Launch Slideshow

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Good as Gold

Good as Gold

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    The Atrium in Victoria, British Columbia, Canada, is the first building in North America to use precast cladding made with ultra-high-performance concrete. The first floor houses cafes, and a full-height glass atrium opens the interior offices to daylight. The building's material and color palette complements its surroundings.

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    Precast panels are cast at the Lafarge Canada plant with ultra-high-performance concrete poured into ribbed forms. Mix designs consisted of cement, silica fume, ground quartz, sand, PVA fibers, and superplasticizer. The microscopic particles precisely replicated the form surface texture and profile. Vibration was not necessary to aid placement.

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    An assembled panel shows the cast-in surface pattern of vertical lines. The ultra-thin precast panels are set on a unitized curtain-wall assembly, which mated various components of the system together. Low mass of the lightweight panels made it possible to reduce the seismic forces on the support structure.

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    The thin profile of the high-strength UHPC precast panels contributed to an exterior building envelope thinner than would be possible with other cladding materials. This allowed space for a conditioned air supply chamber around the building's perimeter used as part of a high-efficiency, displacement air ventilation system.

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    With only minimal fiber reinforcement required for the highly dense precast panels, it was possible to create thin, complex, curved pieces to fit the design. Individual curved forms were used to manufacture the various radii. This technique created smooth curves and curve transitions.

Keeping in mind UHPC's performance under flexure and direct tension loads, the façade design used rectangular panels that are straight, curved, and a combination of both. All panels are 51 inches high and vary in width from 30 to 85 inches, with most at 51 inches.

The panels are thickened around the perimeter on the back side to encompass the support inserts. Panels up to 57 inches wide are supported at four locations. Wider ones, with a mid-height back rib, are supported at six locations.

“PVA (poly-vinyl alcohol) fibers were used in the UHPC instead of steel fibers to avoid potential rust stains,” says Don Zakariasen, director of marketing, concrete products, Lafarge Western Canada. “However, since PVA fibers can exhibit strain softening, which affects the material's post-crack plastic behavior, there was insufficient data to validate design assumptions with the proposed mix design.

“Therefore, it was necessary to conduct testing at the University of Calgary. Prototype panels were fabricated and tested to replicate wind pressure and suction loading. Pull-out testing was also undertaken, because the thinness of the panels precluded the use of standard hardware,” Zakariasen says.

Simple formwork

The formwork for the UHPC panels was not complicated. With careful timing of casting and removal, the desired texture (face ribs 1/3 to ½ inches apart and 3/16 inches deep) was achieved on straight and curved panels.`

All of the panels have a hand-carved texture. The unique finish was created by collaborating with the architect to produce a hand-carved clay panel. This was then used to fabricate a negative plug as a form face for the cladding panels. The surface texture replication was possible due to the moldable properties of the UHPC, its raw material particle size, and the casting method.

The typical ductal mix design for the UHPC panels consisted of cement (1195 pounds per cubic yard), silica fume (385 pounds), ground quartz (350 pounds), sand (1720 pounds), PVA fibers (3 percent by volume), superplasticizer (22 pounds), and water (236 pounds).

The mix was a micro-concrete, with the largest aggregate size being about 500 microns. Compressive strength of the panels was 17,500 psi. Modulus of elasticity was 4.35 x 106 PSI. The water-to-binder ratio was less than 0.2. The percent of silica fume by weight of cement was 0.32; cement also served as a fine aggregate. Neither post-set heat nor pressure was applied to increase the panel compressive strength.

Green attributes

A white UHPC formulation was used to produce the precast panels, providing enhanced energy efficiencies with a solar reflectance index (SRI) of 0.7 and an emittance value of 0.9. The panels reflect the sun's heat energy, helping to reduce the cooling load for the entire building.

Other sustainability features of the design include stormwater management, a green roof system, low-energy air displacement ventilation, high-performance glazing, dimmable LED and T5 lighting, daylighting techniques, low-flow plumbing fixtures, construction materials with a high recycled content, and atrium roof trusses made from reclaimed and beetle-killed lumber. The project includes R28 walls and R30 roof insulation.

The precast UHPC panels also provide superior durability. The UHPC matrix is extremely dense and significantly increases the panels' life expectancy. Consequently, the building's maintenance requirements are greatly diminished because the surface pores are much less susceptible to the absorption of environmental debris.

Roy Diez is executive vice president of JOA (James O. Ahtes Inc.), a virtual marketing/communications agency specializing in the architectural, building, and construction markets. E-mailroydiez@att.net.