Mason Jones via UnsplashFrom the bridges of the Northeast to the stucco buildings in the Southwest, concrete is a dominant component of U.S. buildings and infrastructure.
Structures stand for decades, but the porous construction material doesn’t just sit there – the cement-based products “breathe” in carbon dioxide, uptaking gas from the atmosphere and storing carbon within the nooks and crannies of the concrete.
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The rate and time span of carbon uptake in CBPs can vary dramatically. Everything from the binder composition of cements to the geometry of exposed surfaces can affect carbon uptake. Previously, estimates of carbon uptake for CBPs were based on typical volumes of cement used in infrastructure projects – details about the cement and specific site conditions were not considered.
In a new paper in Proceedings of the National Academy of Sciences, researchers used a novel approach to better estimate carbon uptake in CBPs. They collected high-resolution data on infrastructure projects, including the geometry of structures, cement composition, and site exposure conditions.
Their approach showed that CBPs can sequester a significant amount of carbon and can continue to store it away, even during end-of-life demolition. They note their approach can shift understanding of how construction materials interact with the carbon cycle.
Top-down vs. bottom-up estimates
In the past, researchers estimated potential carbon uptake by cementitious materials with a top-down approach. They used typical cement volumes used in infrastructure projects and assumed standard cement formulas and exposure conditions.
“There are various research papers and quantification of the carbon uptake in cement-based products at different scales – global scales, country-specific, even structure-specific,” said Hessam AzariJafari, Ph.D., an industrial ecology researcher at Massachusetts Institute of Technology and the director of the MIT Electron-Conducting Carbon-Cement-Based Materials Hub, nicknamed the ec3 hub.
He and his colleagues recognized that using the current method of estimating carbon uptake and sequestration did not capture the full picture.
They decided to take a bottom-up approach and dig into the details of CBPs. They focused on integrating the properties of CBPs, like the geometry and cement compositions, and integrated the climatic conditions into their carbon estimation calculations. Each of these factors can change how much carbon can be sequestered within infrastructure.
“For example, in previous studies, they considered a typical thickness and exposure conditions of the walls,” AzariJafari noted. “But we see that the thickness of the walls varies when we think about different types of buildings.”
Additionally, buildings can have multiple geometries and cement compositions. AzariJafari explained that differences between concrete masonry and cast-in-place concrete can produce orders of magnitude of differences in carbon uptake.
The climate a building or bridge sits in also makes a big difference. Carbon uptake is a slow, diffusion-driven process, and local climate can play a big part in sequestration. For instance, there are differences in carbon uptake in a cold climate like Minnesota compared with the hot, humid environment of Florida.
“It's very fascinating to see that kind of study because all these factors really affect the rate of carbon sequestration,” said Ebenezer Fanijo, Ph.D., P.E., M.ASCE, an assistant professor at Georgia Institute of Technology’s college of building construction. He was not involved in the study; however, he said, “Previous studies only looked at one design, one geometry, one thickness – but they looked at different parts of it.”
Calculating carbon sequestering
To calculate these detailed, bottom-up carbon uptake estimates, the team needed a robust record of the built environment.
They tapped into infrastructure records collected by the U.S. Federal Emergency Management Agency and the U.S. Census Bureau’s American Housing Survey and Building Permits Survey to gather detailed, historical information on the geometry, type of cementitious material, and age of structures.
Bert Kaufmann via Flickr“For example, in single-family buildings, almost 70% of the concrete is used for slab on the floor and footings,” AzariJafari said. “Also, because we have access to the structural codes and the construction practices, we know the thickness of all these elements as well.”
Using this information, the team used a model to calculate the amount of carbonation that had occurred in both the U.S. and in Mexico, which has different construction practices than infrastructure built in the U.S. They found that in the U.S., the carbon uptake in 2024 was around 6 million to 7 million metric tons of carbon dioxide. That translates to sequestering around 13.5% of cement processing emissions.
The team found a fourfold variation in carbon uptake per square area of concrete across the U.S. They noted these differences were a result of a mix of building designs, concrete mixes, and different climates. Finally, they discovered that the building sector soaked up nearly twice the carbon of large infrastructure systems like roadways.
The stark difference in carbon uptake between buildings and infrastructure was “fascinating,” said Fanijo. “All this information is powerful. It can help guide our mixed design for the building industry, for instance.” He added that research like this helps engineers think about mix compositions and balancing strength and sustainability.
Beyond in-use CBPs, the team also tracked carbon uptake during the demolition and recycling processes.
From 1940 to 2024, the team found that 25% of the total carbon uptake occurred at the end of life, when infrastructure was demolished and concrete was crushed.
AzariJafari said this discovery “gives us an opportunity to think about how to maximize the amount of carbon update when the concrete is crushed so we can use it as a solution to sequester a larger amount of CO2 from the atmosphere.”
These findings seem to be particular to the U.S. When the team looked to carbon uptake in Mexico, it found that although Mexico uses about half the amount of concrete in its infrastructure, its sequestration levels are only 70%-75% of the U.S. capture.
AzariJafari pointed out that differences in concrete strengths and construction types between the countries account for the differences in uptake.
He said a bottom-up approach can produce more accurate carbon accounting measurements – an important step in climate policy. AzariJafari noted that this approach can also be useful for engineers and designers concerned with environmental product declarations of materials in planned projects.
“The example I usually bring up is a flat floor versus a waffle-shaped one,” he explained. “The waffle-shaped one has more exposed surface area (which leads) to a larger carbon uptake.”
Fanijo also noted that the research sparks a rethinking of end-of-life concrete recycling. Currently, only about 60% of crushed, stockpile concrete is exposed to air and able to be carbonized, but he sees an opportunity.
“We produce about half a billion tons of concrete waste in the U.S. (each year),” Fanijo said, adding that pushing closer to 100% carbonization in stockpiled CBPs would help get closer to the 2050 carbon-neutral goal.
