Lower Wacker Drive
Rebuilding Chicago’s Upper and Lower Wacker Drive
(Building a double-deck street around the Loop to last for the next 100 years)
Restoration took 20-months, $200-million project that was laid down in a monolithic pour – the largest single pour of concrete in downtown Chicago.
In 1909 Daniel Burnham and Edward Bennett produced a long-range plan for the development of Chicago, which included Wacker Drive, a two-level street alongside the Chicago River circling approximately two-thirds of the Loop. But it wasn’t until 1924 that $8 million was committed for the east-west portion of the street and construction started (the north-south portion wasn’t constructed until the 1950s). Workers installed 598 caissons to a depth of 95 feet below the surface to support the upper deck for its 5700-foot length. Over 1,000,000 pounds of reinforcing rods and 116,000 cubic yards of concrete were used in the original construction, completed in 1926.
Today 65,000 vehicles travel on upper and lower Wacker Drive each day, eight cross-streets carry an additional 125,000 vehicles, 57 high-rise buildings depend on Wacker Drive for their primary access (including lower-level loading docks), and over 60,000 pedestrians pass by on the sidewalks. Columns and decks that were rehabilitated in the 1970s were, by the mid-90s, beyond repair, and design and engineering for total replacement were started. The increased use of de-icing salts in recent years was a leading cause of the deterioration. According to David Hurley, the senior project manager for Walsh Construction in Chicago, areas where water leaked through the structure were badly deteriorated. Dry areas remained in good condition, and samples from these areas tested at 4500-psi compressive strength. The caissons, with concrete compressive strengths of 5000 to 6000 psi, were in good condition and are being reused.
During the planning phase, several challenging goals were outlined for the project:
- The new viaduct structure must be designed to last a minimum of 100 years and to resist chemical assault from de-icers (the old Wacker Drive lasted 75 years with heavy use of de-icers over only the past 20 years).
- The lower Wacker Drive clearance must be improved from the existing 12 feet 6 inches to at least 13 feet 9 inches.
- Service to buildings on the street should be maintained during construction, including truck service to loading docks on lower Wacker—even during concrete placements.
- A new Chicago Transit Authority elevated bridge will be constructed over the Wacker Drive intersections with Lake Street and Wells Street.
The project was originally to be precast concrete, but, according to Hurley, “Walsh value-engineered the job for cast-in-place construction, saving the owner significant dollars and taking time off the schedule.” Today the $175 million east-west phase of the project is nearing completion.
The reconstruction is important to the concrete industry for these reasons:
- A unique quaternary high-performance concrete (HPC) mix (using Type I cement, Type F fly ash, ground blast-furnace slag, and silica fume) developed for the project was studied and tested for 2 years.
- Every load of concrete was tested and adjusted on the jobsite for air entrainment and slump.
- Continuous fogging was used during placement with a 7-day wet cure to control shrinkage cracking.
- Post-tensioned (PT) reinforcement was used to help eliminate cracking of the slab (and thereby prevent chloride penetration), and it allowed for a thinner viaduct section.
- A high level of organization was required to serve the needs of the buildings along the route, keep traffic flowing, and maintain pedestrian access during construction.
Developing a high-performance concrete mix design
The high-performance concrete used on this project was not evaluated by its strength but rather by durability and performance over time. Paul Krauss, senior consultant in the materials, science, and engineering group of Wiss, Janney, Elstner Associates (WJE), Northbrook, Ill., directed the process of evaluating and testing mixes and materials for the concrete in 1999 — 2 years before the start of construction. The performance criteria established for the concrete mix included:
- High resistance to freeze/thaw damage
- Low permeability
- High resistance to scaling
- Low shrinkage characteristics
- Compressive strengths of 4200 psi within 3 days for the post-tensioned areas
- Good workability
- Good uniformity
Special requirements were also specified for the materials used in the production of concrete, such as:
- Cement alkalinity not exceeding 0.6%
- Aggregates with low potential for alkali silica reactivity (ASR)
- Well-graded aggregates
To learn how both the structure and the concrete would perform, the Chicago Department of Transportation (CDOT) authorized construction of a full-scale bridge mock-up, which WJE tested to failure. The mock-up performed as expected and as designed— it was loaded to more than six times service load without failure. Stan Kaderbek, deputy commissioner/ chief engineer of the CDOT Bureau of Bridges and Transit, said that they were unable to collapse the mock-up—which inspired everyone with confidence in the design.
