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HPC was specified for both strength and durability.
North Avenue Bridge Reconstruction – A Modern High Performance Structure
Alison Smith, URS CorporationThe North Avenue Bridge spanning the Chicago River has been a part of Chicago history for over 100 years. The original structure was one of the oldest bridges in the city—a steel trunnion bascule bridge, built in 1907. The bridge had significant historical value, but was in a state of deterioration and did not meet the needs of the approximately 30,000 vehicles traveling over the bridge each day between two rapidly changing neighborhoods. The design for a new bridge, with a comparable degree of structural distinction and durability was initiated by the Chicago Department of Transportation (CDOT).
The new structure is a $25 million, suspension and cable-stayed hybrid bridge that will have double the traffic capacity. The 10-in. (255-mm) thick deck and 4-ft (1.22-m) deep longitudinal beams are post-tensioned, high performance concrete (HPC). The exposed portion of the gravitational anchor blocks, used to counter-balance the center of the bridge through the suspension cable, is HPC as well. The beams and deck were cast on shoring until the suspension cable and cable stays were installed and stressed.
The CDOT requires extensive prequalification and rigorous testing of the proposed HPC from each supplier. The mix used was specifically formulated for use on the North Avenue Bridge Reconstruction Project.
The HPC for the project had two quality components: strength and durability. The specification required a 28-day compressive strength between 6000 and 9500 psi (41 and 66 MPa). The average 28-day strength of the concrete was 7400 psi (51 MPa). Test results confirmed that the mix design met the specified properties for resistance to freeze-thaw cycles, salt scaling, shrinkage, chloride ion penetration, and chloride permeability. These characteristics make the mix superior to conventional concrete, in that the concrete is designed to withstand a severe environment and sustain a longer service life.
To achieve the required strength and reduce shrinkage, the specified curing method involved immediately placing cotton mats over the finished concrete and soaking the mats with a mist, placing soaker hoses, and then polyethylene sheeting for a curing period of 7 days. The concrete temperature was monitored to ensure that it was between 50 and 150°F (10 and 66°C). The concrete was placed in the early morning hours so that ambient air temperatures were not above 80°F (27°C) during the placement. The temperature requirements are very important due to less excess available free water than a standard concrete mix and to prevent plastic shrinkage cracking and drying or thermal shrinkage. Ground-granulated blast-furnace slag (GGBFS) and silica fume were used to help achieve the strength and durability properties.
Concrete Mix Proportions
The above table lists the concrete mix proportions as 605 lb of Type I portland cement, 120 lb of Grade 100 GGBFS, 30 lb silica fume, 971 lb of fine aggregate, 1844 lb of coarse aggregate, and 264 lb of water for a water-cementitious materials ratio of 0.35. Air entraining, retarding, and high-range water-reducing admixtures were used as required.
Innovative Construction Methods
Due to river traffic clearance limitations, formwork under the bridge was not feasible, making an off-site construction method more practical. The 109-ft (33.2-m) long center span was, therefore, shored and formed on three barges. The HPC 10-in. (255-mm) thick deck, the 4-ft (1.22-m) deep beams, and the sidewalk were monolithically placed with concrete pumped from the shore. The placement was a challenge because of balancing issues created by the buoyancy of the barges. A detailed placement procedure and constantly surveyed deck elevations ensured a uniform and safe placement. Additional, temporary post-tensioning was added to the beams to provide enough rigidity to lift the center span into place. The 800-ton (7.2-MN) HPC center span was floated up the river, adjusted into position, and jacked up from a temporary structural system consisting of launching trusses sitting on temporary piers. A total of sixteen 100-ton (890-kN) jacks lifted the center span into its final position.
The high performance concrete system is meant to provide a structure that will have a 100-year service life. Long-term, the extended bridge life provides a structure that will be less expensive to maintain and provides Chicago with a unique, signature bridge.
For further information, please contact the author at email@example.com.
The method of construction of the center span of this bridge is an excellent example of a prefabricated bridge system that uses accelerated construction to minimize the impact on river traffic.
The center span was cast on barges and floated into position.
HPC Bridge Views, Issue 48, Mar/April 2008