Under the HSC Canopy

Florida precaster finds success with high-strength concrete and encourages other manufacturers to market it.

By: Sue McCraven

It’s summer 2011. Our sluggish economy lurks overhead like a dark cloud. High unemployment lingers. Construction opportunities are scarce in many regions of the country. Any good news is welcome. For producers who want to learn more about the business potential of high-strength concrete, here is a Florida precaster who has jumped into HSC with both feet.

A little history
Thirty-five years ago in his native Great Britain, Alex McCulloch saw some amazing stone balustrades that were reproduced with precast “dry-pack” concrete. Historical restoration such as the precast balustrades is painstaking work for British producers who repair magnificent structures dating back to the 17^th century. In Wales, Scotland, Ireland and Great Britain, only natural stone is acceptable for building such elite structures as the irreplaceable Westminster Abbey in London, site of the recent royal wedding. When restoration work becomes necessary, only the expensive dry-pack concrete can provide exacting color and a high-quality finish.

Consequently, dry-pack solutions, while pricey, were an easy sell to British architects, especially for McCulloch, who has a background in connections for stone installations. Because he sold masonry fasteners for 20 years, he had ready answers for architects asking about precast connections.

While successful at selling dry-pack in his native country, McCulloch was troubled because there were problems with the concrete mixes. Fine aggregate suppliers sold unwashed sand contaminated with a high percentage of silt. When the company McCulloch represented did nothing to remedy these poor-quality aggregates, he decided to strike out on his own.

New company and New World Symphony
Bringing samples of precast stone to the United States in 1992, McCulloch started Castlestone Inc. in Winter Garden, Fla., catering to architects looking for exact stone replications for historical restoration projects.

A recent example completed in January, the $160-million, 101,000 sq ft New World Symphony Concert Hall in Miami Beach, Fla., was designed by renowned architect Frank Gehry. This modernistic building, located on the newly upscale Lincoln Road and cozying up to art deco beach hotels and unique shops, is both culturally important and architecturally fantastic. Floating in seeming weightlessness, a precast HSC canopy built by Castlestone graces the entrance to this magnificent structure.

Precaster asks, “What if?”
Fishing for different ways to bring in new business, Castlestone bid on the entrance canopy. Initially, McCulloch’s plan was to cast the canopy out of concrete with glass fibers in four 2-in.-thick sections. But at a total length of 40 ft by 16 ft, the canopy would need steel reinforcing bars, and steel with less than 1 in. of concrete cover would readily corrode in the saline air of Florida’s coast.

The only other viable option was to build the canopy in one piece using HSC. The project architects not only were familiar with HSC, but were delighted with the aesthetics of a thin, one-piece canopy. Now the precaster’s challenge was to find a consulting engineer who understood HSC – no easy task as it turned out.

Castlestone interviewed four consulting engineers before finding one who understood HSC. In addition to designing the 3-in.-thick canopy all in one piece, the engineer needed a structure strong enough to withstand Florida’s hurricane winds. Deciding to work with a new concrete material for a high-profile building would be a daunting prospect for any precast producer.

Working with HSC
Castlestone was fabricating a very unusual product. The canopy required:

1. Fabrication based on drawings alone;
2. A superb finish on both sides;
3. Precise inspection of cantilevered support tentacles that had different sizes and orientation; and
4. Exacting laser technology for supports and the formwork’s sinuous geometry.

After familiarizing themselves with the new material, a small-scale prototype of the canopy was made so the general contractor’s engineer could be assured of the strength and durability of the final product. After a rigorous inspection, the contractor was impressed. Castlestone’s HSC fabrication included:

• 1-inch-thick machine-routed formwork to precisely follow the canopy’s flowing lines
• 16 variously sized steel support tentacles with corresponding laser-positioned imbeds
• Steel reinforcing mesh shaped to fit precisely and securely within the concrete’s thickness
• Careful attention to mix temperature

Transportation of the finished 8-ton canopy called for special Florida DOT road permits and a custom truck (used to relocate homes) with police escort. The 300-mile route from the Winter Garden plant to the Miami Beach construction site took about eight hours. Four welders installed the canopy in four hours.

Forging ahead with a novel precast concrete material may induce a bit of nail biting, but if precast producers can convince architects and specifying engineers to consider HSC’s strength and design potential, good news may be on the horizon.

Sue McCraven, NPCA technical consultant and Precast Solutions magazine editor, is a civil and environmental engineer.

Interview with Alex McCulloch
Were you nervous about using HSC on such a high-visibility project?
McCulloch: Of course! The greater challenge was selling a material that costs 60 to 70% more than GFRC (glass fiber-reinforced concrete). HSC has typically been used in bridge construction, but it does not yet have widespread recognition or use by American precasters for architectural work.

What sold you on HSC?
McCulloch: HSC is a very durable material. I was impressed with it the first time I saw it. HSC has superior properties. Very thin panels can be made with HSC that are much stronger than ordinary concrete. Because HSC panels are much lighter than typical precast, this lower weight (and reduced structural load) means less cost for handling, transport and installation. Fewer fasteners needed for HSC also saves money.

What message do you want to send to other precasters?
 McCulloch: The precast industry needs to realize the market potential of HSC solutions, and we all need to advance the use of new material technology. After all, we are here to sell precast, and HSC precast concrete is ideal for historical restoration, architectural details and aesthetic designs. Moreover, I believe precasters can make a strong case for using HSC’s lighter weight and greater strength and durability. Most of all, architects love the stuff! We need to work harder to market HSC to structural engineers because it allows much more freedom in design.

Concrete: The Class Family

By Vic Perry, FCSCE, MASc., P.Eng.

Concrete, the world’s most widely used building material, is typically a locally produced product that provides good jobs for local workers. Portland cement uses raw materials such as limestone, one of earth’s most abundant materials. And 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: 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 to 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, as well as 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 to 14,000 psi.

During the same time period (1980s) that the industry began to commercialize HPC, others were developing the next generation of concretes known as “UHPC.” UHPC includes materials with a cement matrix and 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 properties by:

• Using compact graded materials (sand, quartz flour, cement and silica fume) with a maximum size less than 0.02-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, UHPC 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 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 significant progress toward the development of a better understanding of its 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 being rehabilitated or repaired today was built in an era before HPC or UHPC.

Selecting the correct class of concrete today means that our infrastructure should last hundreds, or 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.