February 22, 2012

Concrete additives speed Saskatchewan tower construction

ALLEN WARREN

The $100 million Mosaic Tower in Regina, Saskatchewan is rising to take it's place in the capital city's skyline.

ALLEN WARREN

correspondent

Regina-based Cindercrete is using various concrete additives throughout Saskatchewan's varied seasons to make the concrete for the $100 million Mosaic Tower.

The firm is a fourth-generation-run concrete supply company and is the supplier of concrete on the Harvard Developments’ project in the provincial capital.

Bill McMillan, district manager with the company, said the key to working with large volumes of concrete in the winter is pretty straightforward: You start by heating your water and then, if need be, you also heat your sand.

In unusually hot weather, you can replace water with ice in the mix.

“Water holds the most BTUs (British Thermal Units), so you can sometimes just get by heating your water. You can actually heat sand four days in advance and have it stockpiled, it holds heat so well,” he said.

But ultimately, mixing a reliable batch of concrete that is going to hold up in all conditions comes down to quality control.

In an average square metre of concrete, which McMillan said weighs about 2,300 kilograms, 350 kg is cementitious materials, 150 kg is water, and the remaining 1,800 kg is split 58 per cent rock and 42 per cent sand. Often times, additives are added to the mix to speed or slow the setting and curing time.

Work started in January 2011 and progressed through a hot Regina summer.

It continued into a mild winter, which included sudden drops and rebounds in temperature.

However, McMillan said there were really no issues with the concrete.

“We are able, through quality control, to put out concrete in the middle of winter that is the same quality as the summer,” he said.

Cindercrete poured a slab about every seven to 10 days, and the company stuck with the same half-dozen, experienced drivers during those runs to ensure reliability and consistency, McMillan said.

On the Mosaic Tower job, Cindercrete also added a “super-plasticizer”, also known as high range water-reducers, to the mix on every slab.

“It’s a chemically-induced slump,” he explained.

The super-plasticizer works to disperse particles within the concrete mixture to improve the flow of the concrete, while reducing the ratio of water to cement needed.

Less water also means harder concrete.

Upper slabs were poured at a strength of 30 megapascals (MPa), while lower floors measured 40 MPa.

Columns, at their base, were as strong at 50 MPa.

When the cold weather hit, they added a calcium chloride-free accelerator as well.

It is widely used to accelerate both the setting and strengthening time of the concrete in cold weather, as chloride-based accelerators have been known to corrode rebar and thus weaken structures.

As soon as the concrete left the chute, however, that is where Cindercrete’s job ended, McMillan said.

The engineering firm of Clifton Associates Ltd. did the slump testing and Ledcor handled the actual concrete work after it was poured.

While concrete usually makes strength at 28 days, the Cindercrete product was making strength at around seven days.

Ken Roy at Ledcor, said the most unique thing about the job was the varying sizes of the column widths in the 18-storey building.

“They’re pretty big at the base of the building, and narrower as you go up,” he said.

Due to the shape of the building on the north and south sides, those columns are set back a little from where they would normally be.

The footprint of the Mosaic Tower, from glass to glass, is 33.1 metres north to south, and 37.6 metres east-west.

With an estimated 97 per cent of the concrete work now done, Roy said the building should be completed by June 2012.

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