June 30, 2008

Ultra high-performance concrete formulation extends material’s limits

Construction Corner

Korky Koroluk

When I first wrote about ultra-high-performance concrete (UHPC) almost seven years ago, it was a new product that had grown out of a concept that was, itself, little more than a decade old.

It was developed with specialized applications in mind, among them anchor plates in coastal applications exposed to sea water that would be much too corrosive for conventional concretes or steel. Another was for long, slender spans that give bridges a lighter, airier appearance, or for other applications where esthetic appeal is important.

By all accounts, it’s a success, but why don’t we see more of it?

In a word, cost. The stuff is expensive, so an engineer or architect needs a solid life-cycle cost estimate to justify the material’s use.

Now, though, there is another reason to consider its use.

UHPC is based on cement and fine particles of sand, ground quartz, silica fume, and a fibre matrix.

The presence of the fibres mean there is no need to reinforce the concrete, thus lessening the chance of corrosion from de-icing salts. UHPC is also highly impermeable, again reducing corrosion.

The material has a compressive strength ranging from about 200 MPa and up — putting it into the same range as steel — and a flexural strength of 45 MPa.

That high compressive strength means it allows for smaller structural members, which in turn means less cement used, and thus fewer of the greenhouse-gas emissions associated with the manufacture of cement.

And that’s one of UHPC’s attributes that is attracting more and more attention as the construction industry pursues the objective of sustainability.

International giant Lafarge offers UHPC under its Ductal brand and has supported its sales efforts with the results of a study it commissioned comparing a bridge made of Ductal with a conventional structure of concrete and steel.

The study showed that the Ductal-only bridge required 50-per-cent less material (by volume), which translated into a 50-per-cent reduction in carbon dioxide emissions.

It took into account not only the embedded energy (from the cement manufacturing process), but also greatly reduced maintenance needed over a projected lifespan of 60 years. That drew a lot of interest from many organizations, including the Institute for Research in Construction (IRC) in Ottawa, and the Federal Highway Administration (FHWA) in the United States.

The IRC has proposed an innovative solution for the twin problems of aging and deteriorating highway bridges in North America and the need for replacement bridges to be both durable and sustainable.

Scientists there have suggested a hybrid superstructure system using UHPC for the girders and normal high-performance concrete, or HPC, for the deck.

Such a system, they said would enable bridges with longer service lives, longer spans, lighter weights, and lower maintenance costs over their life cycles. They said the design they suggest could result in using up to 65 per cent less concrete, depending upon the girder type.

Using less cement in the construction of a bridge lowers its carbon footprint and decreases the true cost of the bridge when costs are calculated over the expected life span of the structure.

So even though the initial cost would likely be considerably higher than it would be if using conventional concrete, a bridge built with UHPC could turn out to be the best buy.

The first UHPC bridge was built in France and opened in 2001, just weeks before a similar structure was opened in Italy. Since then, there have been several others, including one in Australia.

In the U.S., spurred by work done at the FHWA’s research centre in Virginia, a UHPC bridge was built in Iowa.

So far, though, apart from a footbridge in Sherbrooke, Que., there hasn’t been a bridge built with UHPC in Canada.

But as the need for durability and sustainability grows, it seems almost inevitable that such a bridge will be built in Canada — perhaps on a highway near you.

Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@journalofcommerce.com

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