The quaternary mix replaced approximately 25% of the portland cement with other cementitious materials—silica fume, slag, and fly ash (see the table on page 28). Kevin Fitzpatrick, construction manager for Alfred Benesch & Co., Chicago, noted that the design strength was 6000 psi, but test results have averaged between 6500 and 9000 psi. He added that the mix was developed with durability having priority over strength—the contract actually specified a maximum strength not to exceed 9500 psi.
Because the project used a highperformance concrete mix containing silica fume and slag, curing procedures followed the IDOT specification for wet curing of bridge decks, which requires cotton mats. Doug Dirks, IDOT’s engineer of concrete and soils, stated that burlap can dry out too easily. With cotton mats and soaker hoses, curing can start immediately (as opposed to waiting for initial set), and slabs can easily be saturated for the required 7-day period, imparting the following benefits to the concrete:
- Better curing with more complete hydration of cement
- Minimum drying shrinkage, especially with HPC mixes
- Lower permeability
- Minimal risk of plastic shrinkage
On the Wacker project, workers fogged the air over the freshly placed concrete with water from pressure washers. A work bridge followed within 50 feet of the placing operation, and workers placed the cotton mats that were immediately saturated with water from pressure washers. Next, workers placed soaker hoses and then covered everything with a plastic vapor barrier. The mats were kept saturated and after 7 full days were removed.
Over 30,000 cubic yards of concrete will be placed by the time the job is complete. The largest single placement was 2100 cubic yards—completed in 10 hours—an average rate of 2 yards per minute. One pump, two truckmounted conveyors, and two back-outtype conveyors were used to accomplish this. Hurley stated that the decision to use pumps and back-out conveyors was based on accessibility and obtaining the optimum placement rate without compromising quality control.
To ensure the proper amount of air entrainment for both plastic and hardened concrete, Walsh carefully monitored the project, setting up its own quality control section to test every load of concrete for slump and air content. When necessary, the teams adjusted the loads onsite with an air-entraining admixture. Three testing stations of five persons each were required to keep up with the flow of trucks. Walsh expected some loss of air entrainment as a result of placing with pumps and conveyors, so at the start of each concrete placement, several trucks were sent early to determine the actual air loss in transit and through the conveying process. This allowed the remaining trucks to have the air corrected at the plant rather than waste time onsite. Hurley said this effort paid off because all the tests for entrained air in the hardened concrete came back good.
Both epoxy-coated steel reinforcing bars and post-tensioning cables were used for the project. Using post-tensioning as the primary structural reinforcement helped increase the vertical clearance on lower Wacker by decreasing the original thickness of the bridge deck from 2 feet to 131⁄2 inches and the height of the beams from 4 feet to 2 feet.
Workers installed polyethylene ducts on a 2.5-foot grid to house the PT cables. The position of the ducts was critical and was closely monitored by surveyors and ironworkers during concrete placement. According to Eddie He, the principal bridge engineer for J. Muller International (JMI), Chicago, crews started threading cables through the ducts the day after concrete placement— four-strand cable in the transverse direction, five in the longitudinal, and nine at the rib sections over column lines. The cables were tensioned to 160,000 pounds when the concrete reached 3200 psi—placing the concrete under 700-psi compression in the longitudinal direction and 450-psi in the transverse. To ensure that all concrete was in compression, “some cables ran on curved paths,” said He. “One duct made a 40-degree bend on a horizontal curve.”
