Arctic concrete pour defies subzero work conditions

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No contractor likes to pour concrete in the middle of winter. For Nahanni Construction of Yellowknife, N.W.T., pouring concrete when it's -50 degrees Celsius outside is par for the course.
Arctic concrete pour defies subzero work conditions

No contractor likes to pour concrete in the middle of winter. For Nahanni Construction of Yellowknife, N.W.T., pouring concrete when it's -50 degrees Celsius outside is par for the course.

“It’s winter here for three-quarters of the year,” says Kenny Ruptash, estimator and project manager with Nahanni.

“A high of -30 C might be a good day. On a bad one, the wind chill is –70 C.”

Established in 1983, the company employs upwards of 250 people who specialize in sub-Arctic construction, particularly industrial projects involving concrete and steel work.

In the Northwest Territories and Nunavut, that usually means construction contracts on large resource projects.

For Nahanni, logistics is as important as knowing how to work in sub-Arctic conditions. At the Agnico-Eagle Meadowbank Gold Mine project, outside Baker Lake, Nunavut, the company was contracted to perform significant mine foundation and structural concrete work.

Over the course of 25 months the contractor poured more than 35,000 cubic metres of concrete in some of Canada’s most challenging weather and terrain.

“We couldn’t use overland shipping, so everything was initially shipped by barge, at sea, across Hudson Bay,” says Ruptash.

“That includes containers of cement, rebar and a portable batch plant. We had six weeks to mobilize and missing that barge delivery before winter freeze-up is not an option. If you need to bring in even a box of nails by air, you can multiply the price of that item by upwards of 10 times, compared to delivery to the port.”

Due to the rapid deployment and without final drawings, straight bundles of rebar, 18 metres in length, were shipped to the site. The contractor would later build a fabrication shop on-site so the rebar could be bent to design specifications.

From the port in Baker Lake, supplies and batch plant equipment were shipped along a 110-kilometre all-weather road. Only rolling equipment was brought in overland from Baker Lake.

Aggregate was crushed on site from available rock. In extreme cold, both aggregate and water must be warmed to optimal temperatures to ensure a quality concrete product.

“You might be able to deal with ice crystals in the water, but you need to account for that in your batching process,” says Ruptash.

“You have to build structures made of wooden hoarding and tarps around everything that you pour, then heat up the environment to allow warm weather curing.

Depending on the size of the pour, we use anywhere from 10 to 20 diesel-fired Frost Fighter heaters that put out between 350,000 and 500,000 BTUs each and they collectively require as much as 5,000 litres of diesel per day.”

The Meadowbank site offered no soil, so all concrete was set on solid bedrock.

“As our engineers say, for the concrete to adhere properly, you need to clean the bedrock so you can eat off it,” says Ruptash.

The cement was transported from the batch plant in mixers employing insulated drums. The supply lines of concrete pumpers were likewise insulated.

“In some cases, we’ve done long hauls where the cement is transported as far as 45 kilometres from the batch plant,” says Ruptash.

“You have 90 minutes to get that concrete to the site or it’s in the engineer’s hands whether that batch is subject to rejection.”

The greatest project challenge occurred while workers engineered a 300-cubic-metre pour on a rock crusher foundation at an elevation of about 20 metres.

“The wind at the site was relentless and we were working in near-24-hour darkness with temperatures hovering at -35 C,” recalls Ruptash.

About 60 workers were scheduled four weeks on, two weeks off during the entire project. In addition to gold mines, Nahanni has performed work on every major northern diamond mine project in the Northwest Territories, from Ekati, to Diavik and Snap Lake.

The contractor also poured concrete foundations for four wind turbines at the Diavik wind farm, then assisted in the erection of the towers.

“There aren’t a lot of contractors who survive up here for the long haul,” noted Ruptash.

“We sometimes see contractors who come in from the south, bid like it’s a southern project, lose a lot of money and never come back again. We’ve been at this for over 30 years—we’re in our comfort zone.”

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by Peter Kenter

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