The invention of concrete is one of mankind’s greatest construction triumphs, dating back, at least to ancient Egypt. Even as durable and versatile as concrete is, it has one fatal flaw, its tendency to crack.
The most common reasons for cracking are shrinkage during the curing process, settling, load stress, and environmental wear and tear. Over time, moisture and oxygen seep into cracks, causing steel reinforcement rods to rust. Steel expands as it oxidizes, creating pressure within the structure. The result is spalling, a condition characterized by the peeling and flaking of the surface. Large pieces of concrete can separate and fall, leading to costly repairs and even catastrophic structural failure.
According to a 2017 report by the American Society of Civil Engineers, the average age of our country’s dams is 56 years and nearly 4 out of 10 of our bridges are at least 50 years old. The good news is, scientists are testing interesting biological solutions that can prevent cracks and can even repair existing ones.
Healing cracks with mushrooms
Yes, mushrooms. Researchers at Binghamton University in New York are currently experimenting with the spores of Trichoderma reesei, a tropical fungus first noted in WWII for its ability to quickly decompose materials containing cellulose.
Congrui Jin, an assistant professor of mechanical engineering at Binghamton, has been working on this concrete technology for the past five years. Jin studies biomaterial solutions for infrastructure problems, inspired by the way broken bones heal.
Able to tolerate the harsh environment and intense pressure of concrete, the spores are added to the mix where they lie dormant until cracks occur. When moisture seeping through reaches them, they germinate. The resulting growth of fungal fibers called mycelium secrete minerals that fill the cracks and prevent further moisture from penetrating the structure. During this growth cycle, the fungus creates more spores that will repeat the process when necessary.
“Concrete that doesn’t self-heal requires enormous labor and investment in time and money,” Jin told PressConnects in March. “It can also be very hard to find damage in difficult places like tunnels underwater, and in those cases, detection and repairs are very costly. So while it would be more expensive, self-healing concrete will save a lot of money.
Work is also underway to create a mixture of spores that can be sprayed into larger, existing cracks. It will likely take three more years before concrete products containing the fungi are ready for sale.
Bacteria that secrete limestone
Basilisk, a concrete company located in Delft, Netherlands, is already marketing a line of products that heal microcracks of one millimeter or smaller in width. The bacteria and supporting nutrients can either be added to the mix or sprayed onto existing structures. When in contact with moisture, the mineral secretions of the bacteria seal the cracks. Microbiologist, Hendrik Jonkers, developed the system at the Delft University of Technology. Watch the fascinating video here.
Additional research on bacterial additives is also underway in the UK and New Zealand and will soon move into wider scale testing. The process combines nanotechnology with solid-state fermentation, to grow microorganisms on a solid rather than liquid medium. The technology could potentially be used in a variety of materials and applications and will likely become available worldwide in the near future.
Lighter, stronger concrete
Meanwhile, at the University of Michigan, researchers are working with 3D printed concrete containing polymer fibers that exhibits a property known as strain hardening. If you’ve ever worked with copper tubing, you know that the more you bend it, the more difficult it is to bend. Defects, or dislocations in the material caused by applying stress to it make it stronger when deformed.
Polymer-reinforced concrete has been around for some time, but this new, Engineered Cementitious Composite is crack resistant, bendable and self-healing, making it ideal for construction in earthquake-prone areas. It also could eliminate the need for traditional steel reinforcement rods. The first of its kind, this strain hardening concrete is able to tolerate 300 to 500 times the stress and damage of normal concrete.
Look for an upcoming article in the Journal of Cement and Concrete Composites by head researcher David Soltan, who holds a Ph.D. in Macromolecular Science and Engineering for more information about this project.
As concrete technology continues to evolve, the possibilities of new shapes and techniques in construction are exciting to ponder, as is the ability to repair critical infrastructure in a cost-effective manner.