Concrete is tough, but whether its a sidewalk, driveway, roadway or a structural part of a building, the material is subjected to many temperature, chemical and mechanical stresses. Over time, these stresses induce microscopic cracks and fissures that can grow in length and depth. By the time they are visible, these cracks are a signal of impending material failure and possibly irreversible damage. But researchers are developing tantalizing solutions that can stop — nearly as soon as it starts — concrete crack formation and initiate a “healing” mechanism in the material.
Some researchers, such as Victor Li, have proposed self-healing approaches for concrete that rely, for example, on excess or encapsulated unhydrated cement materials in the concrete mix that springs into action when a crack is exposed to moisture and carbon dioxide in the atmosphere.
One concern about many of the self-healing approaches is that there may be limits to the number of self-healing cycles in a particular location within the concrete.
But, a group from the Civil and Environmental Engineering Department at the University of Illinois at Urbana-Champaign, has a novel idea about how to create a renewable form of self-healing concrete based on exploiting the biomineralization characteristics of some bacteria.
The CEE researchers, which include ACerS members Paramita Mondal, Leslie Struble and Bin Zhang, along with Ashna Chopra and Wen-Tso Liu, are investigating the possible use of the common Bacillus pasteurii (also named Sporosarcina pasteurii) to add a self-healing component to concrete.
In a news release from the school, Mondal explains, “The work we are doing puts bacteria in concrete, to mimic the way limestone forms in nature. In nature, bacteria that form calcium carbonate are known to influence the rock formation process of carbonate rocks and sediments such as limestone. The list of bacteria capable of forming calcium carbonate is extensive, but the challenge was finding one that would be active in concrete’s environment of high alkalinity and low oxygen.” She says the B. pasteurii met their requirements.
In the CEE laboratory, the group did some initial testing of the concept. “We provided the bacteria, the food and the right environment. We could see that it was depositing the minerals, which are the basic building block of limestone,” says Mondal.
“Then we made a cement specimen and applied the bacteria with food. We saw the same kind of deposition. We did a chemical analysis of it, and it is the same calcium carbonate that’s forming,” she continues.
At larger scale, the group’s renewable self-healing concept is that once the bacteria are spread into concrete during mixing, they will form spores and go dormant in the highly alkaline condition inside the concrete. Then, if a crack is initiated, the pH drop combined with moisture and gases from the atmosphere will stir the bacteria back to life.
As they awaken, it is hoped that the microorganisms deposit calcium carbonate and fill the crack. Finally, once the crack is sealed, the microorganisms again go dormant until another crack forms.
The chemistry of what should happen is fairly well understood, but the challenge for this team is to test and characterize the “healed” concrete and determine if it will still perform as needed. Filling pores and microcracks via the bacterial process does not necessarily mean that the strength of the concrete has been restored. “That is the specific goal of our project,” Mondal says. “We are testing the specimen to see whether the crack is going through the filling material, through the original material, or through the interface. That will tell us which part is the weakest. … Conceptually, all of this should work, but there is lots more research to be done. It’s an innovative concept—definitely outside the box.”