Magazine

High-Tech Concepts Enhance Precast Concrete Designs

By James G. Toscas, P.E.

As a plant-controlled, engineered product, precast concrete can be fabricated to precise specifications, enabling its use when tolerances are close or specific attributes need to be favored.
Today, precasters are meeting a widening array of design needs for parking structures with new technologies that contribute to the components’ speed of erection, economy, aesthetics and other properties.
Designers are being challenged more often today by the growing interest in mixed-use properties. In blending retail, residential and other functions with parking, developers maximize the efficiency of their project’s footprint and generate traffic throughout the day.
This creates the need for fire-rated separations between parking levels and residential units. Precast concrete double-Ts provide an ideal way to achieve this separation, thanks to their inherent fire rating and structural capabilities.
These projects are enhanced by the growing application of total-precast concrete solutions, in which the building’s entire structural system and façade are constructed using precast concrete components. In these applications, some structural and architectural requirements are combined into one piece. A variety of textures can be created using reveals, form liners and embeds of thin brick, granite or other stones while retaining the component’s structural integrity.
This approach saves material and erection costs while producing striking aesthetic designs. It also eliminates the scheduling complexities and congestion caused by bringing masons or other trades to the site. The precast concrete components are delivered and quickly erected, either directly from the transport or from a staging area accessed by the cranes.
New Technologies Growing
Several new mixtures and casting techniques also are expanding precast concrete’s capabilities. Self-consolidating concrete (SCC), ultra-high-performance concrete (UHPC) and carbon-fiber reinforced concrete (CFRC) help designers expand the range of capabilities.
The concrete mixture in SCC includes high-range water-reducing ad mixtures that significantly increase the material’s workability and fluidity. As a result, it flows quickly into place, fills every corner of a form, and surrounds even densely packed reinforcement. SCC provides a high-quality, smooth finish with minimal if any bugholes, offering speedy casting, the capability to be formed into complex shapes, fewer inspection requirements, and high durability.
UHPC consists of a steel fiber-reinforced, reactive-powder concrete that provides a compressive strength of 30,000 psi, more than twice that of high-performance concrete. Even with lower-psi mixes, precasters today can achieve clear spans in arrangements that can exceed the needs of typical parking structures with double-Ts, eliminating material and erection costs for columns and beams while creating a welcoming, safe interior.
CFRC uses conventional steel for primary reinforcing and a resin-bonded, carbon-fiber grid for secondary reinforcing and shear transfer. The carbon-based product minimizes the potential for corrosion in the precast member. This in turn eliminates the excess concrete cover normally needed to protect the steel reinforcement from corrosion. As a result, costly sealers can be eliminated, and components can be made as much as 12% lighter in weight due to the reduction in cover.
Stainless-steel connections also are being incorporated into designs. They provide more durability and thus a longer service life for the entire facility.
Connections of all types are easier to reference, as the Precast/Prestressed Concrete Institute has just released its first-ever Connections Manual. It features an overview of all types of connections and design examples that aid designers in achieving the best look and more effective option.
Several parking projects have even been certified by the Leadership in Energy & Environmental Design (LEED) program produced by the U.S. Green Building Council. These take advantage of the array of attributes provided by precast concrete, such as lessened site impact, reduced heat-island effect, recycled content and use of local materials.
Maintenance needs, including caulking, can be reduced with precast concrete designs as well. PCI’s Parking Committee has produced a comprehensive maintenance program that can be used by owners to ensure a long service life.
Safety and security in parking structures can be enhanced by lighting techniques that exploit precast concrete’s long-span capabilities. Some designers have questioned the efficiency of lighting in these structures due to the use of double-Ts, which could block some fixtures. However, a 2007 study by Walker Parking Consultants in Denver,
sponsored by the Colorado Prestressers Association, showed there was no difference in horizontal illumination on the floor or vertical illumination on perimeter walls in precast concrete
or cast-in-place concrete structures.
Precast concrete enhancements don’t stop with fabrication. PCI has instituted a Certified Erectors program, similar to the certification program mandated for all products cast by PCI members. Independent auditors track each company’s erection techniques to ensure professional handling. These efforts ensure that owners and designers receive the project they envisioned, with tight tolerances, efficient erection, and long-term quality.
As sustainable design demands grow and available space is reduced, precasters will work closely with designers of parking structures to ensure that their projects are rapidly completed, aesthetically pleasing and cost-effective.
James Toscas is President of the Precast/Prestressed Concrete Institute in Chicago. The organization, which represents precast concrete producers throughout North America, can be found online at www.pci.org. Questions can be sent to info@pci.org.

Article Abstract from July, 2008




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