Alan R. Phipps, FIGG Bridge Engineers, Inc.
Photo: Tim Davis, FIGG
The new I-35W bridge across the Mississippi River in Minneapolis features high performance concrete (HPC) for all concrete components of this post-tensioned, box girder structure. The bridge accommodates 10 lanes of traffic and is designed for future traffic demands including rapid bus or light rail transit and a suspended pedestrian bridge.
The Minnesota Department of Transportation’s (Mn/DOT’s) vision for the bridge included a minimum design service life of 100 years—one third longer than for most bridges. A corrosion protection plan was developed by FIGG to frame the design strategy for achieving this requirement. Some of the key elements of the strategy were:
- Concrete bridge with the superstructure post-tensioned in two directions to provide a residual compressive stress
- High performance concrete containing silica fume and fly ash for low permeability
- Integral concrete wearing surface
- Durable post-tensioning system incorporating polyethylene ducts, prepackaged thixotropic grout, and multiple layer anchorage protection
- Structure health monitoring system, including corrosion potential sensors in the deck
All concrete compressive strengths were specified at an age of 28 days. However, for the mass concrete in the footings, abutments, piers, and portions of the superstructure, the special provisions allowed 80% of the specified strengths at 28 days provided 100% of the specified strengths were achieved at 56 days and no superimposed loads were placed on the element until it had reached the specified strength. Four basic types of HPC mixes were developed for the project as part of the overall corrosion protection strategy to meet the unique requirements of individual bridge elements.
Drilled Shafts
A total of 40 shafts, with diameters of 7 and 8 ft (2.1 and 2.4 m) and depths up to 95 ft (29 m), support the main bridge piers. In addition, sixty-nine 4-ft (1.2-m) diameter shafts, up to 27 ft (8.2 m) long are used for support at the north abutment and at the 2nd Street overpass north of the main bridge. To assure monolithic high quality concrete with tremie placement into slurry-filled heavily reinforced shafts, self-consolidating concrete (SCC) was used. This was the first large-scale use of cast-in-place SCC for Mn/DOT. To control temperatures in the large diameter shafts during curing, pozzolans such as fly ash and slag were incorporated as the majority of the cementitious material. This reduced the heat of hydration by approximately 50%.
Substructure
The concrete mixtures for the footings and piers were proportioned for mass concrete and durability through the use of fly ash and slag. This was important since the least concrete dimension was 13 ft (4 m) for the main pier footings and 8 ft (2.4 m) for the main pier columns. Also, because the substructure elements are adjacent to the Mississippi River, they are subject to significant moisture exposure. Mass concrete plans were developed for the footings and piers. The plans included thermal monitoring, concrete mix modifications (cementitious materials, chilled water, and cooled aggregates), placement sequence, lift heights, form insulation, and use of internal cooling pipes. The combination of a high quality monolithic element and low permeable concrete with these mixes will provide a durable substructure. Average strengths for these mixes were 12 to 23% above the two design requirements of 5000 and 5500 psi (34 and 38 MPa) for the footings and 4000 psi (28 MPa) for the piers.
Superstructure
The HPC mix used for the superstructure was based on a mix that had been used extensively by the concrete supplier in the construction of parking structures in Minnesota. The concrete was designed for a 28-day compressive strength of 6500 psi (45 MPa) and included fly ash and silica fume for low permeability. The corrosion protection plan required the superstructure concrete to have a rapid chloride permeability not exceeding 2000 coulombs at 60 days. The use of pozzolans will ensure that the permeability continues to decrease over time. The use of silica fume increases the impedance of the concrete, thereby inhibiting any corrosion that may occur in the future. The mixture was required to have low shrinkage although a maximum value was not specified. This is important in that it reduces the stresses due to shrinkage resulting in fewer cracks in the structure.
Because of the silica fume, the superstructure mix was stiffer and more of a challenge to place and finish properly. Flatiron-Manson overcame this challenge by staging a series of slab test placements off-site with the concrete mix, crews, and equipment that would be placing the concrete. Once the techniques for working with the mix had been successfully refined, concrete placement on the actual bridge superstructure proceeded easily.
Gateway Features
Two gleaming white concrete sculptures, each comprised of three wavy columns, tower 30 ft (9.1 m) high at each end of the bridge in a vertical interpretation of the universal symbol for water. These symbols serve as markers to identify that travelers are crossing the Mississippi River, a fact that was not evident on the previous bridge. The sculptures were precast using an SCC mix that included a self-cleaning, pollution-reducing photocatalytic cement. The surfaces destroy atmospheric pollutants, resulting in cleaner surfaces with little maintenance. The monuments are the first high-profile North American application of this cement. The SCC mix, with a spread of 20 to 24 in. (510 to 610 mm) and a design strength of 6500 psi (45 MPa), resulted in a marble-like, smooth white finish to the concrete surface. With a low water-cementitious materials ratio, air entrainment, and a rapid chloride permeability less than 1500 coulombs at 28 days, the monument will also be a durable feature in the severe environment adjacent to the I-35W roadway.
Conclusion
High performance concrete was a key element in the strategy for producing a high quality structure for the new I-35W bridge with a service life of at least 100 years. The committed efforts of Mn/DOT, FHWA, Flatiron-Manson, Cemstone Concrete Products, and the design team helped ensure that this important bridge was in service more than 3 months ahead of schedule.
Further Information
For further information about the design of this bridge, contact the author at [email protected] or 850-224-7400. A commemorative book about the project is available for purchase at www.figgbridge.com. All proceeds from the sale of the book benefit the Admissions Possible and the Architecture, Construction, and Engineering (ACE) mentor programs.