Sustainability reaches new heights

September 2009 » Features » MANUFACTURER'S TURN
Nation—€™s tallest LEED-certified building relies on high-performance slag cement
David Johns

Soaring 58 stories and 975 feet above downtown Philadelphia, the new Comcast Center, which was recently awarded LEED Core and Shell Gold Certification, now has the distinction of being the tallest certified green building in the United States.

The Comcast Center boasts many features designed to save energy and reduce environmental impact. The tallest building between New York and Chicago, this high-performance skyscraper’s primary structural system includes a large central concrete core (containing 40 percent Lafarge NewCem slag cement and 60 percent Type I cement) rising the full height of the tower with steel-framed floors.

High strength inner core
According to Thornton Tomasetti, the structural engineers for the Comcast Center, a concrete core provides significant efficiency in a lateral system for this type of building footprint. For a core dimension that is small relative to the height, you can get a much more efficient structure in concrete than in steel, with greater stiffness and good lateral resistance for wind loads. Because the footprint is small and the core relatively slim, the Comcast Center’s exterior core walls are fairly thick — 54 inches up to the 20th floor — to further minimize deflection from wind forces. Other safety benefits of the design are an added bonus, as all of the structure’s elevators, sprinklers, communications systems, and stairwells are encased within the concrete core.

A high-performance concrete mix containing Lafarge’s newcem slag cement helps give the comcast tower’s massive inner core its needed strength.
In mass concrete such as the core of the comcast center, a high percentage of slag cement can help moderate thermal stresses within the concrete and prevent cracking.

Construction of the tower required 50,000 cubic yards of concrete, about half of which was high-strength concrete for its cast-in-place inner core. “Our specifications for the core called for concrete reaching a 10,000-pound-per-square-inch (psi) compressive strength at 56 days to give the desired stiffness and strength to the building,” said Aine Brazil, managing principle in charge of the project at Thornton Tomasetti. “In terms of efficiency, using a higher strength concrete was quite important to producing a cost-effective building.”

Optimizing mix design
After eight months of product development, ready-mixed concrete producer Action Supply Company of Philadelphia and concrete subcontractor Madison Construction Company of Malvern, Pa., received approval for using a self-consolidating concrete (SCC) containing Lafarge’s NewCem slag cement. “Our proprietary mix design reached strengths as great as 14,000 psi, which is one of the key reasons why the NewCem slag cement was selected for this high-performance application,” said Dean Melchiorre, Action Supply’s quality control manager. “Considerable laboratory and in-place testing was conducted on early strength, compressive strength, e-modules, shrinkage, flexural strength, and chloride permeability, and further jobsite testing proved out the results.”

Engineered for high strength and long-term durability, NewCem slag cement improves concrete’s workability and helps structures achieve greater strength potential, reduced permeability, and increased resistance to sulfate attack and alkali silica reaction. “It is also well known that utilizing high replacement levels of slag cement in properly proportioned mixes helps control shrinkage, creep, and cracking in mass concrete structures,” said Brazil. “For these reasons and because the ultimate strengths achieved exceeded our specification requirements, we deemed the structural performance of the mix containing the NewCem slag cement a success.”

Approximately 21,000 cubic yards of the high-strength SCC mix utilizing 40 percent NewCem slag cement were produced for the first 25 floors of the central core walls. In addition, 4,000 cubic yards for the 5,000-psi concrete mat foundation and 38,000 cubic yards for the lightweight decks all used a minimum of 35 percent NewCem in the mix throughout the project.

Green design features
Cutting-edge sustainable design and energy efficiency can be found in nearly every aspect of the Comcast Center. It uses 40 percent less water than a typical office building, and shading on the plaza reduces the heat-island effect caused by pavement by 70 percent. A glass curtain blocks 60 percent of heat while allowing 70 percent of the sun’s visible light, reducing energy used for air conditioning while utilizing natural light to its fullest. An under-floor water loop is used to cool the winter garden and 6th floor atrium floor during the summer and warm it during the winter, saving energy.

The use of NewCem slag cement also made a strong contribution to the Comcast Tower’s sustainable design that helped it earn LEED certification from the U.S. Green Building Council. For structural elements, concretes containing NewCem can contribute to LEED credits in the following categories: Sustainable Sites, Energy and Atmosphere, Materials and Resources, Indoor Environmental Quality, and Innovation in Design.

NewCem is finely ground, hydraulic cement produced from granulated blast furnace slag, a product of the iron-making process. Its use in concrete saves virgin raw materials, consumes less energy, and makes use of a recyclable by-product material of the iron-making process normally destined for landfills. The result is high-performance concrete with less environmental impact.

Project team
The project team for the Comcast Center includes — Owner: Liberty Property Trust; Design architect: Robert A.M. Stern Architects, New York; Architect of record: Kendall/Heaton Associates, Houston; Structural engineer: Thornton Tomasetti, New York; Contractor: L.F. Driscoll Company, Bala Cynwyd, Pa.; and Environmental consultant: Atelier Ten, New York.

Editor’s note: The October issue of Structural Engineer will include a second Manufacture’s Turn article about the Comcast Center. That article will discuss how Hill International successfully managed the construction using project management software.

David Johns is the technical sales representative for Lafarge’s Lehigh, Valley Cement Sales District and can be reached at david.johns@lafarge-na.com. Joe Berlucchi is the territory manager for Lafarge’s Lehigh Valley Cement Sales District and can be reached at joe.berlucchi@lafarge-na.com.

Upcoming Events

See All Upcoming Events