The Atrium, a new mixed-use office and retail structure in Victoria, British Columbia, bills itself as the first building in North America with an exterior precast cladding made with ultra-high-performance concrete (UHPC). The building was also designed to achieve a Gold rating from the Canadian Green Building Council's LEED program early this year.
Developed on a one-acre downtown site, the seven-story structure includes two levels of subterranean parking. Upper floors are built to the street walls, but a set-back and transparent ground floor, housing cafes, and restaurants invite people in and integrate the structure into the urban fabric. An inviting, glass-walled atrium introduces daylight into the heart of the building and is visible from the street during both night and day.
The decision to use precast UHPC components began with the desire to use a material that would contrast the reflectivity of the heavily glazed fenestration, says architect Franc D'Ambrosio, of D'Ambrosio Architecture + Urbanism, in Victoria. The pre-cast panels provide a natural color and texture and were manufactured with a surface pattern of vertical lines of various width, depth, and spacing.
These “ribs” were hand-carved by the architect into a modeling clay master used to make the casting molds. Designed to curve at the corners, the façade also presented challenges for conventional construction systems. Precast UHPC spandrel panels provided a highly moldable cladding material that could easily form tight radial curves.
Ultra-thin, the textured spandrel panel system contributed significantly to the building's high-efficiency envelope. The panels are light enough to be hung from the unitized curtain-wall envelope system and thin enough to allow an air-displacement ventilation system that fits within the exterior wall thickness.
The precast concrete system also provided corollary benefits. The 204,000-square-foot building is located in an area with a high seismic rating. Using the lightweight precast panels reduced the dead weight and mass of the envelope, reducing the potential seismic forces on the support structure.
Lafarge Canada of Calgary, Alberta, produced 690 precast UHPC panels from curved and flat molds using a displacement casting process.
The panels are just ¾-inch thick with 1 1/5-inch ribs, making each one relatively light, immensely strong, and thin. Only minimal fiber-reinforced-polymer bars were required due to the material's ultra-high strength and ductile properties. These were used in the perimeter ribs as a safety precaution against accidental overloading. By eliminating the need for rebar in the panel portion, it was possible to create the thin, complex, curved pre-cast panels.