Chicago’s Wacker Drive is used by 300,000 vehicles and pedestrians every day. Originally built in 1926, the east-west portion was demolished and rebuilt in 2002 and 2003 (see “Rebuilding Chicago’s Upper and Lower Wacker Drive” in the November 2002 issue of Concrete Construction). Many of the same specified features carried over to construction of the north-south portion of this major Chicago street that started in the fall of 2010 and continues today. These requirements included:
- Upper Wacker Drive—the second level and considered a bridge deck—must be designed and built for a minimum 100-year service life, including resistance to chemical assault from deicers.
- The vertical clearance for Lower Wacker must increase from 12 feet 6 inches to 13 feet 9 inches to allow for truck access.
- The deck should have no cracks during its 100-year life.
- Pedestrian access to all building entrances at the “street” level (upper deck) must be maintained throughout construction, including demolition, even though the deck extends to the building walls on both sides.
Brett Szabo, senior project manager for McHugh Construction, Chicago, says there were additional challenges:
- Although most of the east-west construction had buildings on one side and the Chicago River on the other, the north-south construction has buildings on both sides, including the Opera House, Mercantile Exchange, and the Willis (formerly Sears) Tower—a total of 18 skyscrapers along the length of the project.
- To maintain access to all building loading docks and underground parking, traffic on Lower Wacker continued during construction, meaning that upper deck forming operations had to proceed with live traffic beneath.
- The concrete superstructure for the upper deck is built first, at 13 inches thick, then topped with 2 inches of latex-modified concrete to get the final profile and elevation.
Funding for the $54 million project is shared between the Federal Highway Administration (FHWA), the Illinois Department of Transportation (IDOT), and the Chicago Department of Transportation (CDOT). A total of 15 segments—separate 200x140-foot concrete placements—are planned to complete the construction during 2012. As the project progresses, each segment is completed and turned over for public use. The overall length is 2900 feet over seven intersections. This construction used 55,000 cubic yards of high-performance concrete (HPC), 2½ million feet of post-tensioning strand (PT), and almost 7 million pounds of epoxy-coated reinforcing steel bars (all rebar on the project is epoxy-coated).
Forming the deck
To get the vertical clearance needed for Lower Wacker 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 on Lower Wacker 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 they are using Peri forms for the project. 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.
Engineering a zero-tension deck
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. Because Chicago experiences severe winters with numerous freeze/thaw cycles and large amounts of deicing chemicals, eliminating the possibility of cracks is key because chlorides will penetrate cracks, leading to reinforcement corrosion. According to Keaschall, the solution was to use post-tensioned reinforcement to create a zero-tension deck, ensuring cracks wouldn’t develop over time because the deck would always be in compression.
To achieve zero tension, you must overcome shrinkage forces in the concrete, as well as supporting loads wherever they occur. Alfred Benesch engineers worked with Dywidag Systems International (DSI), Bolingbrook, Ill., to design the PT system. “We used the post-tensioning system to balance the gravity loads and provide overall compression so there 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.”
Mike Lally, the iron worker superintendent for McHugh responsible for installing all the rebar and PT reinforcement for the project, says the PT system includes wedge boxes on the form edges attached to plastic PT ducts. The task of installing tendons in the transverse direction was 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 PT duct with the five preloaded cables totalled 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 Lally.
Lally’s crew placed longitudinal wedge boxes and ducts at 2-foot intervals. There are an additional five ducts with nine cables in each beam. Lally says all the PT cables are tensioned from both ends.
The cables are stressed to 41,000 pounds when the concrete reaches 4200 psi compressive strength. Lally says they pull all the cables in a duct 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.
Proportioning the deck mix
In the east-west deck construction, a quaternary high-performance concrete mix was used, including portland cement, fly ash, slag, and silica fume. For the north-south construction, Tristan Tad-y, the quality control manager for Ozinga Ready-Mix Concrete, Chicago, says a ternary HPC mix, including portland cement, slag, and silica fume, was submitted and approved. The ¾-inch top-size “bridge deck stone” coarse aggregate is cleaner and harder than what was used in the east-west mix. Fly ash wasn’t included included in the mix. The water-cementitious materials ratio is being held to 0.38.
