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Concrete with a specified compressive strength of 10,000 psi (69 MPa) was used in the arch.
Photo: Central Federal Lands Highway Division, Federal Highway Administration
The Colorado River Bridge at Hoover Dam—Overview
Dave Zanetell, Federal Highway Administration
High performance concrete (HPC) is at the core of the successful construction of the Hoover Dam Bypass. The nearly 5-mile (8-km) long project includes eight separate and significant bridges including the centerpiece Colorado River Bridge at Hoover Dam and officially designated the Mike O’Callaghan-Pat Tillman Memorial Bridge. This monumental 1905-ft (581-m) long structure includes twin rib arches that are the longest in the western hemisphere. The arches span 1060 ft (323 m) and rise nearly 900 ft (274 m) above the Colorado River. The comprehensive $240 million bypass project will open to the public in November 2010 without dispute or claim and within the original budget.
Project Organization and Structure
The Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration serves as the project manager and leader of the multi-agency and consultant teams. The multi-agency team includes representatives from the Arizona and Nevada Departments of Transportation, National Park Service, Bureau of Reclamation, and the Western Area Power Administration. The CFLHD is responsible for the cradle-to-grave management of all the design, consultant, contracting, and construction activities. The overall project includes six contracts for roadway and bridge structures, including the Colorado River Bridge at Hoover Dam. HDR, Inc. serves as the managing lead of the collective consultant team with T.Y. Lin International as the engineer of record for the Colorado River Bridge. A joint venture of Obayashi Corporation and PSM Construction USA is the prime contractor for the bridge.
Cradle-to-Grave Approach to Management of Risks and Defining Issues
The incorporation of HPC for the bridge required application of the CFLHD’s construction project management approach. Through a two-step process, the team worked together to further assess the feasibility of using HPC. Special attention was given to specification development, feasibility of the final design requirements, and staged pre-bid field reconnaissance to increase competition and reduce bid-day contingencies.
The strategy was to complete many of the significant time-consuming activities such as material source identification and mix viability assessments before bidding because it would have been impractical to complete these during the bidding period. This pre-bid investigative effort, combined with active sharing of information and industry outreach, served to level any competitive advantage and increase competition. It was important to demonstrate that HPC could be batched, cooled, delivered, and placed successfully on this unique project in extreme conditions. This effort also served to validate the design parameters.
The strategy for using concrete involved a series of planned activities that linked design optimization and final requirements with industry capability and the risk associated with specific project requirements. This approach served to integrate requirements, which are typically considered separately, throughout the development, procurement, and construction processes. This strategy further enabled cost-savings through optimization of the design, increased industry competition, and reduced project risks and uncertainties. The overall result was a saving of millions of dollars.
The two-step concrete feasibility and specification development program was planned to ensure quality construction within budget constraints. These were considered to be complementary rather than opposing requirements. To achieve this, a cradle-to-grave approach that integrated all design, technical, contracting, and construction aspects related to the concrete was needed.
Strategic Approach to Concrete Construction
From inception, material selection and application was identified either as a major risk or as an opportunity. The CFLHD and consultant team developed a progressive concrete implementation and risk management strategy. In the first phase, the CFLHD’s laboratory staff, in conjunction with the consultant design team, completed a basic study to validate the design parameters and link the final contract requirements with local capabilities. The first report concluded that multiple local sources of aggregates suitable for incorporation into HPC were available. Statistical analysis of previously produced low strength concrete mixes and structural concrete mixes confirmed that mass production of HPC from local sources and suppliers would be possible.
The second phase included an intensive HPC testing program using local materials, performance based specifications, and concrete trial batches. This testing provided a broad range of information that otherwise would not have been available to the bidders. The intent of this phase was to verify the basic feasibility of achieving the required 10,000 psi (69 MPa) concrete compressive strength in a production setting. This phase also provided information about concrete mix designs that might yield the necessary fresh concrete properties for delivery, placement, and consolidation, while providing other information on hardened concrete properties.
The team took this information and shared it with the industry well in advance of the bid period. The purpose was to seek input and feedback and to raise industry awareness of the project and build pre-bid energy. While this effort did not define a project-specific source of materials or mandatory mix designs, it made the development, production, and placement of the required concrete a non issue during the bidding period. The complete phase one and phase two reports were provided as informational material in the bidder packages.
This progressive approach adopted for the Colorado River Bridge at Hoover Dam removed all doubt as to availability and viability of successfully meeting the project's requirements for the concrete. It further validated the design team's optimization, while raising industry awareness well in advance of execution. Otherwise, stakeholder and project team confidence entering bid day and beyond would have been limited. A true collaboration between owners, consultants, suppliers, and contractors was achieved.
Further information about the project, including construction photographs, are available at www.hooverdambypass.org.
HPC Bridge Views, Issue 63, Sept/Oct 2010