By Sue McCraven
Photos courtesy Paul Cobo Photography, Miami
Cruising together down the newly upscale Lincoln Road during their first visit to Miami Beach, Fla., an engineer and architect slow down to gawk at the radical design of the New World Symphony Concert Hall. Designed by the renowned architect Frank Gehry, the $160-million, 101,000-sq-ft complex was completed in early 2011. With a cursory glance, the two professionals agree that the concert hall doesn’t appear to entail any untried construction designs despite its fantastical appearance.
One element of the edifice, however, causes them to pull to the curb to take a closer look. Floating in the space above the entrance is a gracefully curved white canopy that appears to be one continuous piece of precast concrete. But, perplexingly, it is only 3 in. thick. Such a thin cross section could not accommodate the reinforcement typically specified for a cantilevered 40-ft by16-ft architectural precast panel.
HSC: the only option
Investigating further, the architect and engineer contacted the precaster who fabricated the canopy. Alex McCulloch, president of Castlestone Inc. in Winter Gardens, Fla., explained to his visitors that he originally planned for a four-section entrance structure for the concert hall. When Castlestone bid on the entrance canopy, it was to be made out of concrete with glass fibers in four 2-in.-thick sections. But with a total length of 40 ft and a width of 16 ft, the cantilevered element would need steel reinforcing bars for structural strength. And, as engineers and producers know, steel bars with less than 1 in. of concrete cover in such a thin section would readily corrode in the saline air of Florida’s coast
Consequently, the only option remaining for Castlestone was to build a canopy in one piece with high strength concrete (HSC). The project architects, Frank Gehry Partners, were familiar with HSC, and were delighted with the aesthetics afforded by a thin, sinuous one-piece canopy. Next, the precaster’s challenge was to find a consulting engineer familiar with HSC: no easy task, as McCulloch discovered. The precast producer explained that four consulting engineers were interviewed before one was finally found who understood how to work with HSC.
In addition to designing a 40-ft-long by 16-ft-wide canopy in one continuous section that was only 3 in. thick, the project engineer needed a structure strong enough to withstand Florida’s hurricane-force winds of 120 to 130 mph. Castlestone accepted the challenge of working with the relatively new HSC material to build a remarkable precast concrete entrance canopy for this very high-profile Miami Beach project.
Engineering and Production Details
Further responding to the engineer and architect’s questions about the “floating” HSC canopy, McCulloch described some of the production precision required:
• fabrication based on drawings alone;
• a superb finish on both sides;
• precise inspection of cantilevered support tentacles that had different sizes and orientation;
• exacting laser technology for supports and the formwork’s sinuous geometry;
• assurance of the product’s strength and durability by providing a prototype for engineering inspection;
• 1-in.-thick formwork routed by machine to precisely follow the canopy’s flowing dimensions;
• steel reinforcing mesh shaped to fit precisely within the 3-in. cross section; and
• careful attention to mix temperature.
To demonstrate HSC’s structural viability, a 2-ft by 16-ft prototype was created to test the product’s strength and durability. After a very rigorous inspection, the concert hall’s project engineer was satisfied and impressed with HSC’s superior material characteristics.
McCulloch’s explanation of how and why Castlestone used HSC satisfied many of the questions for the visiting engineer and architect. These industry professionals left the precast facility with real excitement as they discussed the freedom of design that HSC could deliver in as-yet untried precast concrete applications.
Because HSC and similar materials like ultra high-performance concrete (UHPC) are based on relatively new concrete technologies, architects and engineers, like the two gentlemen cruising Lincoln Road, continue to have concerns about this new family of products. In particular, there are no current design standards specifically published for these materials. The structural engineer for the concert hall’s entrance canopy addressed some of these queries for Precast Solutions (see the interview on page 17). For more in-depth technical information, read “Concrete: The Class Family” on page 18.
Sue McCraven, NPCA technical consultant and Precast Solutions editor, is a civil and environmental engineer.
Felix Crommie, P.E., of Richardson Engineering, Orlando, Fla., the project structural engineer who worked on the HSC canopy design, discusses the HSC design.
What is the market potential for UHPC/HSC in your region of the country?
Crommie: In this project, a magnesium-based material, not a portland cement product, was used. I think this material has value in terms of its extraordinary properties, particularly its waterproofing, corrosion resistivity and crack resistance. HSC becomes waterproof at a thickness of only 3/8 in., so it can be used for the construction of extremely thin shells where traditional cover requirements can be waived.
What do you view as the primary advantage of HSC and why?