McHugh hired Flood Testing Laboratories, Chicago, to provide the testing services required by its contract and to represent McHugh’s interests. Other testing is being performed for the funding bodies and other companies involved in the project.
Glen Hodson, a project manager for Flood, says they test concrete according to the IDOT requirements. The first four or five loads arriving each day are tested for air content, slump, and unit weight. Every load is checked for air content, the number of drum revolutions, and concrete temperature. Every 50 yards thereafter is checked for unit weight and slump. Cylinders are taken at the pump discharge on a random schedule determined by IDOT, but approximately every 250 yards. These cylinders are tested at 3, 7, 14, and 28 days. The early breaks help determine when post-tensioning can begin. If another testing company has cylinders with low breaks, McHugh can present its results. Cylinders are stored near the point of placement in climate controlled curing boxes and moved the following day to test labs for standard curing.
Placing and finishing
McHugh uses a truss screed to strike off the widest portion of the street, while finishers using handheld vibratory screeds strike off small areas on either side. Elevations aren’t critical at this point, as long as a minimum 13 inches of concrete thickness is maintained; the 2-inch topping will provide finish elevations. Finishers pass straightedges over the fresh concrete and workers follow about 50 feet behind the truss screed with curing blankets quickly saturated with water to begin the seven-day cure period.
Installing the 2-inch topping
The Henry Frerk Co., Chicago, provides the 2-inch-thick latex-modified concrete topping after each concrete deck is placed and cured. Mike Vandenbroucke, Frerk’s sales manager, says the company’s own mix design, which includes 24½ gallons of latex per cubic yard of concrete, was approved for the project. This mix provides a very dense, impermeable concrete that is resistant to chloride penetration. This topping is intended to be sacrificial and will be replaced as needed.
Vandenbroucke says a mobile volumetric mixer is used to make the concrete. “It doesn’t work to use a barrel mixer since the latex is like glue and the mix sticks to the side of the barrel,” he says. “You have about 15 to 20 minutes of working time between placements.”
A typical approach when installing a topping is to apply a bonding agent to the substrate, usually a mix of portland cement, silica sand, and latex. That needs to happen just ahead of placing the topping. But if the bonding agent dries before the topping covers it, it becomes a bond-breaker instead. Timing is everything because there is such a short window. Frerk places their topping directly on the clean substrate (the structural concrete deck) and relies on the unusually high latex content of the topping mix to provide the needed bond. By doing this, they avoid the potiential problem of a bonding agent drying before workers cover it with the topping.
After Frerk places the topping, McHugh strikes it off with a Bid-Well screed to provide finish elevations. Afterward, wet curing blankets are placed on the topping to start the four-day cure—two days wet, two days dry.
The owner’s perspective
As the owner, CDOT takes a more behind-the-scenes role. Dan Burke, the chief bridge engineer, says they coordinate with numerous agencies and businesses to minimize disruption in this busy area of downtown Chicago—50,000 pedestrians cross Wacker at one intersection and 20,000 pedestrians cross each of the other intersections in the construction path each day. Burke acknowledges that this project is more complicated than the east-west phase in terms of staging and constructibility, but, “at the halfway mark the project is on schedule and on budget. CDOT feels good about the results so far.”
The high-performance concrete used on this project included silica fume. Slight amounts of water loss in silica-fume concrete results in cracking so curing blankets were placed and saturated shortly after the concrete was placed.
The top 2 in. of the deck is latex-modified concrete. It provides an impermeable surface resistant to chloride penetration. The street was diamond grooved to provide good traction.
In 1909, Daniel Burnham and Edward Bennett produced a long-range plan for the development of the city of Chicago, which included the construction of Wacker Drive—a two-level street alongside the Chicago River circling two-thirds of the downtown Loop. But construction didn’t begin until 1924 on the $8 million east-west portion of the street; the north-south section wasn’t constructed until the 1950s.
For the initial two-year project completed in 1926, workers installed 598 caissons to a depth of 95 feet below the surface to support the 5700-foot length of the upper deck. More than 1 million pounds of reinforcing steel, and 116,000 cubic yards of concrete were used for the columns and deck with a thickness as great as 3 feet.