OTTAWA
A new high-performance concrete mix developed by researchers at the Institute for Research in Construction holds out the possibility of extending the life of bridge decks by as much as 20 years over decks fabricated from ordinary high-strength concretes.
Compared with bridge decks of normal concrete, the life-span may be extended by as much as 40 years.
The key, says lead researcher Daniel Cusson, is the use of a porous sand, which traps moisture within each grain, allowing the concrete to cure from within.
Cusson, senior research officer at the IRC who specializes in durable concretes said the mix he and his team have developed is scheduled to be used in part of a replacement span in the Seaway International Bridge near Cornwall, Ont.
The sand Cusson uses in the new mix is derived from shale sourced from a quarry in New York State. But it’s not natural shale, he said in a recent interview.
Although it is shale, it has been produced much the same way that cement is made. It’s fired in a kiln, and the resulting clinker is ground.
The end product is a lightweight, porous material. When saturated with water, it traps very small amounts of water in each grain.
As part of a concrete mix, those tiny amounts of water are released during the curing process — literally curing the concrete internally.
Cusson says conventional concrete is cured by covering the surface with wet burlap for about a week.
This keeps the concrete from losing the water it needs to cure. A good cure means good strength development.
High-strength concrete contains more cement than normal mixes, which lowers the normal water/concrete ratio. As a result, some of the cement deep within the concrete slab is not properly hydrated and the concrete develops cracks.
By using saturated shale sand, he said, water is provided uniformly in the cross-section of the slab, ensuring hydration of the cement within the slab.
“Think of each grain of the shale sand as a small reservoir of water,” he said. As long as the relative humidity is maintained at 100 per cent in the concrete, the water will remain in each grain.
But when the relative humidity starts to decrease, the water is pulled out of that porous sand and is available to react with the remaining un-hydrated cement.
The result is a concrete that is much less permeable than normal concretes, so it’s less susceptible to cracking. Fewer cracks means much less penetration of deicing chemicals, which adds to the service life of the slab. Better hydration also means increased strength, Cusson said.
“In most high-strength concrete there is a portion of the cement that never gets hydrated,” he said.
Using the shale sand, “you can keep our water level — the mix water — low, but you can still have water inside the sand that will compensate for that, so you get the high strength and very low, almost negligible, shrinkage.”
The shale sand is marketed as Hydrocure, Kenlite and Solite. Cusson and his team used Hydrocure, which is delivered, properly saturated, in sealed bags.
The idea of internal curing didn’t originate with the IRC, Cusson said.
The company that provides the material, Northeast Solite Corp., of Saugerties, N.Y., has been in business for about 60 years, and their products that utilize internal curing have been used in specialized applications, including a few bridges in the United States.
But, said Cusson, in Canada, “it is just starting to be known by the engineers.”
The project on which it will be used is the replacement of the North Channel Bridge, a $74-million project scheduled to begin in May.
The superstructure will be built this year and the bridge deck will be installed next year.
Cusson’s new mix is to be used for the sidewalks, although he said there is at least the possibility that it could be used for the entire deck.
“We are in discussion with the engineers to see if it’s possible to build the entire slab plus the sidewalk, not just the sidewalk,” he said. “That’s what we’re hoping for. The owners (the Federal Bridge Corporation) want the slab to be built with our concrete — that’s their wish — but they have to convince the engineers to use it.”
Cusson said the mix costs about four per cent more, “but if you consider the extended service life — 20 years more, perhaps — you may end up saving 30 to 40 per cent over the life of the deck.”
But he said it’s hard to know just how much longer the service life could be because it would depend on the climate in which it’s used, which would dictate the amount of deicing chemicals used and the amount of maintenance needed. And different jurisdictions might have different definitions of service life.
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