Forming the deck

To achieve the vertical clearance required for Lower Wacker Drive traffic, the thickness of the upper deck was reduced from 19 to 13 inches — a thinner and more reinforced deck.

McHugh keeps two 11-foot-wide traffic lanes open during forming and concrete placement operations. Driveways to underground parking and loading docks also are kept open, but redirected around form supports.

Richard Phelen, a general superintendent for McHugh, says the company's using Peri forms. The shores rest on mud-deck or wood timbers and extend 14 feet to support the deck forms. McHugh places by crane 15-foot-wide preassembled form tables with support beams over the traffic lanes and loading dock entrances.

Andrew Keaschall, project engineer for Alfred Benesch and Co., Chicago, says the 100-year service life requirement provides the backdrop for many of the project's engineering decisions.

Chicago experiences severe winters with numerous freeze/thaw cycles and the city's DOT uses large amounts of deicing chemicals. Eliminating the possibility of cracks is key because chlorides penetrates cracks, causing reinforcement to corrode.

The solution? Post-tensioned reinforcement to create a zero-tension deck. Cracks can't develop over time because the deck will always be in compression.

To achieve zero tension, however, you must overcome shrinkage forces in the concrete and supporting loads wherever they occur. Alfred Benesch engineers worked with Dywidag Systems International, Bolingbrook, Ill., to design the post-tensioned reinforcement system.

“We used the system to balance the gravity loads and provide overall compression so there's is no tension in the deck,” says Keaschall. “Tendons are located every 1½ to 2 feet in both directions and are draped between columns to support midspan loads.”

Installing the system

The tensioning system includes wedge boxes on the form edges attached to plastic post-tensioning strand ducts. Installing tendons in the transverse direction is difficult because the deck extends to building walls on both sides, leaving no room to install cables in the ducts after concrete placement. The weight of each transverse post-tensioning strand duct with the five preloaded cables totals 600 pounds.

“We couldn't place them by crane, so the 150 ducts for each deck placement were lifted and placed by several ironworkers,” says Mike Lally, a McHugh superintendent.

His crew placed longitudinal wedge boxes and ducts at 2-foot intervals. There are an additional five ducts with nine cables in each beam. All the post-tensioning strand cables are tensioned from both ends. The cables are stressed to 41,000 pounds when the concrete reaches 4,200 psi compressive strength. All the cables in a duct are pulled at the same time. When the job is complete, the ducts are filled with a special concrete grout mix — about 60,000 pounds of grout to fill all the ducts for each deck placement. “We place grout 10 continuous hours; no stops are allowed,” says Lally.