Launch Slideshow

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Building a 100-year bridge

Building a 100-year bridge

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    Rebuilding Upper and Lower Wacker Drive in Chicago, workers place high-performance concrete on a 2,800-square-foot upper deck placement. The white plastic ducts house post-tensioning cables. Specifications call for a 100-year service life. Photos: Joe Nasvik

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    A worker sets up the form superstructure.

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    A worker puts the finishing touches on a beam form. Notice the close truss spacing for the deck spanning a traffic lane below.

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    Testing and quality control are seen as vital to the project's success. Flood Testing Laboratories represented McHugh Construction, making sure ready-mix trucks met specifications before unloading.

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    The top 2 inches of the deck is latex-modified concrete to provide an impermeable surface that resists chloride. This layer is intended to be sacrificial and will be replaced as needed.

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    The high-performance concrete used on this project included silica fume. Because slight amounts of water loss in silica-fume concrete results in cracking, curing blankets were placed and saturated shortly after the concrete was placed.

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.

Testing

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.

WEB EXTRA

To read about previous work done on these streets, see “Rebuilding Chicago's Upper and Lower Wacker Drive” in the November 2002 issue of CONCRETE CONSTRUCTION, click here.