A major artery in the heart of Chicago is undergoing double-deck surgery, bolstered by a special concrete mix designed to keep Wacker Drive from crumbling for the next generation.
|HISTORICAL Once supported on timber and steel (above), Wacker Drive will be reborn as high-performance concrete showcase. (Photo courtesy of Chicago Dept. of Transportation)|
The two-mile-long, double-deck viaduct section being rebuilt carries about 60,000 vehicles a day. Eight bascule bridges intersect with the upper roadway, as do two elevated transit lines–and 60,000 pedestrians on the Loop. Hammered by Chicago's brutal weather, the original road "was falling apart" from freeze-thaw cycles, road salts and wear, says Denise Casalino, assistant project director for the city's Dept. of Transportation. Despite a 1970s patch-up, the deck continued to show failures, forcing a 15-ton limit on the roadway.
The reborn Wacker Drive is expected to hold up much more sturdily for the next 75 years. Chicago DOT hopes to echo some of the construction achievements in the drive's past by utilizing special building methods and a premium mix of high-performance concrete that emphasizes durability.
In the mid-1990s, the city conducted a three-year study, finding substandard merge lanes, clearances and intersections (ENR 3/30/98 p. 16). The project received federal funding, $10.5 million in federal discretionary aid and a research grant for high-performance concrete mixes.
Walsh Construction, Chicago, holds all three contracts on the reconstruction, divided into sections of $75 million, $43 million and $55 million. The entire two-year project is on schedule for completion by November, says David Hurley, Walsh senior project manager. Contract A, for the $75-million, 2,065-ft-long middle section, is nearing completion and will be open to traffic by Halloween, says Hurley.
|TIGHT FIT Columns are built beneath active elevated tracks while allowing crossover traffic. (Photo courtesy of Chicago Dept. of Transportation)|
The contractor faces $30,000 a day in liquidated damages if it fails to meet various milestones. Fifty-seven high-rise buildings flank the drive, and the lower road provides access for service vehicles. Temporary shoring for construction impedes service entrances, so meeting the milestones is especially crucial.
Walsh has avoided penalties so far and is aiming for a $1-million early completion bonus, says Kevin J. Fitzpatrick, project construction manager for construc- tion manager Alfred Benesch & Co. There are $500,000 in change orders but the job is only 1% over budget, he says. There are no major claims.
The city had never used an outside CM before, notes Fitzpatrick. But due to Wacker Drive's high profile, the city wanted its resident engineers to deal with onsite issues and let Benesch managers handle cost, submittal and coordination issues. Weekly meetings to discuss weekend closures and milestones helped ease the pain for local businesses. The CM had five days to answer design questions.
"It was challenging to fit into the role, working with consultants that we might otherwise compete against," notes Timothy P. Schmidt, a Benesch quality assurance engineer. "The city kept the designers involved even into the construction phase." Because of the uncommon use of post-tensioned design and a novel concrete mix, "it was nice to have the consultants available," he says. Long Beach, Calif-based Earth Tech served as lead designer, with J. Muller International–acquired last year by Earth Tech–serving as lead bridge designer.
IN THE MIX. The project features the first major high-performance concrete (HPC) mix in the state, says Stan Kaderbek, deputy chief commissioner for the city DOT's bureau of bridges and transit. When the agency was studying options for Wacker Drive, he read about New York state colleagues using HPC. "It seemed tailor-made for our environment," he says.
Local firm Wiss Janney Elstner Associates Inc. researched HPC mixes used in other states. Working with the University of Illinois at Chicago, WJE developed and tested performance criteria for the project. "A lot of the HPC in the past has been done related to high strength," says Paul Krauss, senior consultant with WJE. "In this case, that was not what we needed. HPC can be defined by durability too....We were looking for a durable mix that can be placed without a lot of problems."
The firm looked at some 40 projects, different state criteria and federal guidelines. Mixes were developed and tested for criteria including reaching 6,000 psi in 28 days, while attaining a maximum compressive strength of 9,500 psi. "We didn't want a mix that had very, very high strength," notes Krauss. "That can cause...cracking tendencies and placeability issues."
Other requirements included air voids with maximum spacings, a moderately low water/cement ratio and consistency of aggregates, materials and admixtures over time. "Once we had a sense of the specifications for the HPC, we did trial batches to make sure we could achieve those properties," says Krauss. The city then conducted a prequalification process for concrete suppliers who could choose from the recommended mix design or from their own. Fourteen mixes were tested. Bridgeview, Ill-based Prairie Group won the supply contract using the city's specs. "It's the most difficult job we've ever done," says Gary Hall, quality control technician for Prairie.
|TRAVELER A customized rolling framework system speeds cast-in-place sections. (Photo courtesy of Chicago Dept. of Transportation)|
Air entrapment within the mix was particularly troubling, notes Kaderbek. "There was a lot of rejected concrete," he says. "Prairie is a good supplier...but they were used to producing standard concrete. This was a challenge." The mix includes portland cement, 10% Class F fly ash, 5% silica fume and 15% ground-granulated blast furnace slag, says Krauss. Class F fly ash ties up alkalis in the cements that could cause future problems and increases impermeability, adds Jim Connolly, WJE principal.
