Charles Dwyer, South Carolina Department of Transportation
The Cooper River Bridge Project involves the replacement of two existing river crossings between Charleston and Mount Pleasant on the coast of South Carolina. The new crossing has an overall length of approximately 3 miles (4.8 km) and includes two interchanges, two high level approach structures, and a cable-stayed bridge with a main span of 1546 ft (471 m) — the longest in North America.
The $531 million design-build contract is the largest and most complex project ever completed by the South Carolina Department of Transportation (SCDOT). Construction costs for this project alone are comparable to the average annual statewide construction budget for SCDOT.
SCDOT classified the bridge as a critical structure because it crosses a busy shipping channel and provides a link to the city of Charleston, which has the only hospital in the area with a trauma center. The critical bridge classification meant that the bridge needed to meet the highest standards and be designed to withstand hurricanes, earthquakes, and ship collisions.
Project Criteria
Because this bridge is unlike any other bridge on SCDOT’s highway system, we did not rely solely on our established standards, specifications, and design criteria. Project-specific criteria were developed for many components ranging from the design of the stay cables to the development of the corrosion control plan.
As with all aspects of SCDOT’s design-build contracts, primary control of the design was given to the contractor. This included concrete mix designs as the contract provided that mix proportions in the standard specifications were for guidance only.
To ensure that the bridge would meet the needs of the state, SCDOT included criteria for a corrosion control plan. Firstly, the criteria set the service life of the bridge at 100 years. Secondly, this plan was one of the few stop points written into the contract. The contractor needed SCDOT’s approval of the corrosion control plan before proceeding.
The corrosion control plan outlined how the bridge will meet its 100-year service life and documented the process of material selection. The contractor was free to select epoxy-coated reinforcement or high-performance concrete provided that the service life could be met with reasonable life-cycle costs. The selection of low permeability concrete by the contractor led SCDOT and the contractor to agree on a plan for testing and acceptance of concrete under this criterion.
Construction Quality Control
The contract emphasized the requirement for contractor quality control (CQC), which included both construction inspection and materials testing. SCDOT still performed its own materials testing at twenty percent of the standard frequencies, or one for every five CQC tests. These test results were the acceptance tests and were used for quality assurance of the CQC results.
The concrete was tested for permeability as part of the mix design process. The potential for variations in the permeability test results meant that we did not have the confidence to require it as an acceptance test. Instead, the low permeability concretes were tested before and periodically during their production. If any mix had failed an interim test, the mix design would have been adjusted and retested. Fortunately, all interim permeability tests met the criterion.
Project Status
The new bridge opened to traffic on July 16, 2005, more than one year ahead of the required completion date in the contract. Demolition of the two old truss bridges is now underway.
Editor’s Note
This edition of HPC Bridge Views focuses on one design-build project from the perspectives of the owner, designer, general contractor, and concrete supplier. A previous article on this project appeared in Issue No. 29.