Mark A. Gaines, Washington State Department of Transportation and Michelle L. Tragesser, Parametrix
The Hood Canal Bridge provides an important link between the Olympic and Kitsap Peninsulas in northwestern Washington State. The bridge is the longest floating bridge over saltwater in the world and has a movable 600-ft (183-m) wide draw span that provides access for marine traffic vital to national security. Extensive use of prestressed concrete and high performance materials are required to withstand the large tidal fluctuations, strong winds, and wave action of Hood Canal’s harsh marine environment.
The most significant project work item is constructing 14 new floating prestressed concrete pontoons to replace the existing east-half structure. The high performance concrete (HPC) pontoons are being constructed in four separate cycles at a graving dock in Tacoma, Washington. The largest of the cellular box structures is 60 ft (18 m) wide, 18 ft (5.5 m) tall, and 360 ft (110 m) long. They are all heavily reinforced with both conventional reinforcing steel and longitudinal, transverse, and vertical post-tensioning tendons.
Mix Selection
The HPC used for the pontoons was originally developed for the I-90 Lacey V. Murrow (LVM) floating bridge project in the 1990s. This mix design, now used by the contractor, Kiewit-General Joint Venture, is as follows:
The above table lists the concrete mix proportions as 625 lb of Type I / II cement, 100 lb of Class F fly ash, 50 lb of silica fume, 1350 lb of fine aggregate, 1680 lb of coarse aggregate, 255 lb of water, and 0 to 80 fl oz of high-range water-reducing admixture for a total water-cementitious materials ratio of 0.33.
The approximately 31,000 cu yd (24,000 cu m) of concrete required for the pontoon construction must adhere to the contract specifications by achieving a minimum 28-day compressive strength of 6500 psi (45 MPa) and a maximum 56-day permeability of 1000 coulombs. To date, these standards have been exceeded. The actual 28-day compressive strengths are approximately 11,000 psi (76 MPa) and the 56-day permeability of this mix is less than 800 coulombs.
Placement
Early in the project, the contractor realized that the LVM concrete placement in the pontoon walls would be challenging because the walls are up to 21 ft (6.4 m) tall, only 10 in. (255 mm) thick, and heavily congested with reinforcing steel and post-tensioning ducts. To improve concrete placement and consolidation, the contractor requested to add more high-range water-reducing admixture and exceed the previously defined maximum 9-in. (230-mm) slump. After conducting a series of qualification tests and constructing a mock-up pontoon wall, the contractor successfully demonstrated that this “new” mix could be placed without segregation. Testing and acceptance of this concrete was accomplished using the spread test that is common with self-consolidating concrete. A spread of up to 23 in. (585 mm) with a visual stability index of 0 or 1 per ASTM C 1611 was allowed. To date, approximately 70 percent of the total LVM concrete quantity needed has been placed with virtually no consolidation or segregation issues.
11,000 psi (76 MPa) was used to cast the pontoon walls.
Curing
One of the challenges encountered during pontoon construction was crack formation in the wall concrete. On the first cycle of pontoons, vertical cracks were observed that extended the full height of the wall. The cracks had a spacing of 8 to 10 ft (2.4 to 3.1 m), average widths between 0.006 and 0.030 in. (0.15 and 0.76 mm), and extended through the full thickness of the wall. Washington State Department of Transportation (WSDOT) investigated and found that the likely mechanism for the crack formation was thermal expansion and contraction of the wall concrete in conjunction with restraint provided by the base slab. At the time the wall concrete was placed, the base slab concrete was at an ambient temperature of about 60°F (16°C).
The high cementitious materials content of this concrete resulted in a high heat of hydration and associated thermal expansion. Within 24 hours of placement, the wall concrete reached its peak temperature and also achieved final set. As the wall concrete slowly cooled, thermal contraction was resisted by the base slab, creating tensile stresses in the wall that ultimately led to cracking. After the first cycle of construction, WSDOT allowed the contractor to remove forms and apply curing water after only 12 hours. This lowered peak temperatures in the concrete and reduced thermal expansion and contraction. During subsequent cycles, the number of cracks was reduced by about 10 percent and the cracks that did form were narrower. This improvement decreased the amount of crack repair required.
Cracks with a width larger than 0.006 in. (15 mm) after post-tensioning was completed were repaired with epoxy injection. Cracks measuring 0.006 in. (15 mm) or less were sealed with a crystalline waterproofing product.
The main lesson learned from the cracking in the first cycle of pontoon construction was the importance of early removal of the wall forms and application of water on the surface to lower the peak concrete temperatures. It was also learned on the first cycle that the longitudinal post-tensioning closed these cracks by 50 to 75 percent.
Quality Product
Lessons learned during each stage of pontoon construction continue to be shared with the project team and applied in order to facilitate constructing a quality product. To date, 12 pontoons have been successfully constructed and are floating in the Puget Sound near Seattle. These pontoons are currently being joined together to form the movable draw span portion of the bridge. Work continues to move forward on schedule to meet the May-June 2009 east-half bridge replacement date.
Further Information
For further information, please contact the authors at [email protected] or [email protected].