Crommie: Its primary advantage is its strength and its ability to be formed, troweled or sprayed. It has a natural fast cure rate (20 minutes to become stiff) that can be retarded using additives. The material becomes attractive whenever strength, waterproofing or thinness of section is desired.
What is the main disadvantage of HSC?
Crommie: There is currently no published design standard. Designers are using the ACI method and applying it to the product. There is research available, but it is sparse and not comprehensive. The material really needs an organization dedicated to the codification of the product.
Any specific challenges in the Castlestone project?
Crommie: We were working for an architectural team that was very focused on making sure the end product matched the virtual three-dimensional model contained in their design. Castlestone divided the model into sections to obtain a fine resolution and used radar surveys of the as-built skeleton. The computer-controlled cutting machines were able to create cross sections that matched the model perfectly.
The biggest production hurdle of all was the irregular support tentacles that approached the warped shell at odd angles. Since the skeletal frame was constructed prior to the construction of the thin shell, we used radar surveys of the as-built skeleton to identify the three-dimensional coordinates of the end points of the columns. The embed plates in the shell were placed precisely to accommodate the end points of the tentacles per the survey. Another challenge was the thinness of the shell and the architect’s desire for it to be thinned to near-zero thickness at the edge. The shell is reinforced with mesh-shaped No. 3 bars throughout, and we specified hardware cloth to reinforce the extremely thin edges.
Concrete: The Class Family
By Vic Perry, FCSCE, MASc, P.Eng.
Concrete, the world’s most widely used building material, is typically a locally manufactured product that provides good jobs in the community. The portland cement that binds the concrete uses raw materials such as limestone, one of earth’s most abundant materials. When the right concrete is selected, the result is a long-lasting piece of infrastructure that, over its life (measured in hundreds of years), has one of the lowest environmental impacts of any construction material. Using the correct class of concrete for a particular piece of infrastructure is very important to achieving a long-lasting, durable solution.
The terminologies used to describe concretes can be confusing, for example: strength vs. performance, or high-strength concrete (HSC) vs. high-performance concrete (HPC) or ultra high-performance concrete (UHPC).
Until the early 1980s, concrete was normally supplied and specified in terms of its compressive strength at 28 days. This family of concrete is referred as normal strength concrete (NSC) that typically has 28-day compressive strengths up to 6,000 psi. With improvements in concrete technology in the late ‘70s to early ‘80s, strengths started to increase. Technological improvements in cement manufacturing, admixtures, supplementary cementing materials (fly ash, silica fume and slag) and “know-how” (the science of cement hydration) resulted in the ability to supply higher-strength concretes.
Early pioneers referred to this new family of concretes as “HSC.” However it quickly became evident that, in addition to high strength, it could provide superior characteristics such as resistance to sulfate attack, freeze/thaw and salt scaling, plus other benefits. Since high strengths also provide higher performance in many concrete properties, this family of concretes became known as “HPC,” which normally categorizes concretes with compressive strengths ranging from 6,000 psi to 14,000 psi.
During the same period of time (1980s) that the industry began to commercialize HPCs, others were developing the next generation of concretes known as “UHPC.” UHPC is defined as materials with a cement matrix and characteristic compressive strengths in excess of 20,000 psi, possibly attaining 36,000 psi; fibers in order to achieve ductile behavior under tension; and the potential to dispense with passive (non-prestressed) reinforcement. UHPC differs from HPC by:
• a compressive strength greater than 20,000 psi;
• systematic use of fibers to ensure non-brittle behavior;
• high cement content; and
• special aggregates.
Furthermore, UHPC achieves its ultra high-performance properties by:
• using compact graded materials (sand, quartz flour, cement and silica fume) with a maximum size less than 0.02-in. in diameter;
• low water-to-cement content; and
• fibers, resulting in a dense, low-permeability ductile product.
Unlike HPC, which requires reinforcing to ensure non-brittle failures, UHPCs can be used without passive reinforcing (such as rebar) due to its tensile ductility.
Concretes with strengths in the gap between HPC and UHPC (the range of 14,000 psi and 20,000 psi) are sometimes referred to as “very high-performance concrete” (VHPC).
Over the past 30 years, concrete technology advancements have led to a better understanding of the material’s performance capabilities and clearer determination of how to provide the right class of concrete for each type of application or piece of infrastructure. Much of the deteriorating infrastructure that we are rehabilitating or repairing today was built in an era before HPC or UHPC.
Selecting the correct class of concrete today means that our infrastructure should last hundreds, and perhaps thousands of years into the future. Now that is sustainability!
Vic Perry, FCSCE, MASc., P.Eng., is vice president and general manager for the Ductal division at Lafarge North America Inc., Calgary, Alberta.