Bendable Concrete?

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.
Photo: University of Michigan, School of Engineering

 

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.

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.

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.

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.