The average concrete placement was 25,000 square feet. Hurley stated that there were five post-tensioning crews working when it was critical to accelerate the schedule. Grouting the strands within the ducts followed the tensioning operation and required as little as 2 days to complete for a placement. Fitzpatrick added that the goal for the post-tensioned design is for zero tension in the concrete slab and to fully balance the dead load.
Installing the latex-modified concrete overlay
To further ensure durability, CDOT specified a sacrificial latex-modified concrete wearing surface to be installed over the HPC deck. Fitzpatrick stated that the overlay will provide a toughwearing surface and have high flexural strength. Its primary purpose is to protect the structural concrete from salts and chlorides. Though the topping mix is less permeable than structural concrete, chlorides will gradually penetrate the 21⁄4-inch-thick topping, so this will be monitored and the topping replaced when chlorides penetrate to a certain depth. Krauss thinks this might happen every 20 years or more.
The overlay installation process starts with base slab surface preparation. To determine the best way to do this, Walsh performed “pull-out” tests to check for the bond strength between the topping and the structural concrete. From these tests it determined that the best way to remove laitance and lightly expose the sand aggregate in the mix was to hydroblast the surface of the base concrete with 30,000-psi water. This process occurs after slabs are posttensioned and the 7-day wet cure is complete. Just before placement, the slab is moistened with water (no standing water) and kept in that condition until the topping goes down. The mix, a standard IDOT latex grout, is batched from a mobile mixer. Workers use stiff brooms to scrub topping into the surface of the concrete for a good bond. It is screeded with the same Bid-Well screed used for the placement of the structural concrete. After placement, the surface is misted with pressure washers, and workers place cotton mats and soaker hoses to wet cure for 2 days before allowing the slab to dry for 2 additional days. Before opening a section of the street to traffic, the surface is grooved with diamond cutters to provide traction.
Electronic monitoring of structures to proactively monitor the maintenance needs of a structure over time is becoming more common. WJE installed several types of sensors in one intersection area to provide information on the health of the entire structure. Strain, chloride penetration, cracking, corrosion, strand breakage, sealer effectiveness, and mid-bay deflection are among the conditions that will be tracked.
Because timing was critical to the success of the entire project, Walsh used Collins Engineering, Chicago, to help engineer a forming system that could be easily moved from one location to the next. Hurley said that rolling false work allowed the formation of nine bay sections in the same time required for seven bay sections.
Collins vice president Michael Garlich explained that the “tabletop” forms that his company designed make use of towers that rest on the crash-wall projections of the new columns located on lower Wacker. The towers support a rolling deck form. The deck form is designed with adjustable framing members to accommodate both longitudinal and transverse deck slopes as they vary over the deck. These large deck forms also allow traffic to pass beneath. After a slab is cast, the deck form is lowered onto the towers, and the entire deck form is rolled onto the next bay, where towers have previously been positioned. Garlich credited Walsh for not having the deck form stick to the concrete when it was released. At each end of the project, the deck design and layout are irregular, and for these areas a conventional falsework arrangement was used. These sections required aisles wide enough for vehicular traffic. Due to settlement concerns, foundation load tests were run to ensure that final geometries were within the specified allowance.
Hurley is convinced that the success of this project is a result of the collaboration among all parties, especially the owner, CDOT. “The typical dayto-day activity takes an unusually high amount of coordination due to the location of the project,” he said. “The biggest task was accommodating the buildings and the pedestrians. We meet with building management every week to work out logistics.” Kaderbek adds that each time there is a phase change in the project, 20,000 copies of the notice are hand delivered to all offices affected by the change. Kaderbek also credited his staff of Denise Casalino, project director; Irwin Blumensaadt, construction manager; and Andy Balke, project structural engineer, for their commitment and foresight to proactively solve problems before they become stumbling blocks and for helping to keep the project on schedule and free from the complaints that normally accompany a project of this size.