Solidifying the Future
Concrete Innovations Across the Country Get the Job Done

By Craig Barner, editor of Midwest Construction. The McGraw-Hill Construction publication can be read online by visiting www.midwest.construction.com

Consumption of Portland cement, a main ingredient in concrete, is expected to reach a record 124 million metric tons, in part because of projects such as Chicago's Trump International Tower & Hotel. (Photo courtesy of James McHugh Construction Co.)

Concrete Stands Tall in Chicago
Concrete is a key element in the 92-story Trump International Tower & Hotel under construction in Chicago in part because it forms the building's structural support.

Indeed, 20 concrete mixes are used, and about 300 trucks full of concrete are received every week, said Brett Szabo, senior project manager of concrete for James McHugh Construction Co., the concrete contractor.

The stiffness of the concrete was specified on Trump so that the eventual residents of the condominiums and hotel do not perceive sway in the building after it is completed. The modulus of elasticity specification of the concrete was measured.

"It's pretty unheard of," said Bob Sinn, associate partner of Chicago-based Skidmore, Owings & Merrill LLC, the architect and structural engineer. "Most concrete suppliers know about strength tests of concrete. Modulus of elasticity is a different animal for them."

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Numerous details keep the crews busy.

For instance, the floor-slab thickness of the completed garage is 14 in., Szabo said. The depth will be 9 in. for a typical residential floor and 18 to 20 in. for a transfer level.

Supporting the garage are 30 6-ft.-diameter concrete columns, and 47 thinner concrete columns will support the residential portion.

Transfer levels are needed because setbacks at levels 16, 29 and 51 require column pressure to be transferred from some lines to adjacent lines, Sinn said. The transfer levels, which also hold mechanical equipment, are on 15, 28, 50 and 90.

Usually, mammoth outrigger beams that are typically 17-ft., 6-in. deep and 5-ft., 6-in. wide are on each transfer level. They provide lateral stability against the city's winds.

The outrigger beams extend from the perimeter columns to the 197-ft.-long, 49-ft.-wide central core, also concrete. The core, which is composed of four I-shaped walls and one C-shaped wall, will hold the elevators and stairs. More important, it provides additional lateral stability.

The care taken with the 10-ft.-thick mat slab shows how important concrete recipes on the project are.

The building has four below-grade levels, and the 200-ft.-long, 60-ft.-wide slab, which is below Lower Level Four, was poured over a 22-hour period between Sept. 30 and Oct. 1, 2005. The slab serves as a juncture between the below-ground caissons and the above-grade core.

The mix was composed of self-consolidating concrete that features specialty additives and low-water content, said Tim Snyder, construction manager in Chicago for New York-based Trump, the developer.

Key benefits include strength and less labor during the pour. Because of the chemistry, the material is poured without having to be vibrated. And, as the name implies, the concrete has a high slump rate.

"That concrete is so sticky and dry because it has so little water and so much cement and other additives that within 15 to 20 minutes of striking it off, it goes hard," Snyder said.

More important, the concrete's temperature and strength were carefully monitored to ensure strength and solidity.

Readings were taken every minute at 36 points and downloaded onto a computer so a curve could be plotted.

Within 24 hours, the temperatures spiked at 150 degrees because of hydration but receded over weeks. After 10 days, the concrete averaged around 120 to 125 degrees.

Similarly, six full-depth cores were made in the mat to measure the concrete's strength. The concrete was poured at 10,000 psi. It measured at 7,000 psi after seven days due to heat gain but averaged 13,000 psi after cooling.

"It was a 56-day design mix, but we were reaching design strength in less than 28 days," Snyder said.

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