Thin UHPC Cladding Exhibits Green Curves

The Atrium is setting a new stage for ultra high-performance precast solutions.

By Kelly A. Henry, M.Arch., and Peter J. Seibert, P.Eng.

The Atrium is a unique, seven-story, mixed-use building located in the vibrant downtown core of Victoria, British Columbia. Aptly named for the large, free-form atrium space at its core, the building’s exterior is clad with ultra high-performance concrete (UHPC), a material at the cutting edge of innovation for new architectural applications. Thanks to its combination of superior properties, UHPC makes it possible to design and produce thin, complex shapes, curvatures and customized textures; concepts that were previously difficult or impossible to achieve with traditional reinforced precast concrete elements (see the sidebar “A Decade of UHPC”).

Developed by Jawl Properties and designed by D’Ambrosio Architecture + Urbanism (both of Victoria, B.C.), the Atrium is a state-of-the-art, ‘Class A’ commercial/retail building boasting energy efficiency and environmental controls. A public space by day, the Atrium remains functional at night as it transforms into a bustling facility offering a range of dining and entertainment options complete with acoustical and theatrical lighting features.

Using passive energy saving and day-lighting strategies, the project is targeted to achieve a Leadership in Energy and Environmental Design (LEED) Gold rating through the Canada Green Building Council (CaGBC). Green roof technology, street-edge rain gardens and a high-efficiency envelope also contribute to the environmentally sustainable design.

Freedom of form
Compared with the highly curved walls of the atrium space, the street-side facade is very rectilinear. The contrast of these two spaces led Franc D’Ambrosio to seek a cladding system that could adapt to provide the freedom to form tight radial curves and straight lines. UHPC was chosen for the spandrel panel section of the unitized curtain wall system in order to marry the flat and curved forms. By using UHPC precast panels that can be produced in fully customized curvatures (rather than adapting a segmented flat-panel system), the architect could control the number of seams and openings in the facade, ensuring the building envelope would not hinder the energy-efficient performance. Also, white UHPC was used to produce the 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, reducing the cooling load for the entire building.

Another important factor in the decision to utilize UHPC was its thin profile, which assists in reducing the weight of the overall panel and reinforcing steel. Minimal fiber-reinforced polymer (FRP) support was required due to the material’s ultra-high strength and ductile properties. By eliminating the need for a concrete covering over rebar, it is possible to create precast panels with a skin thickness of only three-fourths of an inch!

According to the architect, the panels are extremely light and strong for their thickness. Aside from allowing for tight-radius curving walls, the thin slabs contribute to making the perimeter walls thinner, and provide extra internal wall space for the displacement-conditioned air supply system. The light weight of the UHPC panels allows for two design options that contribute greatly to the reduction of material and energy consumption for the building as a whole:
• The building’s structure is reduced, because it no longer carries a higher dead weight;

• The reduced mass makes it possible to keep the seismic design requirements to a minimum (Victoria is located in one of the most seismically active areas in Canada).

Additionally, UHPC is utilized because of the color, texture and varied surface finishes that the architects were able to control. A large, smooth and pristine white archway serves to usher visitors through the main doors, while the spandrel panels display a unique texture created by the architect, through hand-carving a positive clay panel mold. This mold was then used to create several negative rubber molds to cast approximately 690 panels. The surface texture replication is possible due to the material’s fluid characteristics and fine aggregate size in its matrix. Consequently, all the panels appear to have been hand-carved.

Structural analysis
The UHPC cladding was assembled into a glass curtain wall system in an offsite factory. Lafarge’s precast team in Calgary, Alberta, produced the panels from curved and flat molds using a displacement-casting process. According to Gamal Ghoneim, structural engineer and panel designer with DIALOG (integrated design firm), the facade design uses rectangular panels that are straight, curved and combinations of both. All panels are 4-ft-3-in. high and vary in width from 2 ft 5 in. to 7 ft, with 4 ft 3 in. widths being most typical. The curved texture of the panels is, in fact, deep grooves that essentially create an aesthetically appealing face rib. The textured ribs account for additional weight, but in order to simplify the finite-element mesh of the structural model, the weight of this ribbed texture was not considered in the analysis of panel stiffness.

The panels are thickened around the perimeter on the backside to allow for enough material to embed support inserts. This perimeter rib is also required to properly seal the joint between the precast and the zinc and glass panels that are part of the wall assembly. The smaller panels, up to 4-ft-9-in. wide, are supported at only four locations, while the wider panels with a rib running up the centerline have six insert locations. Additional embeds located 1 in. from the top and bottom edges of the panel are used to attach the zinc and glass panels that are part of the overall wall system. Panel thickness varies slightly, depending on the width and shape.

