Self-healing concrete for
safer, more durable infrastructure
Thursday, April 23,
2009
A concrete material developed at the University of Michigan can
heal itself when it cracks. No human intervention is
necessary---just water and carbon dioxide.
A handful of drizzly days would be enough to mend a damaged bridge
made of the new substance. Self-healing is possible because the
material is designed to bend and crack in narrow hairlines rather
than break and split in wide gaps, as traditional concrete
behaves.
"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 Engineering and a
professor of Materials Science and Engineering.
A paper about the material is published online in Cement and
Concrete Research. It will be printed in a forthcoming edition of
the journal. The professor says this new substance could make
infrastructure safer and more durable.
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. That means they stretched the specimens to
3 percent beyond their initial size. It's the equivalent of
stretching a 100-foot piece an extra three feet---enough strain to
severely deform metal or catastrophically fracture traditional
concrete.
"We found, to our happy surprise, that when we load it again after
it heals, it behaves just like new, with practically the same
stiffness and strength," Li said. "Self-healing of crack damage
recovers any stiffness lost when the material was damaged and
returns it to its pristine state. The material can be damaged and
still remain safe to load."
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 or by routine overuse, Li said. But flexible ECC bends
without breaking. It is studded with specially-coated reinforcing
fibers 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.
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 to heal and
form a thin white scar of calcium carbonate. Calcium carbonate is a
strong compound found naturally in seashells. In the lab, the
material requires between one and five cycles of wetting and drying
to heal.
To test the healed concrete, the researchers used resonant
frequency measurements to determine the stiffness and strength
before and after inducing the cracks. These tests send sound waves
through the material to detect changes in its structure.
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 deicing 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, so it eliminates corrosion.
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."
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University of Michigan
Thanks to University of Michigan
for this article.