Imagine a bridge that requires no maintenance, a road that never need costly repairs. No traffic jams or tailpipe emissions due to reconstruction activities. Such infrastructure is a step closer to reality if concrete has self-healing ability.
A concrete material invented in the Advanced Civil Engineering Materials Research Laboratory (ACE-MRL) at the University of Michigan can heal itself when it cracks. No human intervention is necessary—just water and air. A handful of drizzly days would be enough to mend a damaged bridge made of the new substance.
“It’s like if you get a small cut on your hand, your body can heal itself. 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. Even if you overload it, the cracks stay small,” said Victor Li, the E. Benjamin Wylie Collegiate Professor of Civil and Environmental Engineering and a professor of Materials Science and Engineering. Julie Tibbitt, a UROP student working with Professor Li, was the first to document self-healing in a sample of this self-healing concrete exposed to the natural elements.
A paper about the material is published online in Cement and Concrete Research. It will be printed in a forthcoming edition of the journal.
In Li’s lab, self-healed specimens recovered most if not all of their original strength after researchers subjected them to a 3 percent tensile strain, enough strain to severely deform metal or catastrophically fracture traditional concrete.
The engineers found that cracks must be kept below 150 micrometers, and preferably below 50, for full healing. To accomplish this, Li and his team improved the bendable engineered cement composite, or ECC, they’ve been developing for the past 15 years.
More flexible than traditional concrete, ECC acts more like metal than glass. Traditional concrete is considered a ceramic. Brittle and rigid, it can suffer catastrophic failure when strained in an earthquake, Li said. But flexible ECC bends without breaking. It is studded with specially-coated reinforcing fibers and other ingredients that hold it together. ECC remains intact and safe to use at tensile strains up to 5 percent. Traditional concrete fractures and can’t carry a load at .01 percent tensile strain. ECC is emerging in full-scale applications in transportation, building, water and energy infrastructure systems.
The average crack width in Li’s self-healing concrete is below 60 micrometers. That’s about half the width of a human hair. His recipe ensures that extra dry cement in the concrete exposed on the crack surfaces can react with water and carbon dioxide in air to heal and form a thin white scar of calcium carbonate. In the lab, the material requires between one and five cycles of wetting and drying to heal.
Self-healing concrete works because it can bend. When it's strained, many microcracks form instead of one large crack that causes it to fail. Here, a specimen is bending as a force of five percent tensile strain is being applied. Regular concrete would fail at .01 percent tensile strain.
The white lines on this slab of bendable concrete show where the material has healed itself with no human intervention. Only water and carbon dioxide is necessary.
The professor says this new substance could make infrastructure safer and more durable. By reversing the typical deterioration process, the concrete could reduce the cost and environmental impacts of making new structures. 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.
“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 five 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 infrastructure. As civil and environmental engineers, we are stewards of these mega-systems. Advanced materials technology is one means to keep them healthy.”
The paper is called “Autogenous healing of engineered cementitous composites under wet-dry cycles.” This research is funded by the National Science Foundation and a China National Scholarship. Li will give a keynote address on self-healing concrete at the International Conference on Self-Healing Materials in Chicago in June 2009. The University is pursuing patent protection for the intellectual property, and is seeking commercialization partners to help bring the technology to market.
This story is based on material from a UM press release: http://www.ns.umich.edu/htdocs/releases/story.php?id=7106.