As the United States government contemplates more than a trillion dollars in overdue repairs to roads, bridges, dams and other structures, one material will be front and center in most of those work orders: concrete.
Since the ancient Egyptians and the ancient Romans, concrete has been synonymous with strength. But it leaves a heavy carbon footprint. The production of one of its major components, cement, creates a whopping 5 percent or so of all global carbon emissions.
The recipe for concrete hasn’t changed in centuries: water, aggregate and the cement that binds it together. And concrete’s vulnerabilities haven’t either. The main one: Over time, concrete tends to crack due to factors such as shrinkage, rapid drying, settlement and applied loads. Even small cracks in concrete can reduce its durability.
Labor costs can be expensive when making concrete repairs to infrastructure. Current fixes involve injecting cement, epoxy or chemical grouts, or jacketing deteriorating structures. And of course, replacing cracked concrete structures with new concrete takes a new toll on the environment.
Making existing concrete longer-lived would help on both fronts. The good news: Soon, cracked concrete may be able to heal itself.
Civil and environmental engineering professor Marwa Hassan at Louisiana State University is researching one approach to self-healing concrete.
“What we are trying to do is mimic the human body. We have in the body white blood cells that go to the problem and start to heal. We want our process to act like these white blood cells, to close the crack and heal the concrete,” she said.
Her team is working on pilot projects that use cheap, simple compounds, such as calcium nitrate and sodium silicate, as the “healing agent,” which is embedded in a microcapsule shell made of polyurethane/urea-formaldehyde. The microcapsules are distributed throughout the cement matrix. “When a crack starts to propagate, it breaks the shell, releases the healing agent and closes the crack,” said Hassan.
Other researchers are testing biological components, from polymers and resins to bacteria, as the crack healer.
Certain bacteria, such as Sporosarcina pasteurii, can be embedded in the concrete along with a starch as a food source. If a crack forms and air gets in, the bacteria begin to grow and reproduce, excreting a form of calcium carbonate that seals the crack.
Hassan claims that while the chemical and biological components are both showing promise, her team’s use of sodium silicate results in concrete with some superior properties.
The key with concrete cracks is to fix microscopic fissures before they become cracks large enough to let in water, salts, and ice that can reduce strength.
“You want to seal any cracks that are 0.1 millimeter or larger, especially if they are on critical structures,” said Karthik Obla, vice president of technical services for the National Ready Mixed Concrete Association. He said it’s especially important when dealing with the possibility of corrosion in the reinforcing rebar. The LSU researchers have succeeded in sealing cracks of 0.1 millimeters.
Self-healing concrete could lead to significant cost savings over the lifecycle of a structure. It could eliminate the need for recurring maintenance and rehabilitation. First uses are likely to be in dangerous and difficult locations, such as underwater tunnels, bridges and pipes that are hard to access.
Less intervention for repairs could help cash-strapped states and municipalities save precious infrastructure dollars for new projects. Hassan’s team is currently working on quantifying the cost benefits of self-healing concrete versus traditional repairs.
Meanwhile, the motto of people like Hassan: Concrete, heal thyself.
Marsha Walton is a science, technology and environment reporter and broadcast producer. She’s worked for the National Science Foundation, the American Association for the Advancement of Science, the PBS conservation show “This American Land” and CNN’s Science and Technology Unit.