The New ‘Stone Age’ - What’s New in Precast

By Terry Olsen, AIA, CSI, LEED AP

To view April meeting's photos, click here

Precast concrete seems to be a timeless material, used by the ancient Romans, and still in use today.  You’d think this well-established material wouldn’t have any new surprises for us.  However, Tom Kelley, President of Gage Brothers, introduced us to a couple of new technologies for precast wall panels to update our current Stone Age technologies... 

Carbon Cast

Concrete works well in compression, and steel works well in tension, which is why reinforced concrete has become such a staple for construction.  When it comes to precast wall panels, there are some basic requirements we’ve come to appreciate, like minimum thickness and significant weight.  However, a new system, called Carbon Cast, replaces steel with carbon fibers for reinforcing.  Unlike steel, which is a thermal conductor, carbon does not transfer temperature to the same degree.   Carbon bonded to the insulation can improve the thermal performance of a precast wall panel.  Carbon can also be used in lieu of steel for reinforcing mesh. Carbon does not rust, so the minimum code-required thickness of concrete cover for steel to prevent corrosion is not applicable and this allows thinner sections.  Carbon also has about 8 times the strength of steel and permits load-bearing wall panels in very economical dimensions.  By replacing the steel reinforcing with carbon and by reducing the required amount of concrete cover, the weight of the precast wall panel can be reduced.  This saves shipping and fuel costs of the panels and also compounds the savings by reducing the required capacity and size of the structure.

Ericksen/Roed Open Space Truss

Trusses are a familiar structural device in the World of Steel.  Now, precast concrete trusses, a hybrid bowstring-Vierendeel truss, spaced up to 45 feet on center can span up to 85 feet and carry up to 25 stories.  These structural devices have an inherent 2- or 3-hour fire-resistant rating and an STC above 50.  These are highly effective tools for residential buildings.  In addition, these trusses can be staggered every other floor, further minimizing structure and increasing cost savings.  The downside is that these can weigh 60,000 to 80,000 lbs, so the contractor needs a good-sized crane to unload and install.  However, installation is quick, within 20 to 45 minutes to erect from the back of the truck, and can save significantly on labor costs of alternate structural methods.

Thin Brick

This market is growing phenomenally, with about 30% annual growth.  Last year 1/3 of the precast wall panel projects included thin brick.  Bricks have dove-tail keyways in back or a sawed back face and thus the poured concrete becomes the bond to the back. The concrete also becomes the “grout” joints between the bricks.  The faces of the bricks are coated with a wax to prevent the concrete from bonding to the face.  This wax is simply washed off with a high-pressure hot water rinse.  Brick has been shown to bond well in these panels, and bond failure has not been observed.  If anything, the brick itself may fail.  Thin brick in precast panels is durable; there are buildings in Canada with thin brick surviving 30 years of freeze-thaw cycles without pulling out.

Thin-set Stone

Stone is also making an appearance in the precast panel market.  Unlike brick, the thermal expansion-contraction of stone is not similar to concrete and can cause failure.  Therefore, a 6-mil poly bond-breaker must be placed behind the stone, which is then mechanically fastened to the precast.  Soft joints must be used behind and between stone pieces.

Faux Brick

Precast panels now come in colors and textured molds that can actually simulate brick.  Some projects are so convincing that developers of adjacent buildings seek the brick to match on their projects, only to find out there is no brick to match!

A Rainbow of Colors

No longer are precast concrete wall panels limited to a monolithic single color somewhere in the beige or grey range.  Multiple colors within the same wall panel are no longer unusual requests, and the palette of available choices has widened as well.  An upper Canadian black rock has led to a true, striking black, and contrasts well against a stark white. 

Examples - What’s Old is Now New Again

Many photos of new technology precast panels in place were presented, including the high-profile TCF Bank Stadium for the University of Minnesota Golden Gophers and the Minnesota Twins Ballpark in downtown Minneapolis.  It appears, despite what we already knew about precast, there are new opportunities.  What’s old is now new again.  Welcome to the new Stone Age.