The Department of Transportation (DOT) is now using concrete injected with carbon dioxide (CO2) for the construction of ADA curb ramps. This technology reduces the carbon footprint of our construction activities.

What is CO2-injected concrete?

CO2-injected concrete traps the most common greenhouse gas inside Portland cement concrete used for typical projects, such as sidewalk curb ramps. The CO2 is first captured from large emitters. Then, it is delivered and refilled by industrial gas companies in liquid form. CarbonCure manufactures a technology that recycles this CO2 into fresh concrete. Once injected into concrete, the CO2 undergoes a chemical reaction known as mineralization. Mineralization forms calcium carbonate (CaCO3), which is permanently embedded in the concrete.

How does the CO2 affect the properties of concrete?

The addition of CO2 to concrete does not seem to significantly affect the properties of concrete, either in its fresh or hardened state. We have tested it on five curb ramps so far, which resulted in no issues with workability. The CO2 has also been shown to have a positive effect on the concrete’s strength. The CO2 potentially increases strength by up to 10 percent.

Why use it?

Concrete is a mixture made up mostly of cement, aggregate, and water. It is the most widely used construction material in the world. Cement is the largest carbon-emitting component of concrete. It is a very energy-intensive product, responsible for about eight percent of the world’s CO2 emissions. If the cement industry were a country, it would be the third largest emitter in the world, behind China and the U.S. Once post-industrial CO2 is permanently stored in concrete, it is not released into the atmosphere.

DOT has begun piloting a project to construct new ADA-compliant curb ramps with Spencon Construction and Central Concrete. We will use a sustainable mix design with CO2, lower cement content and supplementary materials in lieu of cement, but produce less CO2. All three are meant to reduce the carbon footprint of concrete.

What are the environmental benefits?

Concrete producers are now quantifying how “green” their concrete mix is. Producers use the Global Warming Potential (GWP) metric, which measures the heat absorbed by a greenhouse gas. The reported GWP covers the production stage of the concrete. A typical concrete mix with no cement replacement or CO2 has a high GWP score. With the addition of CO2, lower cement content, and cement substitution, the GWP can be significantly reduced by about 40%.

To put this into perspective, DOT constructs approximately 2,000 curb ramps annually, which could save about 1,000 metric tons of CO2eq. An average vehicle emits 4.6 metric tons CO2 per year. Using a more sustainable concrete mix would be equivalent to removing more than 200 cars from the road for one year!

We are excited to begin this pilot project with CO2-injected concrete. And we look forward to continue to work toward our goals for a more sustainable San José.