S.L. Kaderbek, Chicago Department of Transportation
Chicago’s Wacker Drive is a two-mile (3.2-km) long, two level roadway originally built entirely of reinforced concrete, used by 60,000 vehicles a day, and bracketing the north and west sides of Chicago’s downtown “Loop.” The older east-west section was built in 1926 and the newer north-south section was built in the early to mid 1950s. In late 1998, as the Chicago Department of Transportation’s (CDOT) Bureau of Bridges and Transit was beginning final design for the new Wacker Drive Viaduct, the specific characteristics of the materials to be used for the construction came under debate. The proposed replacement superstructure was designed to be a post-tensioned concrete slab resting on discrete concrete columns. There was also a strong consensus that a segmental type design might speed construction. Considering the cost of the replacement structure and the inconvenience that the reconstruction would cause, it was CDOT’s desire to build a structure that would last for 100 years. The goal for the high performance concrete (HPC) was a minimum service life of 75 years.
CDOT strongly believed that the use of HPC in combination with the post-tensioned structural design would best ensure a long-lasting structure. Chicago’s environment is extremely harsh, particularly in the winter. Temperatures vary widely above and below freezing. These freeze-thaw cycles, combined with an aggressive salting program to control snow and ice, reduce the life span for conventional concrete bridge structures. Developing a concrete mix that could resist chloride penetration, did not achieve excessively high compressive strengths, and could be used in both post-tensioned segmental and traditional cast-in-place construction was desired.
CDOT, using FHWA’s Innovative Bridge Research and Construction (IBRC) Program funding, engaged consultants to research HPC mixes already in use around the country and, if necessary, to develop a mix design specifically for this project. Constituent materials were screened to determine their durability and susceptibility to alkali-silica reaction. Accelerated testing was performed on “off the shelf” mixes provided by producers and new mix designs to ensure that long-term performance would be satisfactory. Finally, a full-scale mock-up of the superstructure was built to test actual performance under load and to refine construction techniques.
Research showed that previous HPC mixes did not have all the characteristics that CDOT was seeking. Consequently, a quaternary HPC mix using low-alkali cement, Class F fly ash, silica fume, and ground granulated blast-furnace slag as cementitious materials was developed.* The concrete has a moderate compressive strength of 6,000 to 9,000 psi (41 to 62 MPa) at 28 days, develops strength quickly to allow stripping of formwork or post tensioning, has excellent placement and consolidation characteristics, and, for the most part, uses locally available materials. The mix stays workable without the addition of retarders over a wide range of temperatures. Concrete was placed with ambient temperatures as high as 80°F (27°C) without the use of a retarder. Attention was also paid to the curing of the concrete once it was in place. Wet cotton blankets, soaker hoses, and plastic sheeting were placed on the concrete and the concrete was kept wet for seven days. The result is a well-hydrated deck that is free of shrinkage cracks.
CDOT believes that the time and cost involved in the development of this HPC mix was a good investment. The HPC, combined with the posttensioned deck design, will ensure that the new Wacker Drive Viaduct will be faithfully serving the citizens of Chicago into the twenty-second century.
Editor’s Note
This edition of HPC Bridge Views focuses on one project from the perspectives of the owner, construction manager, general contractor, and concrete supplier.
*See HPC Bridge Views, Issue No. 19, January/February 2002.