Self-healing building products

The development of new materials and intensive research into existing ones is leading to a range of exciting applications that could have a significant impact on construction products. The introduction of self-healing materials could mean safer and more durable products, and considerable cost savings.

Concrete

A team led by Victor Li, of the Advanced Civil Engineering Materials Research Laboratory (ACE-MRL) at the University of Michigan has developed a form of concrete that can heal itself when it cracks, needing just the addition of water and air.

"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," says Victor Li, the E. Benjamin Wylie Collegiate Professor of Civil and Environmental Engineering and a professor of Materials Science and Engineering.

In testing, 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.

To work, the cracks must be smaller than 150 micrometres, and preferably below 50, to heal fully. Using a bendable engineered cement composite (ECC) developed by Prof. Li and his team, the average crack width can be kept below 60 micrometres. Dry cement in the concrete exposed on the crack surfaces reacts with water and carbon dioxide in air to heal, and form a thin white scar of calcium carbonate. Test show that the material requires between one and five cycles of wetting and drying to heal fully.

Currently, concrete structures are reinforced 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. This self-healing concrete needs no steel reinforcement to keep crack width tight, so eliminating corrosion. According to Prof. Li, 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.

Metal coating

In separate research, teams at the Fraunhofer Institute for Manufacturing Engineering and Automation and the University of Duisburg-Essen in Germany have developed a metal coating that can repair itself after sustaining damage.

Around 15 micrometres thick and containing polymer capsules a few hundred nanometres in diameter, the new coating could eliminate metal corrosion. The coating is applied using electroplating; when this plating is scratched, the tiny capsules burst, releasing their contents, which could be a polymer capable of sealing the crack, or corrosion-inhibiting liquids.

So far, nanocapsule-infused coatings have been made from metals or alloys including nickel, copper, and zinc. Theoretically, it should be possible to make them from any metal that can be electroplated, and so introduce them into a widely used industrial process.

Related NBS information:

Articles:

• Cements – standards, third party certification, and chromium (VI)
• Lime mortar: an introduction

Selected links:

• Concrete, cement, binders
• Metals

Written October 2009

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