Sometimes, the numbers really do speak for themselves, and the numbers for Comcast Center the media giant’s new headquarters in downtown Philadelphia,
Steven Prokopy
Sometimes, the numbers really do speak for themselves, and the numbers for Comcast Center Û the media giant’s new headquarters in downtown Philadelphia, set to open in fall 2007 Û are impressive: 57 stories (60, if you include three underground floors), nearly 1,000 ft. tall, 1.24 million sq. ft. of floor space, with an estimated price tag of $435 million. The high-rise will not only be the tallest in the city (besting One Liberty Place by 30 ft.), it will be the nation’s highest structure outside of New York and Chicago and the tallest Leadership in Energy and Environmental Design (LEED)-certified building upon completion. And, some of the numbers regarding the tower’s construction are equally impressive.
Comcast wanted New York architect Robert A.M. Stern to design a headquarters that challenged every aspect of skyscraper construction and make the building a benchmark. Concrete contractor Madison Construction Co. of Malvern, Pa., and ready mixed producer Action Supply Co., Philadelphia, took note, formulating proposals for the tower’s cast-in-place core, which will consume 44,000 yd. After nine months of testing different cementitious material and mix designs Û including some done with Lafarge’s blended silica fume SF Cement Û they were determined to secure approval from Stern and project engineer Thornston-Tomasetti Group for a self-consolidating concrete (SCC) mix using slag cement.
The job’s most important specification is the high-strength concrete, with mixes exhibiting a minimum 20-in. flow and reaching 10,000 psi compressive strength at 56 days, says Action Quality Assurance Director Dean Melchiorre. Our mix design ultimately hits about 13,000 to 14,000 psi in 28 days with a 26- to 28-in. flow. We decided that SCC would be easier to place under these conditions.
Action was so convinced that SCC was the way to go on the Comcast project that the company said it would only sign on to the work if that spec carried. A representative from the project contractor went to see SCC in use on other high-rise jobs in Las Vegas and the Pacific Northwest and returned convinced.
STRUCTURALLY SOUND
Situated between Arch Street, 17th Street, and John F. Kennedy Blvd. and built atop a commuter train station, Comcast Center features an exterior steel skeleton with concrete and metal deck floor plates and the SCC center core. According to Melchiorre, the core wall thickness runs approximately 54 in. through floor 35, tapering to 36 in. above. With average set times in the four- to five-hour range, forms are removed in about two days.
The mat foundation pour started last November, requiring 4,000 yd. of regular-slump concrete supplied by two of Action’s four area batch plants, and soon work began on the core. Although pours on the three below-grade floors went fine using the standard boom pump, concerns began to grow about safety and pump pressure when pours reached the sixth and seventh floor. There were concerns about using (7- to 9-in.) high-slump concrete from the beginning, says Robert Haas, regional manager for Sika Construction, which supplied the job with its ViscoCrete 2100 superplastisizer for the SCC. There is an enormous amount of reinforcement, plus the use of vibratory units on higher floors was a worry. SCC reduces safety concerns, unnecessary labor, and vibration issues. Plus, the concrete looks like glass.
While segregation was never an issue even when pumping to upper floors, the contractor had decided to begin using a trailer pump into a form-mounted placing boom beginning with the eighth story.
Another element to the SCC’s high-strength property is the use of slag cement in the proprietary mix design Û Melchiorre won’t even reveal his design to the contractor Û which Action insists is nothing particularly exotic. There’s a little more sand than normal, he says, with about a 50-50 mix by weight of coarse aggregate and cementitious products, which results in a high amount of fines and high compressive strengths. It’s almost all local products Û Type I cement from Keystone Cement [owned by Giant Cement Holding], sand from Hanson’s operation in South Jersey Û with the exception of the Lafarge’s NewCem slag cement, which is shipped in from Sparrows Point, Md. Unlike other SCC mixes, we do not need a Viscosity Modifying Admixture due to the very high quality of the materials we use.
At the plant level, Action is providing continual quality testing, running ASTM C-1202 Rapid Chloride Permeability, with very low results at an average of 702. According to Haas, the modulus of elasticity results also are excellent.
Pours are typically done once every five work days, with one pour taking care of one floor. Due to tight space constrictions, one of the property’s surrounding roads is dedicated to construction traffic. We can usually unload a truck in six minutes. And with one of our plants only about five miles from the job site and the other three about 12 miles out, traffic isn’t usually a problem. Eventually, the dedicated road will run through the finished building, says Melchiorre.
The use of SCC on a building this tall has caught the attention of more than a few visitors since the project broke ground last year. Haas estimates as many as 100 industry folks have made their way to the site since November with dozens more booked for the coming months. This job is about tight quality control, he explains. Not everyone is willing to make that commitment. But most of the people that come to see this tower are.
Perhaps the most encouraging sign that SCC is the wave of the future for local high-rise construction is the recent announcement of the construction of the 42-story Murano high-rise condo building on Market Street, with a strength requirement of 12,000 psi. Haas speculates that in order to hit that specification, silica fume will probably have to be added to the mix, but this addition should take the SCC to about 16,000 psi. In addition, Comcast has the option of building a second, 250,000-sq.-ft. office building along side its towering headquarters.