Biomineralization: Key to Self-Healing Concrete
1/9/2012 10:04:00 AM
A bacteria-induced mineral deposit. Photo courtesy of Paramita Mondal
By Leanne Lucas
A crack in the sidewalk outside your front door is not that big a deal when you first notice it. But before long the crack has grown, and you know if you don’t deal with it soon, you could be looking at a major repair. Now fast-forward a few years. You notice a crack on Monday, but you’re not worried. By Friday it will have "healed" itself. Sound impossible?
Maybe not, according to a team of CEE researchers. Assistant Professor Paramita Mondal, graduate students Bin Zhang and Ashna Chopra, and professors Leslie Struble and Wen-Tso Liu are studying biomineralization, the process by which living organisms produce minerals as a possible method to promote concrete crack remediation. In other words, self-healing concrete.
“Concrete is basically weak in tension and strong in compression,” Mondal said. “People have tried to deal with the problem over the years in a variety of ways. The most common solution has been to use steel rebar to reinforce concrete. Reinforced concrete is stronger, but it still cracks,” she said. “And you might find this crack when it happens, one month later, or six months later. As this delay increases, the repair cost increases. Or the area might be so damaged you need to replace the structure partially.”
Another method suggests that some of the cement powder that was initially used to make the concrete still resides in the structure; contact with water (rain water or ground water) could cause the remaining powder to react and fill up the space. Of course, there is a limit to how big a crack could be filled, so researchers at Michigan have tried putting fibers in concrete to minimize crack width and control the damage. Yet another method puts glass spheres or tubes filled with glue in the concrete. If the concrete cracks, the glass cracks and releases the glue to fill up the space. This method has been effective, but limited. If there is another crack in the same location later, the glue is gone.
“The work we are doing puts bacteria in concrete, to mimic the way limestone forms in nature,” said Mondal. “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.”
The family Bacillus suits all the conditions, said Mondal. B. pasteurii is a non-pathogenic microorganism commonly found in soil and is not known to have any effect on human health.
The first step was to test the bacteria’s growth in the laboratory.
“We provided the bacteria, the food and the right environment,” said Mondal. “We could see that it was depositing the minerals, which are the basic building block of limestone. This has been achieved in a test tube.
“Then we made a cement specimen and applied the bacteria with food,” Mondal continued. “We saw the same kind of deposition. We did a chemical analysis of it, and it is the same calcium carbonate that’s forming.”
Eventually, the team hopes to prove that after introducing these microorganisms into concrete during mixing, they will form spores, or hibernate, in the highly alkaline condition inside the concrete. Once a crack occurs, the pH level at the cracked surface will drop due to the exposure of surface to air. The combination of the pH drop and a flow of oxygen and carbon dioxide at the crack face will activate the microorganisms and will provide the conditions favorable for growth. The microorganisms will deposit calcium carbonate, and as the crack fill up, the supply of oxygen and carbon dioxide will be interrupted, causing the microorganisms to hibernate again, ensuring the continual effectiveness of the microorganisms in filling up cracks at the same location.
Mondal said there are a few ongoing research projects in several parts of the world on the use of microorganisms in developing self-healing construction materials, and researchers are reporting that the biomineralization technique is promising based on its effect on compressive strength due to filling up pores inside concrete. However, Mondal said that an increase in compressive strength is not sufficient proof that the bond between the newly developed deposit and the cracked material is sufficient to regain part of the strength lost due to cracking.
“That is the specific goal of our project,” she said. “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,” Mondal concluded, “but there is lots more research to be done. It’s an innovative concept—definitely outside the box.”