Veins of bacteria could form a self-healing system for concrete infrastructure

Hoping to produce concrete structures that can repair their cracks, researchers at Drexel University’s College of Engineering are putting a new twist on an old trick aimed at improving concrete’s durability.

Fiber reinforcement has been around since the first masons mixed horsehair with their mud. However, the Drexel research team is taking this method to the next level by transforming the reinforcing fibers into a living tissue system that rushes healing bacteria from the concrete to the crack site to repair the damage.

Recently reported in the journal Construction and building materials, Drexel’s “BioFiber” is a polymer fiber encased in a bacteria-laden hydrogel and a protective, damage-resistant shell. The team reports that a grid of BioFibers embedded in a concrete structure can improve its durability, prevent crack growth and enable self-healing.

“This is an exciting development for ongoing efforts to improve building materials with inspiration from nature,” said Amir Farnam, Ph.D., associate professor in the College of Engineering and head of the research team.

“Every day we see damage to our aging concrete structures, reducing their functional lifespan and requiring critical and costly repairs. Do you imagine that they could heal themselves? In our skin, our tissues do this naturally thanks to a multi-layered fibrous structure impregnated with our self-healing fluid: blood. These biofibers mimic this concept and use stone-producing bacteria to create a self-healing living concrete that is responsive to damage.

Extending the lifespan of concrete is not only a benefit to the construction industry, it has become a priority for countries around the world working to reduce greenhouse gases. The process of making concrete ingredients—burning a mixture of minerals, such as limestone, clay, or shale at temperatures above 2,000 degrees Fahrenheit—accounts for 8 percent of global greenhouse gas emissions.

Concrete structures can deteriorate in as little as 50 years, depending on their environment. Between replacements and the growing demand for new buildings, concrete is the most consumed and demanded building material in the world.

Producing concrete that can last longer would be a big step forward in reducing its contribution to global warming, not to mention reducing the long-term cost of repairing infrastructure. That’s why the U.S. Department of Energy recently launched efforts to improve it.

Over the past decade, Drexel has led the way in researching how to improve the durability and durability of concrete, and Farnam’s lab is part of a team participating in a Department of Defense effort to fortify its aging structures .

“For several years, the concept of bio-self-healing cementitious composites has been developed within the Advanced Infrastructure Materials Lab,” said Mohammad Houshmand, a doctoral student in Farnam’s lab and lead author of the research.

“The BioFiber project represents a collaborative, multidisciplinary effort, integrating expertise from the fields of civil engineering, biology, chemistry and materials science. The primary goal is to pioneer the development of multifunctional, self-healing BioFiber technology, setting new standards at the intersection of these diverse disciplines.

The team’s approach to creating BioFibers was inspired by the self-healing capacity of skin tissue and the role of the vascular system in helping organisms heal their own wounds. It uses a biological technique that they developed to enable self-repair of concrete infrastructure using biomineralizing bacteria.

In collaboration with research teams led by Caroline Schauer, Ph.D., Margaret C. Burns Chair in Engineering, Christopher Sales, Ph.D., associate professor, and Ahmad Najafi, Ph.D., assistant professor, all from the College of Engineering, the group identified a strain of bacteria Lysinibacillus sphaericus as a bio-healing agent for the fiber.

The durable bacteria, typically found in soil, has the ability to trigger a biological process called microbe-induced precipitation of calcium carbonate to create a stone-like material that can stabilize and harden to form a area of ​​exposed cracks in concrete.

When induced to form an endospore, bacteria can survive the harsh conditions inside the concrete, remaining dormant until they spring into action.

“One of the amazing things about this research is how everyone approaches the problem from their different expertise, and the solutions for creating new biofibers are much stronger because of that,” Schauer said.

“Selecting the right combination of bacteria, hydrogel and polymer coating was central to this research and the functionality of BioFiber. Taking inspiration from nature is one thing, but translating that into an application made from organic ingredients that can all functionally coexist. The structure is quite an undertaking, which required a multi-faceted team of experts to succeed.

To assemble the BioFiber, the team started with a polymer fiber core capable of stabilizing and supporting concrete structures. He coated the fiber with a layer of endospore-laden hydrogel and covered the whole thing with a polymer shell susceptible to damage, like skin tissue. The whole thing is just over half a millimeter thick.

Placed in a grid throughout the concrete as it is poured, BioFiber acts as a reinforcing support agent. But its true talents are only revealed when a crack penetrates the concrete far enough to pierce the fiber’s outer polymer coating.

As water enters the crack, eventually reaching the BioFiber, the hydrogel expands and makes its way out of the shell toward the surface of the crack. During this time, the bacteria are activated from their endospore form in the presence of carbon and a nutrient source in the concrete. By reacting with the calcium in the concrete, the bacteria produce calcium carbonate, which acts as a cementitious material to fill the crack to the surface.

Healing time ultimately depends on the size of the crack and the activity of the bacteria – a mechanism the team is currently studying – but early indications suggest the bacteria could do its job in just as little time. only one to two days.

“While much work remains to be done to examine the kinetics of self-healing, our results suggest that this is a viable method for stopping the formation, stabilizing and repairing cracks without external intervention,” Farnam said. “This means that BioFiber could one day be used to create ‘living’ concrete infrastructure and extend its lifespan, avoiding the need for costly repairs or replacements.”