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For new Rogers Foods Ltd. Chilliwack flour mill
Insects, rain, earthquakes lead to slip forms
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Concrete finishers work the surface as soon as it's exposed beneath the slipform. Once the walls rise above standing height, work platforms (one is being readied in the background) will be suspended from the slipform on cables.
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Three concerns when contemplating the construction of a flour mill are how to avoid dark corners that might encourage insect infestations, how to be certain you can keep rain water out, and - at least in earthquake-prone BC - how to create seismic stability.
The answer for Rogers Foods Ltd. of Armstrong, BC, was a relatively uncommon form of reinforced concrete construction using slipforms to enable the building to be raised in one, continuous pour.
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David Dick, right, FWS senior vice-president, and Kelly O'Brien, Rogers Foods' mill operations manager, watch the proceedings on the first day of the pour in June. |
Rogers Foods wanted a 250-tonnes-per-day flour mill, complete wih eight storage silos for an expansion of their Chilliwack facility. They turned to ICBA member FWS Construction Ltd. - one of the leading design-build contractors of slipform grain mills on the Canadian Prairies and the upper Midwest of the United States.
"One of the great advantages of this form of construction is the avoidance of any joints or connections where water might enter the building - the whole building is, in effect, poured as one, complete, impervious unit," explains David Dick, senior vice-president of FWS.
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| Rounded corners are easily maintained during the pour and eliminate areas favoured by insects and rodents. |
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"And all the internal corners can be kept rounded, avoiding recesses that encourage insect infestations."
The main mill has a footprint of 5,760-square-feet and contains five floors in its 85 feet height.
The eight silos - each over 20 feet in diameter and 95 feet high - were poured as a separate unit, also using a continuous-pour slipform.
The slipform appears much like any other concrete form - four feet high and eight inches wide. But on closer inspection, the formwork stretches across the building's whole footprint and there is a series of steel-girder "workhorses" throughout. Actually, the "workhorses" are supports for the slipform's working surface or platform.
The whole unit is in fact one complete platform - the slipform - which supports the workforce of up to 90 workers and all their supplies, equipment and tools.
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| Guillotine-like boards are used to contain the concrete where there is an opening on one floor but not the next. A temporary piece of ply covers the gap in the slipform until the wall is once again being poured. |
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The slipform is suspended from an overhead crane, which in turn sits atop a series of 22-tonne, five-tonne and three-tonne hydraulic jacks with a combined lifting force of 500 tonnes.
The slipform, supporting crane, and hydraulic jacks all sit on a well-reinforced, 30-inch concrete slab poured in advance of all other work.
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A reinforced, precast beam is attached to a steel column prior to being encased in the rising concrete column being created within the slipform.
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A constant supply of readymixed concrete is pumped into a central hopper and then distributed around the site in wheeled carts.
Once started, the pour takes six days - 24 hours a day - of uninterrupted work before the building's structure is complete.
Redundancy is key to ensure smooth workflow: There's a backup power generator, two motors available to run the hydraulic pumps, and several alternate sources for the readymix concrete.
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| A 22-tonne jack - one of many with combined lifting power of 500 tonnes - moves the slipform up seven-eighths of an inch at a time at a rate of nine to 12 inches per hour. |
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The jacks raise the slipform by seven-eighths of an inch at a time, and at a rate of about nine- to 12-inches per hour, meaning the concrete is in the form for a minimum of four hours.
By the time the concrete is exposed at the bottom of the slipform, it is sufficiently set to support the concrete above (it only has to support its own weight as everything else is carried on the jacks).
As the concrete is exposed, a team of 10 finishers - working from platforms suspended on cables hanging below the slipform - apply a finishing coat inside and out for a smooth, impervious surface.
Walls, columns and floors - along with their corresponding reinforcing - are created on the move. Doorways are formed simply by placing a steel frame where the opening is to be and then pouring the concrete for the wall up to the sides of the frame. Once the slipform reaches the top of the frame, the wall just continues across the top of the steel frame. If there is to be an open area on one floor, but a wall on the floor above, the ends of the walls in the open area are fitted with guillotine-like boards that contain the concrete.
Large open-floor areas are more work, requiring precast beams to be laid on top of steel columns embedded in slipform concrete columns. The beams sit on steel brackets welded onto the steel column before being encased in the slipform's concrete column.
Eight beams are added per floor, each 800mm deep by 300mm wide, seven to 8.5 metres long, and spaced four metres apart.
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| A slipform is being readied in preparation for pouring the series of eight, 95-foot-high silos, that are part of the new flour mill in Chilliwack. |
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Watching the work progressing is Rogers Foods' mill operations manager, Kelly O'Brien.
He comments the slipform process keeps everything sanitary, but checks to make sure no ledges are being created and that all the corners are properly rounded "so nothing can accumulate," he says.
FWS started on site in October 2003 and is on target for finishing this November. Progress can be followed on FWS's on-site webcam at www.fwsgroup.com.
Other ICBA member companies involved include Bert's Electric (2001) Ltd., Gowest Drywall Inc, and Pioneer Precast Products Ltd.
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