In recent coverage of how an Atlanta suburb banned wood construction, we quoted their bylaw promoting concrete construction because of its purported “increased building quality, sustainability, durability, and longevity.” But there has been a lot of research and not a few articles recently that call all of those so-called virtues into question.
The sustainability argument is the easiest and and most important one. The Economist summed it up recently:
The cement industry is one of the world’s most polluting: it accounts for 5% of man-made carbon-dioxide emissions each year. Making this most useful of glues requires vast quantities of energy and water. Calcium carbonate (generally in the form of limestone), silica, iron oxide and alumina are partially melted by heating them to 1450°C in a special kiln. The result, clinker, is mixed with gypsum and ground to make cement, a basic ingredient of concrete. Breaking down the limestone produces about half of the emissions; almost all the rest come from the burning of fossil fuels to heat the kiln.
The Economist doesn’t mention that cement is only 10 to 15 percent of concrete; the bulk of it is aggregate, or sand and crushed rock. In 2014 in the US, 1.26 billion metric tons of crushed stone was produced by 1,550 companies operating 4,000 quarries and 91 underground mines.
Aggregates are heavy, and get carried in heavy trucks running on diesel and pumping out CO2 at the rate of 0.14645 kg CO2e per Tonne-Mile; according to Wikipedia, transport alone accounts for 7 percent of concrete's CO2 emissions. When you add up the full impact of aggregates and add it to the impact of cement, the picture is far worse.
For building construction, the aggregates and cement are delivered to the Ready-mix people who mix the concrete to order and deliver it to construction sites in cement mixers, again heavy trucks that have to drive through city streets on deadline- they only have so much time between mixing cement and when it starts to set. They are deadly.
Then there is the question of durability and longevity. Writing in Architect Magazine, Blaine Brownell questions the myth of concrete durability in an article titled Concrete's Moment of Reckoning:
Concrete faces not only a problem in the production of a key ingredient, but also one of longevity. Steel-reinforced concrete, the most widely used building product in the world, is inherently flawed. The reason? Unprotected steel corrodes. Standard practice dictates a shielding the steel rebar or welded wire fabric with a layer of concrete to safeguard the metal from the oxidation and degradation that would occur if exposed to the elements. However, engineers are finding this method is inadequate, as evidenced by the number of deteriorating bridges and roadways in this country, designed for decades of use, that are now threatened by the premature failure of the reinforcing.
There is not a balcony or parking garage built with unprotected reinforcing that’s not going to need remediation at some point; just like exposed wood, exposed concrete deteriorates. But Brownell goes further, quoting author Robert Courland, who claims that “virtually all the concrete structures one sees today will eventually need to be replaced, costing us trillions of dollars … in the process.”
There's lots that can be done by the concrete industry to reduce their carbon footprint, and to make their concrete more durable. Many of the big companies are trying, and there are alternatives to unprotected steel reinforcing.
Everyone recognizes that there is a critical role for concrete, and it is not like we can do without the stuff; We are not likely to start building bridges and highways out of wood, although that has been done. But where we can replace concrete, we should be doing so. And buildings are a logical place to start, using established or new wood construction technologies.
The architecture firm Skidmore, Owings & Merrill LLP (SOM) has been working on its Timber Tower Research Project where they “examined solutions that could utilize mass timber as the main structural material to reduce the embodied carbon footprint of buildings by 60 to 75 percent compared to a benchmark concrete building.” They designed a hybrid wood and concrete system and recently did destructive testing on a floor slab.
The tested floor specimen—36 feet long by 8 feet wide—was modeled on a portion of a typical structural bay.….The floor system provided greater stiffness than required by code and supported an ultimate load of 82,000 pounds—approximately eight times the required design load.
SOM Associate Benton Johnson remarked that the successful test “highlights the real benefits of the composite timber approach. We took a small amount of concrete that was necessary for acoustic and fire performance and used it to enhance the structural performance of the floor. This move allows mass timber to reach its full potential, allowing it to compete in the market while also reducing the carbon footprint of cities.”
Seriously, when the Ready-mix industry says “Build with strength”, the wood people can just show them these photos from SOM and Oregon State University. Now that’s building with strength.