Style and Sustainability of Precast Concrete
Precast Design Challenges
Part of the Gate Precast Company's challenge in fabricating the individual concrete panels was the sheer scale and weight of each individual panel. (Each tower panel, for example, weighs about 8 tons.) Because of their size, the solid concrete panels needed to be as lightweight as possible—for obvious reasons of logistics, staging and erection. Yet any changes made to affect weight had to allow the cast panels to retain their structural and compositional integrity.
The project team had estimated that about 4 million pounds of grey-cement-based material were required to form the distinctive panels. By using only an expanded shale coarse aggregate, the project's precasters were able to reduce by 20 percent the weight of each panel. While the resulting panels were still relatively large and heavy, those same specifications allowed for a relatively quick installation process that cut down on project costs and construction waste.
Fabricating concrete panels that were not only uniform in color but unique in their textures presented another challenge for the project's precasters, who used a series of specially designed molds of varied sizes to create the more than 650 concrete panels that make up the exterior of the building. Precast concrete is versatile enough that it can be molded into virtually any shape and given whatever appearance and texture design specifications call for. But Mayne's design was intricate and had few repetitive conditions.
About a dozen modular molds were developed that could be arranged as needed to produce the appearance designed by the architects. The standardized molds improved the efficiency of the fabrication process, cutting both time and cost by allowing repetitive concrete pours. By inserting rubber molds into the panels as needed, and caulking between the molds, the team could cast the proper profiles. Some panels required a many as 130 mold modules to produce the correct design.
| Careful Cement Specs |
Portland cements are classified by type, and there are six main types in the standard ASTM C150 / C150M - 12. Type I (and Type IA) designate general purpose cements suitable for all uses where special properties are not required, according to the Portland Cement Association (PCA). Type II cements contain 8% or less tricalcium aluminate. This type of cement can help produce concrete with moderate sulfate resistance. For more information on all cement types, see www.cement.org For the precast panels on the Perot Museum in Dallas, the cement specified met the Type II requirement for sulfate resistance while also meeting all Type I traits for compressive strength. In total, about 2,400 tons of the Type I-II cement was delivered for use on the Dallas building project. While many architectural precast panels are pigmented on the exterior portion and sandwiched against the grey concrete, which typically comprises the interior, the Perot Museum panels are designed so they are composed entirely of grey concrete. To develop the concrete mix, Gate Precast’s team met with Holcim’s senior market manager, Tim Mummey, and the local senior technical service representative, Tony Sorcic to discuss the project requirements:
Concrete placement began in the summer of quarter of 2010 with the laying of the museum’s piers and foundation. Later came the building’s signature precast panel façades. |
One major challenge was inspecting the molds and casts to ensure that in spite of the numerous different profiles and conditions, the exposed surfaces would be consistent and within allowed tolerances.
The team that fabricated the precast concrete forms and panels had a lot to do with the successful installation, says Todd Petty, Gate Precast Company's vice president for operations. “Architectural precasters are skilled craftsmen for many reasons, but they have to be cerebral for complex projects like this,” Petty says. “The in-house panel tickets used on projects similar to the Perot Museum are difficult to digest. And the precasters have to be able to work within tight, industry-mandated tolerances.”
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