The mix is pumped onto the deck and placed with a finishing machine. After hardening, the concrete is abrasively blasted. A 2.75-in.-thick latex layer of concrete is placed on top to act as a short-term sacrificial layer that soaks up winter road salts and chemicals. It can easily be milled off and replaced every 25 years or so.
Hurley says 25,000 cu yd of concrete is being poured on the job, up to 220 cu yd an hour. Pours reached up to 240 ft long and 110 ft wide. HPC pours cannot be done in the winter, due to quality control issues. Hurley says the HPC mix costs about twice as much as regular concrete, but the city hopes life-cycle cost savings will more than make up for it. "As [the mix] ages, it will continue to become more impermeable to chlorides," says Krauss.
MAINLINE. Some 30 million ft of lumber and 1 million steel reinforcing rods supported the original Wacker Drive, with about 600 caissons excavated to an average of 95 ft. On Aug. 10, 1925, workers poured 1,080 cu yd of structural concrete in one nonstop shift, starting at 6 a.m. and ending at 1:30 the next morning. According to the city DOT, the feat set a record that was broken just 12 days later when crews placed the same amount in 6.5 hours for the elevated structure.
Last year, Brandenburg Industrial Service Co., Chicago, began demolition for Contract A, pulling out the deck, columns and original timber pilings. The segment follows the original route and maintains some of the road's existing foundation. Walsh value-engineered a cast-in-place method to replace original precast segmental design when the start of construction slipped from fall to spring (ENR 8/27 p. 14). "It saved half a million dollars for the city," says Hurley. Casalino adds that "December bids meant there was no winter in which to precast."
|DECK POURS Concrete mix achieved a minimum of 6,000 psi and a maximum of 9,500 psi. (Photo courtesy of Chicago Dept. of Transportation)|
The contractor, with local firm Collins Engineering, developed a rolling formwork system that moves steel falsework towers on rollers from section to section. First, the bases of existing foundations were chipped away to expose the rebar, which was preserved, says Irwin Blumensaadt, city construction manager. Using decades-old documents, "we had to assume the amounts of reinforcing steel, then often found that the configuration was different or the damage extensive," notes Fitzpatrick.
Once foundations were secured, square blocks measuring 4 ft, 6 in. were placed as new caps. When new columns were poured, crews set up shoring towers around them to help place the steel framework for the new deck. After the concrete was pumped in for the cast-in-place closure pours, post-tensioned cables were pulled to 210,000 lbs of tensile force.
Each section took about 13 weeks from demolition to reopening, says Hurley. "Concrete pours took as little as six hours or up to 12 hours" and were done mostly at night, to minimize traffic closures. Eight fixed span sections of the cross streets were repaired. The city also spent $6 million to save the historic limestone facade from the old viaduct and reapply to the new one. The new deck is cast with imbedded areas to receive the facade.
The remaining major piece in Contract A is the replacement of a Chicago Transit Authority truss bridge in March. Delgado Erectors Inc., Dolton, Ill., plans to use a motorized cart system that can pick up a new 400-ton steel truss and tracks together and bolt it into place with a quarter-inch tolerance. The work is scheduled to be done in one weekend; there is a $1,000-per-minute late penalty, notes Hurley.
|HEART OF CHICAGO Reconstruction of Wacker Drive is flanked by high rise buildings and river. (Photo courtesy of Chicago Dept. of Transportation)|
Utility relocation work cost some $4 million, adds Hurley. This included a 24-in. water main replacement and a new utility corridor about 5 ft wide and 5 ft deep.
Contracts B and C call for construction of new columns and new, relocated foundations to replace the monolithically poured deck and original octagon- shaped columns. Work started in October on contract B, the $43-million, 1,000-ft-long section, using steel ribs to transfer loads onto the new columns, not the beams. The new post-tensioned concrete allows for a thinner deck, widening clearances by about 10 ft.
Demolition has just begun for the 1,400-ft, $55-million C contract, which shifts the viaduct about 40 ft to modify its sharp curve. "Some of the most intensive work is on the C contract," says Hurley, noting that the curve doesn't allow for traveling formwork. When done, the drive will have two 11-ft lanes and a 10-ft turn lane in each direction, meeting FHWA standards.
Kaderbek says the city is standardizing the HPC design for more bridge decks. Krauss adds "reviews have been excellent....This particular mix should not be blindly applied to other decks, but the ideas would be similar."
Chicago officials plan post-Wacker Drive reconstruction development of the riverfront, including reconfiguring the interchange with Interstate 290. On the lower roadway, the city is continuing a study on construction of a row of shops and a floating riverwalk. Ideally construction would begin in 2003, says Kaderbek.