Bendable Concrete and Carbon Sequestration
Anyone who deals with infrastructure knows concrete is an excellent construction material. It ranks as one of the most-used resources on Earth, with an estimated 26 billion tons produced annually worldwide. It works well for constructing bridges, roads, dams, airports and buildings, and we know it lasts, as evidenced by the concrete structures built by ancient Romans that still stand.
Why, then, have researchers embarked on a mission to replace the everyday concrete we know and love? “The main shortcoming of the material is that concrete is brittle—meaning it cracks—and it brings deterioration and damages that require repeated maintenance,” explains Victor Li, a materials engineering professor in the Civil & Environmental Engineering Department at the University of Michigan. In addition, producing concrete has a huge carbon footprint and contributes heavily to greenhouse gas emissions.
Room for Improvement
Fortunately, steps are being taken to increase the flexibility and strength of concrete as well as reduce its environmental impact, and it turns out you can do both at the same time with the same measures.
Since the 1990s, Li and his fellow researchers have developed a type of flexible concrete known as an engineered cementitious composite (ECC). “We’re trying to create a new generation of concrete that if you put it under excessive loads, it bends but doesn’t fracture,” he explains.
The material has a compressive strength similar to that of regular concrete. But while normal concrete has a strain capacity of 0.01 percent, ECC has a tensile strength capacity of 3 to 5 percent, or about 300 to 500 times as much, making it far more ductile.
In addition, Li says, “when ECC gets damaged by excessive loading, the micro cracks are self-controlled, and the crack widths are limited to less than 50 microns. In structures like a bridge deck, we don’t want water or deicing salts to get through the cracks and attack the steel. This kind of deteriorating mechanism is greatly delayed or eliminated.” Corrosion tests show that rebar in ECC exhibits a lower corrosion rate, and it distributes more evenly along the bars.
The net result, according to Li: “Improved durability means less maintenance, and that means lower lifecycle costs, particularly for infrastructure like bridges and roadways, where a lot of maintenance is required.”
Meanwhile, Jessica Wilson, strategic account manager with CarbonCure, one of the companies trying to improve concrete, notes, “the industry has recognized that sustainability is the future of concrete. The major trend we’re seeing is the demand for more transparency around the embodied carbon or global-warming potential of materials. The ultimate goal is achieving carbon neutrality for the industry.”
Local and state governments as well as the federal government have taken the first steps. Rules aimed at reducing embodied carbon in concrete and projects to reduce the amount of cement in concrete have cropped up around the country, including in Marin County, Calif.; Hastings-on-Hudson, N.Y.; and Portland, Ore. In New York and New Jersey, lawmakers have proposed state-level policies that would provide price discounts in the bidding process to proposals with the lowest emissions from concrete.
What’s the Difference?
Conventional concrete is made by mixing sand, cement and aggregates such as gravel, and then activating it by adding water. To make cement, calcium carbonate (usually in the form of limestone) is heated to approximately 1,480 degrees Celsius to extract calcium oxide. Concrete typically has steel or fiberglass reinforcing bars (rebar) running through it for added tensile strength and to reduce cracking. This results in a material about 10 times as strong in compression as it is in tension or bending.
ECC resembles regular concrete but can weigh up to 40 percent less, as it consists mostly of the same ingredients except for the coarse aggregates. It has small polyvinyl alcohol (PVA) fibers embedded in it that are 8-12 millimeters long and about 40 microns in diameter, about half the thickness of a human hair. The fibers have a nanometer-thick surface coating that allows them to slip rather than break under heavy loads. In place of coarse aggregates, it relies on fine sand, as aggregates disturb placement of the fibers and destroy the ductility. In some applications, rebar can be eliminated.
ECC can be used in the same applications as regular concrete. According to Duo Zhang, a professor in the School of Water Resources and Hydropower Engineering at Wuhan University in China and a former colleague of Li’s at the University of Michigan, “the technology has been spread out across more than 10 countries. We have recognized dozens of applications worldwide, spanning a broad range of civil infrastructure, typically in building seismic design, bridge link slab, tunnel lining, irrigation channel, etc., including both new construction and repair applications.”
The original application of ECC came with seismic structures, particularly in Japan, which lies directly on seismic faults. “The material can deform during the earthquake and absorb the energy without fracturing,” reports Li. In Osaka, ECC was used in a 60-story residential tower, the tallest anti-seismic building in Japan.
Another area where ECC can result in savings is on bridge decks, as major problems occur when expansion joints between deck sections jam frequently. In a demonstration project Li’s charges performed in Ypsilanti, Mich., for the Michigan Department of Transportation, the bridge had regular expansion joints replaced by a slab of ECC material 17 feet wide crossing four lanes of traffic. Known as link-slab in this application, the ECC actually expands and contracts as the deck moves with temperature fluctuations. It eliminates many of the common problems associated with conventional expansion joints such as joint jamming and leakage, which results in water and deicing salts passing through the joints and corroding the steel supporting the structure.
ECC can be mixed and placed by the same equipment used for traditional concrete. Li’s group has worked with suppliers and Michigan DOT to develop procedures so regular ready-mix trucks can deploy the material. Placement of ECC is actually easier, because it’s self-consolidating and needs no vibration.
Financially, bendable concrete costs about three times as much upfront as the conventional variety, but because it has a longer life than regular concrete, ECCs are expected to cost less in the long run, especially when experience is gained in large-scale production.
