Green Options

The old saying “step on a crack, break your mother’s back” may not apply to sidewalks for much longer now that MIT researchers have figured out why concrete breaks down. As a result of the discovery, structures like buildings, bridges, and yes, sidewalks, could last for hundreds of years longer than they currently do. A nuclear waste container built to last 100 years could, for example, last 16,000 years.

Concrete that maintains itself by healing cracks improves the sustainability of infrastructure through its longer service life and lower maintenance inputs. Now researchers have developed flexible, self healing cement that won’t suffer catastrophic failure when strained in an earthquake.

For several years, Professor Behrokh Khoshnevis at the University of Southern California has been working on systems for rapidly creating buildings with system that is essentially a very large 3-dimensional printer.  Called Contour Crafting, the equipment is able to rapidly build up walls.  Already, test runs have been able to produce six-foot high concrete walls.

One goal of the team developing this technology is to be able to build a house in a day, a goal that they hope to reach within the next few years.  There is some merit to this goal, and the benefits of being able to rapidly and inexpensively produce houses are obvious, though not without some attendant problems.

Now here’s an innovation that might make me consider moving back to New York City: air-purifying concrete. The small Dutch town of Hengelo is testing out the concrete paving stones, which contain a titanium-dioxide based additive that binds to nitrogen particles emitted by car exhaust and turns them into harmless nitrates.

A South African company called Moladi is promoting their system for quickly building homes. Using their system, the exterior walls for a single-family dwelling can be built in a matter of just a day or two, and it can be done using unskilled labor. But, while their goals are admirable, it’s a question whether or not this is a really green method of building.

The system uses lightweight formwork panels, much like those used for poured concrete walls. Once erected, the forms are filled with mortar (concrete without stone) which can be hand-mixed and hand-placed, or which can be mechanically mixed and pumped into place. Typically, the formwork can be removed the day after the mortar is poured. The result is a smooth finish material, little more than a roof is needed to complete the building.

On the downside, this material uses 250 kilograms of portland cement per cubic meter (about 420 pounds per cubic yard), making it fundamentally the same as a concrete building with an enormous carbon footprint stemming from the extensive use of cement in these buildings.

The Association of Collegiate Schools of Architecture (ACSA) and the Portland Cement Association (PCA) have announced their “Concrete Thinking for a Sustainable World” student design competition.

This is the third year for the competition. The concrete industry is looking for opportunities to boost the perception of concrete as a green material, since the mining, processing and transportation of concrete is responsible for 8% of all carbon dioxide produced by human activity.