September 18, 2006

Innovation

Discovery sends engineers around a bend

New formulation provides users flexibility and product strength

PETER KENTER

correspondent

Fibre-reinforced concrete has been used since the ancient Egyptians added straw to clay bricks for resilience and strength. Today’s modern engineers have taken the concept much further, developing a product they call “bendable concrete.”

Dr. Victor Li, professor of Civil and Environmental Engineering at the University of Michigan, and his team of researchers have developed the product, called Engineering Cementitious Composite (ECC).

While traditional concrete has proved its strength under compression, it lacks inherent tensile strength. The addition of fibres made of polyvinyl alcohol (PVA) allows concrete to stretch and bend, while making ECC up to 40 per cent lighter than traditional mixes and up to 500 times more resistant to cracks.

“What makes it different is the deliberate choices in the amount and type of size of these components,” says Dr. Li.

“It isn’t just the use of the PVA fibres, because ECC technology is not tied to one particular fibre additive. The ingredients aren’t exotic, just normal cement, sand, water, fly ash and polymers. But each of the components must be mixed in a particular way to give the product its ductile characteristics, although the product can be mixed by trained contractors using conventional construction equipment.”

While ECC is three times more costly by volume than traditional concrete, typical applications use far less of the product.

The Mihara Bridge in Hokkaido, Japan used ECC to reduce the thickness of the road deck from 230 millimetres to 50 millimetres. ECC is being by the Michigan Department of Transportation (MDOT) in various real-life applications.

“The longest trial is a patch repair in Ann Arbor that has gone through three years of freezing and thawing cycles,” says Dr. Li. “The product has come out very well and remained in good condition under heavy truck traffic. The adjacent concrete that was applied within a day of the ECC is already showing cracks.”

He estimates that the repair will hold for at least 10 years.

This summer, MDOT has used the bendable concrete to repair a bridge in Ypsilanti.

“In this case, we’re using ECC in a large strip to replace a conventional expansion joint linking two adjacent slabs of regular concrete,” says Dr. Li.

“Because of its flexibility, this material will be asked to stretch and perform all of the functions of a traditional expansion joint, which usually requires extremely high levels of maintenance. The product looks and feels like regular concrete, so motorists driving over the bridge won’t even feel the expansion joint under their tires. It will be like a continuous ribbon of concrete.”

“The ingredients aren’t exotic, just normal cement, sand, water, fly ash and polymers. But each of the components must be mixed in aparticular way.”

Dr. Victor Li

University of Michigan

Because ECC can be mixed to various tolerances, the expansion joint test is quite experimental, says Steve Kahl, supervising engineer, experimental studies with MDOT.

“If we can replace the expansion joints, we can prevent road salt from getting through the bridge into the pier and the bridge supporting structure beneath. Our initial assessment showed the material was more flexible than regular concrete, but there was cracking. While the first round of tests doesn’t show spectacular results, that’s a huge amount of stress put on the material in that application. We did some tweaking of the formula to see if the ECC will stop those cracks from growing.”

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