A handful of drizzly days would be enough to mend a damaged bridge made of a new self-healing concrete developed by researches at the University of Michigan. Self-healing is possible because the material is designed to bend and crack in narrow hairlines rather than break and split in wide gaps like traditional concrete.
“It’s like if you get a small cut on your hand, your body can heal itself,” said Victor Li, a professor of materials science and engineering at the university. “But if you have a large wound, your body needs help. You might need stitches. We’ve created a material with such tiny crack widths that it takes care of the healing by itself.”
Li and his team have been working for the past 15 years on a bendable, engineered cement composite, or ECC. In his new recipe, extra-dry cement in the concrete on the crack surfaces can react with water and carbon dioxide to heal and form a thin white scar of calcium carbonate—a strong compound found naturally in seashells.
By reversing the typical deterioration process, the concrete could reduce the cost and environmental impacts of making new structures, Li said. And repairs would last longer. The American Society of Civil Engineers recently gave the country’s roads, bridges, water systems and other infrastructure a “D” grade for health. The federal stimulus package includes more than $100 billion for public works projects.
In the lab, self-healed specimens recovered most if not all of their original strength after researchers subjected them to a 3 percent strain—enough to severely deform metal or catastrophically fracture traditional concrete. Traditional concrete fractures and can’t carry a load at .01 percent strain.
“When we load it again after it heals, it behaves just like new, with practically the same stiffness and strength,” Li said.
Traditional concrete is considered a ceramic. Brittle and rigid, it can suffer catastrophic failure when strained in an earthquake or by routine overuse, Li said. More flexible than traditional concrete, ECC acts more like metal than glass. Studded with specially-coated reinforcing fibers that hold it together, ECC bends without breaking and remains intact and safe to use at tensile strains up to 5 percent.
Today, builders reinforce concrete structures with steel bars to keep cracks as small as possible. But they’re not small enough to heal, so water and de-icing salts can penetrate to the steel, causing corrosion that further weakens the structure. Li’s self-healing concrete needs no steel reinforcement to keep crack width tight, which eliminates corrosion.
“Our hope is that when we rebuild our roads and bridges, we do it right, so that this transportation infrastructure does not have to undergo the expensive repair and rebuilding process again in another 5 to 10 years,” Li said. “Also, rebuilding with self-healing, bendable concrete would allow a more-harmonious relationship between the built and natural environments by reducing the energy and carbon footprints of these infrastructures.”
The University of Michigan is pursuing a patent on the ECC formula and is seeking commercialization partners to help bring the technology to market.
A paper about the material is published online in Cement and Concrete Research. The research is funded by the National Science Foundation and a China National Scholarship.
—By Leslie Fink/NSF from material provided by the University of Michigan.