete wisely: reduce the cement content, look at different designs of foundation systems, avoid toxic admixtures. Given the material's benefits of thermal mass, its fire resistance, and its ability to be a final finish of floors and walls, they make a good case for its use, if it is used right.
I have a few quibbles; the authors buy the concrete industry's party line that the material has a low embodied energy. That's because the table shows the embodied energy per pound. In real life, you want to compare the embodied energy of the material needed per square foot of construction; A concrete slab might weigh a hundred pounds per square foot while a steel structure might weigh a tenth of that. Fiberglass will be a couple of ounces. The embodied energy per pound is completely irrelevant.
I also think that the authors don't give enough exposure to the issue of the aggregate that makes up 65% of concrete. The excavation and transport of the stuff is a huge issue in many parts of the world. 20,000 people showed up last weekend north of Toronto to protest the proposed Melancthon Mega Quarry, and others have been fighting to protect the Niagara Escarpment, a recognized biosphere reserve, from being dug up for gravel. It is a big issue that doesn't get enough attention.
On balance, though, BuildingGreen's Guide not only tells you just about everything you need to know about concrete and green building, it gives great recommendations on how to get the best performance out of the stuff with the least environmental impact.
What You Need to Know About Concrete and Green Building is free to subscribers of BuildingGreen, and is $ 49 to non-members. Joining is a better bet; the site is full of great stuff about green building.