Building Even Better Concrete

Manufacturers, scientists, and designers strive to reduce a vital material's environmental footprint while exploiting its many beneficial qualities.
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From Architectural Record
Joann Gonchar, AIA

The whole building

Though cement production is energy-intensive and CO2-emitting, if architects focus exclusively on these aspects of the material, they run the risk of forgoing performance efficiencies that concrete can provide, especially when used as part of a whole-building approach to design. One example of a project that is the outcome of such a process is the recently completed U.S. Federal Building in San Francisco [record, August 2007, page 96]. Half of the cement in the 18-story tower's exposed reinforced-concrete structure was replaced by blast-furnace slag, preventing release of about 5,000 tons of CO2 into the atmosphere, according to estimates from Morphosis, the project's architect. But even more noteworthy is the building's reliance on natural ventilation to cool its upper 13 floors. The concrete structure and its thermal mass are key components of this strategy.

The tower has a building automation system (BAS) that controls operable components in its exterior window walls. The BAS opens these apertures to cool, or "charge," the concrete during the night, when warm weather is expected the next day. Once the structure's temperature has dropped sufficiently, the BAS closes the openings. Then, during the day, heat generated by occupants, computers, and lights is transferred to the slab by radiation.

The Morphosis-designed San Francisco Federal Building relies on natural ventilation and an exposed reinforced concrete structure to cool the upper floors of the 18-story tower (middle). The building's structure is "charged" at night when operable components in the exterior window walls are opened (bottom). The office areas (top) feature exposed slabs that have a wave profile in section. The configuration provides a large surface area and aids absorption of heat.
Photography: © Nic Lehoux

The architects and engineers worked together to find the structural configuration that would provide maximum cooling. One decision made in order to enhance the performance of this system was to use normal-weight concrete despite the possible structural benefits of using lightweight concrete in the highly seismic region. The entrained air in the lighter material would have made the structure act like an insulator. Instead, a structure that quickly absorbed and released heat was required, explains Steve Ratchye, now an associate in the Los Angeles office Thornton Tomasetti. Ratchye was formerly project structural engineer for Arup, the m/e/p and structural consultant on the Federal Building.

Another feature of the structure that aids the natural ventilation strategy is its raised floor/upturned beam configuration. By avoiding perimeter beams, the designers enhanced the flow of air, the penetration of daylight, and access to views. The slab has a wave profile in section, providing a larger surface area than a flat slab, enhancing its ability to absorb heat. In the design of this element, the requirements of thermal mass, architecture, structure, and daylighting all coincide, says Ratchye.

In addition to more emphasis on an integrated approach to design like that employed for the Federal Building, new technology, such as ultra-high-performance concretes (UHPC), could help architects produce more environmentally benign buildings. One UHPC is Ductal, introduced several years ago and developed by chemical company Rhodia, the construction arm of Bouygues, and building products manufacturer Lafarge. It incorporates metallic or organic fibers and does not require reinforcement. Because its compressive strength is 6 to 8 times greater than conventional concrete, the material allows for smaller structural members and therefore has fewer associated greenhouse-gas emissions. According to a study commissioned by Lafarge comparing a bridge made of Ductal to one with a conventional concrete-and-steel structure, the Ductal-only solution required 50 percent less material by volume, which translated into a 50 percent reduction in CO2 emissions, explains Vic Perry, Ductal general manager, Lafarge North America. The study takes into account not only embedded energy, but also reduced maintenance needs over a projected 60-year life span, he explains.

Highway barriers by fieldoffice, made of photocatalytic cement, could protect adjacent areas from traffic-generated air, sound, and light pollution.
Rendering: Courtesy Fieldoffice

Another recently introduced cement, TX Active, has environmental benefits discovered almost by accident. The material, which contains titanium dioxide, was first developed by Italian manufacturer Italcementi for Richard Meier's Jubilee Church, in Rome [record, February, 2004, page 101], to help to maintain the building's brilliant white appearance in perpetuity. But scientists later discovered that the product acts as a photocatalyst, using light to help break down airborne pollutants, such as particulate matter, volatile organic compounds, and nitrogen oxides. To some extent, conventional concrete also breaks down pollutants, explains Dan Schaffer, product manager for Essroc Cement, the U.S. distributor. "But the titanium dioxide accelerates the process," he says.

Since completion of the Meier building in 2003, the product has been used on several European projects, but has not yet been specified in the U.S. One prospect is a "superabsorber" developed by fieldoffice, a Clemson, South Carolina−based architectural practice. The firm first proposed using the material to create spongelike highway barriers that would dissipate the light, sound, and air pollution generated on heavily traveled roadways. But now several school districts are interested in deploying the porous panels to help improve air quality at urban sites, according to Doug Hecker, fieldoffice principal. Hecker is exploring both 3D printing and more conventional forming methods for producing the panels, which at least in his initial concept, were to be nonrepeating. But more practical concerns have taken over: "We are now investigating ways a limited number of panel types can be combined to produce different effects," he says.

 

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Originally published in Architectural Record
Originally published in December 2007

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