“The most expensive ingredient of bendable concrete is fiber,” adds Zhang. “In traditional bendable concrete, PVA fiber is mostly used, and its cost has not changed significantly over the past years. But we have demonstrated the success of using low-cost polypropylene fiber and waste-derived polyethylene fiber for making bendable concrete at lab scale. These studies are ongoing.”
Concrete as Carbon Sink
On another front, researchers are focusing on ways to use carbon dioxide (CO2) as an ingredient in concrete, locking it away and preventing it from entering the atmosphere. This also improves the strength of concrete and allows thinner, less-brittle structures that require less steel reinforcement, further reducing carbon emissions.
CO2 can be added in the form of aggregates or injected during mixing. It also can be added after concrete has been cast in a process known as “carbonation” or “CO2 curing.” These processes turn CO2 from a gas to a mineral, creating solid carbonates.
“We can use concrete as a carbon sink because of its makeup,” explains Wilson. “At the chemical level, when that CO2 is injected into concrete as it’s being batched, the calcium oxide mixes with the water in it to form calcium carbonate.”
Injecting CO2 into concrete allows producers to reduce the amount of cement needed. This is important, because cement accounts for about 80 percent of concrete’s carbon footprint, and it accounted for 7 percent of total global CO2 emissions in 2018.
From the Private Sector
To meet the call, companies such as CarbonCure and CarbiCrete have emerged that inject CO2 into concrete to sequester it and improve the strength.
CarbonCure retrofits its technology into concrete plants and enables producers to inject CO2 into concrete during mixing. “We license our technology to concrete plants,” says Wilson. “A lot of engineers will come on our producer-partner plan, and they’ll work hand in hand to install the control system.”
The company does calculations with its customers to determine how much CO2 to add to batches of concrete. Typically, it’s about a pound or two per cubic yard of ready-mix concrete. On average, CarbonCure claims producers reduce cement content by 4 to 6 percent with no compromise on quality.
Batching is controlled by the CarbonCure Control Box, which is integrated with the plant’s batching software. In a ready-mix dry-batch application, the CO2 is injected into the hopper; while in a central mix or masonry application, the CO2 is injected into the central mixer.
The CarbonCure Valve Box is connected to the CO2 tank stored onsite and automatically injects a precise dosage of CO2 into the concrete during mixing. Telemetry gathered from each control box syncs with the CarbonCure Command Centre in real-time. “That allows us to track and measure the amount of CO2 being injected into batches,” adds Wilson. “We also measure the amount of cement that is reduced. It is very data driven and scientific in terms of reducing cement in their mix designs.”
Wilson notes that CarbonCure has two levels of customers: “We have ‘mom-and-pop’ producer plants all the way to large national companies. The end users consist of the architectural, structural engineering and general contracting communities as well as concrete contractors.”
Some projects that have used CarbonCure concrete include the General Motors Spring Hill Assembly Plant in Spring Hill, Tenn.; Amazon HQ2 in Arlington, Va.; LinkedIn Middlefield Campus in Mountain View, Calif.; and Kendeda Building for Innovative Sustainable Design in Atlanta.
One of CarbonCure’s customers is Clark Construction Group, which has used other techniques such as injecting slag and fly ash into concrete to replace the cement. “The carbon dioxide injection into concrete began for us in 2020 with the requirements of the Amazon HQ2 project,” reports Fernando Arias, the company’s director of sustainability. He says injecting CO2 in concrete reduces the logistical challenges of the other admixtures such as fly ash. “This is something we were excited about, especially when we aligned all the value-chain partners.”
“We have on average maybe 200 projects across the U.S., and about a third use a carbon-reduction strategy,” states Arias. “There’s definitely been a lot of opportunity to implement this material here on the East Coast. We’re starting to hear more about requirements on the West Coast. We’re seeing a lot more demand.”
CO2 was injected into concrete at the Amazon HQ2 project in Arlington, Va., using a system supplied by CarbonCure. (Clark Construction Group)
Arias says Clark approaches the application of reduced-carbon concrete in two ways: “The first one is performance specification and directions straight from the client about a specific project. In the case of the Amazon HQ2 in Arlington, it had climate pledge goals to accomplish. That’s one of the big reasons we sought to reduce carbon at every stage of the construction process, including materials,” he explains. “The second one is federal procurement guidelines. What we’re seeing now, especially with the GSA, are buy-clean policies both at the federal level and with jurisdictions such as California.”
For the Amazon HQ2 project, most of the concrete used “has the embodied carbon solution with CarbonCure, so it will be one of the most visible and widely studied executions of that material,” notes Arias. “Being in Arlington, it will be studied by environmental and construction groups in D.C. It will create a textbook area where it will establish baselines for the industry.”
Although carbonation curing works well with conventional concrete, bendable concrete is even more reactive to CO2, owing to its high content of mineral admixtures that promote CO2 diffusion, according to Zhang. “Additionally, due to the absence of coarse aggregate, the ingredients of bendable concrete are finer than those of regular concrete,” he adds. “This leads to an increased surface area for carbonation.”
Further Academic Research
CO2 can be injected in two ways: through the mineral carbonation of its ingredients or during the curing process. Work is being done through the Center for Low Carbon Built Environment at the University of Michigan; Li serves as director.
In recent years, Li and his team conducted research to improve their bendable concrete by adding smart functionalities. One objective is for the material to know when it has been damaged so it can heal itself when it cracks.
It all points to a bright future for bendable concrete and carbon sequestration in concrete. “With the help of public policies and lifecycle cost reduction, we believe bendable concrete will be more deployed in both general and specialized infrastructure applications,” notes Zhang.
Infrastructure everywhere will benefit … as will the environment.