Can Concrete Be Sustainable?

By Matt Baker

concrete_1Concrete, industry proponents will tell you, is one of the most sustainable building materials on the market. It is incredibly durable, made from abundant materials, has a low thermal transmittance and it is recyclable. All of these are true. Unfortunately, it is also an enormous source of carbon dioxide emissions.

Cement, to be precise, is the real culprit. Portland cement typically composes a little more than 10% of concrete’s volume, but 90% of its carbon footprint. Given how ubiquitous concrete is as a building material, something needs to be done to lower these emissions.

One step that concrete company Ozinga has taken is to change its fleet of diesel-run ready mix trucks to compressed natural gas (GNG). The CNG ready-mix trucks are an effort to both source the company’s energy demand domestically and use a cleaner-burning fuel. Ozinga initially ordered 14 CNG trucks, which at the time, according to Brian Lutey, Vice President of Green Building, was the largest fleet in the world. “Now we have 70, with a commitment to switch over all 400 plus of our trucks to CNG,” said Lutey. This represents a large investment for the company. As Lutey points out, “There’s not a big market for used ready-mix trucks.” They also have a contingent of support vehicles that run on the alternative fuel, such as Honda Civics designed for CNG.

While CNG vehicles emit up to 95% fewer greenhouse gases, there is one major drawback: logistics. There are two CNG fueling stations currently open to the public in Chicago, and only a handful more in the entire state.

Ozinga now operates two CNG fueling stations of its own at the Chinatown location and another in south suburban Mokena, with a third planned for Gary, Indiana. They make the stations available to a few other commercial fleets and hope to open them to public at large as well. The stations feature fueling pumps where the drivers can leave their trucks overnight to fill, as well as “fast fill” pumps where the CNG is pressurized close to 4,000 psi for a quicker refueling.

Distribution—from the quarry to the plant to your foundation footing—has a definite impact on concrete’s carbon footprint and switching from diesel to CNG will go a long way toward curbing carbon emissions. But the cement manufacturing process has much larger consequences. The key ingredient in cement is “clinker,” limestone that has been heated to over 2,600° F. The limestone off-gases carbon dioxide as part of this process, in addition to what is released from the fuel burned to reach those temperatures.

Some concrete mixes are designed to replace the clinker with other materials. IDOT allows up to 5% of clinker to be replaced with ground, un-fired limestone. Cement can also be completely replaced with fly-ash, a byproduct of coal burning, and a few chemical admixtures.

These changes to a centuries-old recipe do alter concrete’s structural specifications, so this more environmentally-conscious concrete can’t be used in all applications. And of course, it’s usually more expensive. “The most important leg of being green is economic stability,” said Lutey. “If you’re green and you can’t stay in business, the gray guys are going to take over.”

concrete_2The other main concern when considering the environmental impact of concrete is water use. “In most of the towns where there’s a concrete plant, generally it is in the top for water use,” said Lutey. Water goes into the concrete mix and eventually evaporates out as the concrete cures. But water is used on site, to clean the equipment of excess concrete, wash aggregate material and for other uses.

Because federal regulations levy hefty fines on any water leaving the plant site, this water is reused, sometimes 20 or 30 times a day. “It’s a valuable resource,” said Lutey. “We don’t waste any water. Our problem is using all the water we do have.” Rain falls on concrete plants just like everywhere else, but unlike most sites, it can’t leave. This water goes into Ozinga’s washwater system, where aggregate is cleaned before being added to concrete.

One seemingly obvious use for this excess water is in the concrete itself. Ozinga is pushing for the use of some washwater in its mixes and has conducted many studies to show that, done properly it will have no impact on the final product. “Let’s reduce the potable water we put in concrete,” said Lutey.

The concrete plant’s washwater might even have a second life as a component of sewage treatment. Lutey and Mike Repkin, who both work for the non-profit Urban Habitat Chicago, have discussed the idea of using pre-treated washwater to settle biosolids out of wastewater at water treatment plants. “Leave it in contact for 24 hours, it’ll kill all the bad stuff,” said Lutey. “You can actually drink it.”

Water can also be a concern at the end-use site as well. The reason that the Chicago region’s water treatment systems are so overtaxed and basements flood during heavy storms is the abundance of impervious materials like concrete that shed the water straight into the sewers. One answer to this is pervious concrete.

Pervious concrete has been around a long time, but it isn’t more popular in part because it’s different. “It doesn’t tolerate fools,” Lutey said. “If you try and cut corners, it will fail.” The basic idea behind pervious concrete is that aggregate is held apart, creating cavities and crevices for water to pass through into the soil below. If the mix is too wet, cement drains down and seals the bottom; if it’s too dry, the concrete will fall apart.

And of course, getting the chemistry of the mix just right costs more than traditional concrete. “People think since pervious concrete is 20% voids, it should be 20% cheaper,” said Lutey. “Getting those voids to be the right size, the right shape and to be connected properly is expensive.”

Because it is more prone to failure when used improperly, Ozinga offers training on how to install pervious concrete and only sells to trained contractors. They also perform quality control on the back end as well; if a contractor has three failed jobs with pervious concrete, they are blacklisted from purchasing it.

We have been building with concrete for centuries and will it will likely be around for centuries to come. If, that is, we pursue the technology and muster nerve necessary to make it a truly sustainable material.

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