Stan L. Kaderbek, Chicago Department of Transportation and Sharon L. Tracy and Paul D. Krauss, Wiss, Janney, Elstner Associates
Wacker Drive is a major two-level viaduct bordering the north and west sides of Chicago’s downtown “Loop.” The existing 75-year old structure is being replaced due to severe corrosion of the embedded reinforcing steel and spalling of the concrete cover. The columns and deck of the new structure are being built using cast-in-place high performance concrete (HPC). The deck is post-tensioned HPC with a latex-modified concrete overlay.
Reconstruction of Wacker Drive is a joint project by the Chicago Department of Transportation, the Illinois Department of Transportation (IDOT), and the Federal Highway Administration. A lengthy process for prequalification of concrete materials and suppliers began in 1999, when a plan was initiated requiring testing and evidence that raw materials and HPC mixes would exhibit properties to ensure long-term durability, quality, and performance in the field. These requirements provided the groundwork for the HPC specifications.
The requirements for the HPC focused on durability, not strength. The minimum nd maximum specified concrete compressive strengths at 28 days were 6000 and 9500 psi (41 and 66 MPa), respectively. The water-cementitious materials ratio was specified as 0.36 to 0.38. The upper strength imit and the moderately low water-cementitious materials ratio were specified to educe the risk of cracking or placement problems that often accompany very high strength concrete. Durability requirements included testing for freezing and thawing resistance, chloride permeability, chloride ion penetration, deicer scaling resistance, and shrinkage. The HPC was also proportioned to be easily placed using conventional concreting practices. Two mix designs were suggested in the specifications, with a contractor-designed mix as a third option. Tables 1 and 2 on page 4 list the HPC mix performance and durability criteria. The HPC specification also included high performance raw materials. The portland cement had to be an ASTM C 150 Type I or I/II and IDOT approved, meeting requirements for total and water-soluble sulfate contents, total alkali content, fineness, and early stiffening behavior. The coarse and fine aggregates were required to be IDOT approved, Class A, alkali-silica nonreactive, and with water-soluble chloride contents less than 0.04 percent. Class F fly ash, silica fume, and ground granulated blast-furnace slag (GGBFS) were included in the preferred mix design.
A prequalification process was followed whereby interested concrete suppliers submitted their raw materials for testing. In total, ten cements, five coarse aggregates, six fine aggregates, five fly ashes, one GGBFS, and three silica fumes were tested. Concurrently, a total of 14 HPC mixes were batched and samples were cast at the plants for durability testing.
Many potentially harmful issues were rought to light by the testing. For example, many of the fine aggregates were found to be potentially alkali-silica reactive, containing moderately high amounts of potentially deleterious chert. Many local cements and some fly ashes were found to have high alkali contents. The HPC mix testing emphasized the importance of having good air void systems in the hardened concrete for freeze-thaw durability. Some suppliers’ concrete exhibited poor performance in the chloride ponding and chloride permeability tests, had high shrinkage, or did not meet the strength requirements. It was clear that specifying “high performance” concrete alone was not adequate to achieve the performance goals. Verification testing of concrete cast in plant conditions using job materials was required.
A list of acceptable raw materials and suppliers was generated from the prequalification program. After the contract award, the contractor was required to place a large trial slab using concrete pumps and other bridge-finishing equipment to gain experience placing the HPC mix. Deck curing required 7 days exposure to water-soaked cotton mats covered with plastic sheeting
Finally, a quality control and quality assurance plan specific to the HPC was developed. It specified that the contractor was responsible for much of the quality control testing. Representatives of the City of Chicago performed quality assurance tests. Job-site testing included frequent monitoring of the plastic concrete properties. Hardened concrete specimens were routinely tested for compressive strength, coulomb values, and air void parameters. Control charts and limits were maintained on water-cementitious materials ratio, aggregate gradation, air content of plastic concrete, and strength.
The placement of the HPC columns and decks at Wacker Drive has been very successful. The mix, as given in Table 3, has proved to be very workable, easily consolidated, and lacking early age cracks. Laboratory data indicate the HPC has the long-term durability characteristics for which it was designed. The success has been due to careful planning, testing, and a commitment to high performance. This commitment should result in durability and a lifetime of 75 years. High performance cannot be achieved solely by specification. Prequalification testing of raw materials, testing of HPC mixes, and thorough quality control and quality assurance programs are necessary.
Further Information
Further information on the project may be obtained by contacting Sharon Tracy at 847-272-7400 or e-mail at [email protected].