Michael J. Abrahams, Parsons Brinckerhoff Quade & Douglas, Inc.
The Admiral Clarey Bridge was a design/build project for the US Navy to improve access to the largely undeveloped Ford Island in Pearl Harbor. Due to the need to provide an opening large enough to allow the passage of aircraft carriers while minimizing the impact on the nearby Arizona Memorial, a unique solution was developed that utilized a low-level fixed concrete trestle span combined with a floating concrete draw span.
The almost one-mile (1.6-km) long structure made extensive use of precast and prestressed concrete for both economy and durability. Given the aggressive marine environment of Hawaii, the Navy’s requirement for a durable structure included the use of high performance concrete (HPC) throughout the project. Except for some incidental concrete, all concrete was required to contain a minimum of 5 percent silica fume by weight of cementitious materials, and to have a maximum water/cementitious material ratio of 0.38. Other means to improve durability included increased concrete cover to the reinforcement, zero tensile stress in all prestressed concrete except for extreme load cases, a pipeline-type epoxy for coating reinforcing bars, and a maximum tricalcium aluminate content for the cement of 8 percent to improve sulfate resistance.
The concrete came from several sources. The pontoons and precast, prestressed concrete beams came from Tacoma, Washington. The precast piles and deck panels were produced in Hawaii. The cast-in-place concrete was produced locally. While there were initial concerns that concrete from such different sources would have non-uniform colors, this was not the case and the color was uniform throughout. In Hawaii, there are some limitations to achieving HPC strengths above 9000 psi (62 MPa). These include the Hawaiian DOT Standard Specifications limiting the cement content to 900 lb/cu yd (534 kg/cu m) and the type of available aggregates. All aggregate in Hawaii is manufactured and the angular shape of the aggregate takes away from the concrete workability. For the 24-in. (610 mm) octagonal prestressed concrete piles, the water/cement ratio was 0.36 or less and a special mix using 3/8-in. (10-mm) maximum aggregate size was needed because of the close spacing of the reinforcement.
In Hawaii, summer temperatures can exceed 90°F (32°C) with a hot sun and prevailing trade winds. We were aware of the lack of bleed water and finishing problems that are associated with the use of silica fume concrete. To sort out these issues, a test pour was used to demonstrate finishing techniques. A 7-day wet cure was mandatory for all concrete. The use of fogging and wind screens was recommended.
Finishing of HPC provides a challenge to be overcome in the field. The fresh concrete mix has a sticky texture and develops very little bleed water. Misting or fogging were required to provide surface moisture when finishing. Otherwise, floating had a tendency to cause tearing of the surface. A curing compound was applied to try and retain the moisture and to allow the concrete to set prior to starting the wet cure. The mixes also had varying consistency when placed. Concrete from the same truck or from one truck to the next truck had a wetter or drier mix consistency.
In Tacoma, the concrete for the pontoons had similar problems with a lack of bleed water. The use of fly ash in the mixture helped to promote bleed water and ease the finishing problems. An evaporation retarder was sprayed on the surface of the wet concrete after screeding to help retain the surface moisture.
Further Information
For further information, see: Abrahams, M. J. and Wilson, G. “Precast Prestressed Segmental Floating Drawspan for Admiral Clarey Bridge,” PCI JOURNAL, July/August 1998, pp. 60-79.