May 27, 2009

Cracks are OK in this concrete

Economic constraints and environmental concerns have led to a growing emphasis on life-cycle costing. That, in turn, is spurring scientific efforts to devise new materials that might seem costly at the outset, but which turn out to be cheaper over the long haul.

The idea is to spend more now so you can spend less later and come out ahead of the game. That’s why it sounds like good news when we hear that a flexible 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 few drizzly days would be enough to mend a damaged bridge made of the new substance.

The material is the result of 15 years of research by a team headed by Victor Li, who teaches materials science at the university.

Self-healing is possible, Li explained, because the material is designed to bend and crack in narrow hairlines rather than break and split in wide gaps.

Li likens it to getting a small cut on your hand. If that happens, he said that 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,” he said.

In Li’s lab, self-healed specimens recovered their original strength after researchers subjected them to a three-per-cent tensile strain.

That means they stretched the samples to three per cent beyond their initial size. It’s the equivalent of stretching a 10-metre piece an extra 30 centimeters.

Li said his team found to their happy surprise, that when the concrete is loaded again after it heals, it behaves just like new; the material can be damaged and still remain safe to load.

But to achieve that performance, the cracks must be small—less than 150 micrometers in width.

That’s not much wider than the diameter of a human hair, which is usually about 120 micrometers.

Li calls his material an engineered cement composite, or ECC. Much more flexible than traditional concrete, which can suffer catastrophic failure when strained in an earthquake, or, eventually, by routine use, ECC flexes without breaking.

Patent applications for the material are still not complete, so Li is circumspect in what he says about it.

But specially coated reinforcing fibres are the source of ECC’s ability to remain intact and safe to use at tensile strains of up to five per cent.

From what little he said about its composition, it may also contain a somewhat higher proportion of cementitious material than most concretes.

A carefully worded news release from the university describing the healing process.

“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,” it said.

Calcium carbonate is a hard, strong compound found in seashells. The release said that in the lab “the material requires between one and five cycles of wetting and drying to heal.” So a few showery days would do the trick. So would a crew hosing down the damaged area a few times.

Li foresees the use of his ECC in places like bridge decks where corrosion resistance is important or in any structure that might be susceptible to earthquake damage.

But the initial cost would be high. ECC costs about three times as much as conventional concretes. The saving in its use would come from longer life with fewer repairs.

So far, the material has performed well in a bridge built in 2006 in Michigan, where it eliminated the need for traditional expansion joints and resulted in reduced road noise.

Li will be keynote speaker at a conference on self-healing materials in Chicago next month.

Korky Koroluk is a regular freelance contributor to the Journal of Commerce. Send comments to editor@journalofcommerce.com

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