Standardized codes for UHPC cladding projects have not yet been finalized, so casting and testing of prototypes must be done to verify designs. The Civil Engineering Department at the University of Calgary performed these tests on the 4-ft.-3in.-square panels (the typical panel size used on the facade). Test data show that a stone or granite panel with the same dimensions would have been three to five times heavier, because a greater panel thickness would be required due to the lack of tensile resistance of such materials. The UHPC panels, in their vertical position within the curtain wall frame, support their self-weight in the wall’s plane as well as resisting wind loads acting out-of-plane or perpendicular to the wall. Panels also transmit seismic forces that act in any direction to the backing frame.

Because of the different thicknesses within the panel and configuration of the face texture and back ribs, in-plane forces develop from the arching action. To minimize such forces and still place the panels as close as possible, the 2007 Canadian Precast/Prestressed Concrete Institute (CPCI) Design Manual guidelines for designing panel connections were used in obtaining the boundary conditions. These conditions were used in analyzing the panels supported at four points to permit more accurate determination of force distribution and to minimize internal stresses. It also permits movement in the panel’s plane to accommodate story drift and volume changes due to shifting temperature effects. The expected movement was accommodated in the support angles that are part of the backing frame.

A key success factor of this project (as with any innovative solution) was the strong collaboration of all key participants – owner, architect, developer, panel designer and precaster. Good communication early in the design phase is vital to complete prototyping and obtain approvals that avoid delays or problems with the project schedule.

The New Era
UHPC’s combination of superior properties provides unparalleled creative potential along with the freedom to design thin, complex shapes, curvatures and highly customized textures. As a first of its kind in North America, this project, completed in the fall of 2010, is truly innovative and leads the way toward a new era in high-performance envelope solutions.

In addition to envelope/cladding applications for new buildings, UHPC can work very well for challenging restoration projects. In 2008, the century-old Hotel Rialto, a municipal heritage site in Victoria, was restored to its original beauty with UHPC columns on the building’s exterior. Because of the material’s design flexibility and superior moldability, it provided a strong, durable alternative to the difficult-to-find, costly terra cotta tile that was used in the original columns. To date, numerous unique UHPC cladding/facade projects in a range of different design styles (perforated panels, double-skin textured rain-screen systems) have been successfully completed in North America and Europe.

NPCA quality assurance
In North America, the National Precast Concrete Association is leading the way with development and further enhancement of quality assurance specifications for architectural UHPC precast elements by adding new sections to the NPCA Quality Control Manual for Precast and Prestressed Plants. The Architectural Subcommittee leads this effort, and the section additions are expected to appear in the new manual by 2012.

A Decade of UHPC
Going strong with ultra-thin, durable and sustainable solutions.

With desirable properties that include superior strength, ductility, durability and aesthetic design flexibility, ultra high-performance concrete (UHPC) facilitates the ability to design and produce innovative structures with longer spans that are lighter, more graceful and innovative while providing improved durability and impermeability against corrosion, abrasion and impact.

The superior durability and strength properties of a UHPC building envelope make it sustainable. UHPC’s extended service life will result in less environmental impact over time as compared with an envelope using conventional materials. Ion-transportation predictive modeling tells us that it would take a thousand years for UHPC to have the same level of chloride penetration as high-performance concrete would have in less than 100 years.

With UHPC, any mold’s texture, form and shape can be replicated with the highest precision. Reinforced with high-carbon metallic or polyvinyl alcohol (PVA) fibers (depending on the application), UHPC produces compressive strengths up to 29,000 psi and flexural strengths up to 6,000 psi. Due to this impressive combination of properties, UHPC solutions can be designed with smaller elements without the use of passive reinforcement (reinforcing steel) and, in most applications, without prestressed or post-tensioned reinforcement.

Over the years, UHPC has been subjected to a significant amount of rigorous testing and field trials. In the United States, Canada, France, Japan and Australia, several official documents now prescribe UHPC materials for building, infrastructure, manufacturing, artistic and industrial designs. Today it is used for a seemingly endless variety of applications around the world including bridge beams and decks, joint fill (in precast bridge deck systems), solid and perforated wall panels/facades, urban furniture, louvers, stairs, floors, pipes, piles, marine structures and more.

Kelly A. Henry, M. Arch., MBA, LEED AP, has worked as an architect in Building Information Modeling Technologies, and held an adjunct professor at Georgia Tech on the topic of BIM theory. Based in Calgary, Alberta, she is the architectural project manager for Ductal, Lafarge’s ultra high-performance concrete.
[email protected]

Peter J. Seibert, M.Sc., MBA, P.Eng., is an active member of the NPCA Building Products Committee and Architectural Subcommittee. Based in Calgary, Alberta, he is the technical director for Ductal at Lafarge and is responsible for the manufacture, supply chain, quality control and technical aspects of Ductal throughout the United States and Canada.
[email protected]