OWNER: CHICAGO DOT
DOT FUNDING: Federal Highway Administration and Illinois DOT
Architect: Site Design Group, Chicago
ENGINEER OF RECORD: ALFRED BENESCH, CHICAGO
GENERAL CONTRACTOR: James McHugh Construction, Chicago
READY-MIX PRODUCER: Ozinga Ready-Mix Concrete, Chicago
POST-TENSION REINFORCEMENT SUPPLIER: Dywidag Systems International, Bolingbrook, Ill.
LATEX-MODIFIED CONCRETE CONTRACTOR: Henry Frerk, Chicago
|Historical Wacker Drive|
In 1909, Daniel Burnham and Edward Bennett produced a long-range plan for the development of the City of Chicago that included Wacker Drive: a two-level street alongside the Chicago River circling two-thirds of the downtown Loop. Construction on the east-west portion of the street didn't begin until 1924; the north-south section wasn't built 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 5,700-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.
TESTING: FLOOD TESTING LABORATORIES, CHICAGO
Running along the Chicago River as it curves south toward St. Louis, Chicago's dual-level Wacker Drive is used by 300,000 vehicles and pedestrians every day. The top level is for local traffic; the bottom level for through-traffic and trucks servicing buildings along the street. Originally built in 1926, the east-west portion was demolished and rebuilt in 2002 and 2003. Many of that project's specifications carried over to reconstruction of the north-south portion, which began in the fall of 2010 and continues today. These include:
Brett Szabo, senior project manager for McHugh Construction of Chicago, says there were additional challenges:
Funding for the $54 million project is shared between the Federal Highway Administration (FHWA), the Illinois DOT (IDOT ), and the Chicago DOT (CDOT).
A total of 15 segments — separate 200 x140-foot concrete placements — are planned to complete the construction in mid-2012. As the project progresses, each segment is completed and turned over for public use.
The overall length is 2,900 feet over seven intersections. This construction used 55,000 cubic yards of high-performance concrete , 2½ million feet of post-tensioning (PT) strand, and almost 7 million pounds of epoxy-coated reinforcing steel bars (all rebar on the project is epoxy-coated).
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.
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.
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.
Proportioning the deck mix
In the east-west reconstruction, a quaternary high-performance concrete mix was used that included portland cement, fly ash, slag, and silica fume.
This time, a ternary high-performance concrete mix, including portland cement, slag, and silica fume — but not fly ash — was submitted and approved. According to Tristan Tady, quality control manager for Ozinga Ready-Mix Concrete of Chicago, the ¾-inch top-size “bridge deck stone” coarse aggregate is cleaner and harder than what was used in the mix for the east-west reconstruction. 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.
Flood Testing Project Manager Glen Hodson says the company tests concrete according to state DOT 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 the state, but approximately every 250 yards. The 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
Chicago-based Henry Frerk Co. provides the 2-inch-thick, latex-modified concrete topping after each concrete deck is placed and cured. Sales Manager Mike Vandenbroucke says the company's own mix design, which includes 24½ gallons of latex per cubic yard of concrete, was approved. This mix provides a very dense, impermeable concrete that resists chloride penetration. This topping is intended to be sacrificial and will be replaced as needed.
A mobile volumetric mixer is used to make the concrete. “It doesn't work to use a barrel mixer because the latex is like glue and the mix sticks to the side of the barrel,” says Vandenbroucke. “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. If not added just before the topping is placed and the agent dries before it's covered by the topping, it becomes a bond-breaker. Timing is everything because there's such a short window of opportunity.
Frerk places the 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 necessary bond. This eliminates the potential of the 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 on all this
Every day, 50,000 pedestrians cross Wacker Drive at one intersection and 20,000 pedestrians cross each of the other intersections in the construction path.
To minimize disruption, Chicago DOT Chief Bridge Engineer Dan Burke's team coordinated with numerous agencies and businesses. The north-south reconstruction is more complicated than the east-west phase in terms of staging and constructibility, but, he says “at the halfway mark the project is on schedule and on budget. We feel good about the results so far.”
This article originally appeared in the September 2011 issue of CONCRETE CONSTRUCTION.