Authored by South Carolina Department of Transportation’s Charles Dwyer, this article was adapted from the September/October 2005 issue of HPC Bridge
Authored by South Carolina Department of Transportation’s Charles Dwyer, this article was adapted from the September/October 2005 issue of HPC Bridge Views, published jointly by the Federal Highway Administration and the National Concrete Bridge Council.
For its largest and most complex bridge project Û a $531 million design-build contract Û the South Carolina Department of Transportation set high design standards: 100-year service life at the lowest life-cycle cost. The use of high performance concrete in addition to a high-performance contract between the owner and contractor made possible the fulfillment of design objectives, plus completion of the project in July 2005, more than one year ahead of schedule.
Replacing two existing river crossings between Charleston and Mount Pleasant on the coast of South Carolina, the nearly 3-mile-long (4.8-km) Cooper River Bridge includes two interchanges, two high-level approach structures, and a cable-stayed bridge incorporating a 1,546-ft. (471-m) main span Û the longest in North America. SCDOT classified the crossing as a critical structure because it spans a busy shipping channel and provides a link to Charleston, the site of the area’s only hospital with a trauma center. Given the critical bridge designation, the crossing needed to meet the highest standards to withstand hurricanes, earthquakes, and ship collisions besides the stress of heavy daily traffic.
The designer’s main challenge lay in the lack of a precedent: no code was available for a 100-year design. Because the structure is unlike any other bridge within SCDOT’s highway system, relying solely on established standards, specifications, and design criteria was not an option. Accordingly, project-specific criteria were developed for many components and functions, ranging from the stay-cable design to a corrosion-control plan.
The towers of the cable-stayed bridge, for example, were designed with sufficient reinforcement to withstand hurricane-generated winds. For ductility to satisfy seismic demands, however, reinforcing steel in the towers was limited so that hinges could form at their bases without requiring excessive amounts of steel or exerting undue forces on the drilled shafts.
To demonstrate the design would provide a 100-year service life, an appropriate analytical model had to be devised, incorporating such environmental factors as water salinity, annual amounts of chloride applied to the deck, and level of airborne chlorides. The design team was significantly aided in developing its data by SCDOT chloride-level measurements collected on the adjacent 1929 Grace Memorial and 1956 Pearman Bridges. Thus, research and considerable engineering judgment led to the adoption of an approach utilizing uncoated reinforcing steel, while specifying the permeability required to achieve a 100-year service life in view of the assumed rate of chloride application and concrete cover specified in the design criteria. In the splash zone, two alternatives were developed, utilizing (1) concrete with a maximum permeability of 500 coulombs and a minimum 4-in. concrete cover over reinforcement; or, (2) concrete with a maximum permeability value of 1,400 coulombs and a minimum 6-in. cover.
To meet the permeability values in addition to strength and material specifications, local suppliers offered either slag cement or Class F fly ash. As an accelerated curing method used by Virginia DOT was allowed in the design to accommodate the longer period Û approximately one year Û required for fly ash concrete to reach its long-term permeability index, the more economical fly ash mix was selected. The use of performance-based specifications provided the contractor complete freedom to create the most economical concrete mix to satisfy both design and placement requirements.
High-performance demands coupled with placement limitations prompted the contractor to use every imaginable placement technique: hoppers, pumps, tremie concrete, and mass concrete. Marine transportation of concrete was accomplished with a system of custom-made 125-cu.-yd. hoppers mounted on barges. Two traveling-hopper barges received concrete at shore and moved to the placement site assisted by tugboats. A third holding hopper and a 180-ft. pump truck were mounted on a barge stationed at the placement location. A high-speed conveyor transferred the contents of the traveling barges, once positioned, to the holding hopper, where a small conveyor at the bottom of the receptacle directly fed the pump.
For every structural element, proper placement depended largely on mix design to accommodate marine, trestle or land conveyance in addition to set requirements. Specified for the marine-drilled shafts, for example, was a tremie mix with high slump and small aggregate size, plus hydration-stabilizing admixtures to maintain plasticity during transport and placement often lasting 18-plus hours. As main-span tower footings required a continuous placement of approximately 5,000 yd. each, that mix needed an initial long life for transportation and then had to begin setting to reduce pressures on the 20-ft.-high formwork. For the 575-ft.-high main-span towers, a 7,000-psi concrete was required with long plastic life for marine transportation; yet, a strength of 2,500 psi was demanded in 12 hours following placement to maintain the construction schedule. Similarly, placements of 700 to 1,000 yd. for 160-ft.-wide bridge decks, typically pumped long distances over newly placed sections, called for a pumpable mix retaining plasticity during six- to eight-hour pours.
Due to the specifications for mass concrete, heat of hydration was closely monitored. For smaller pours, mix designs were optimized, using the lowest possible concrete temperature at placement. Exterior insulation on formwork controlled temperature gradients. Yet, most placements required a closed-loop internal cooling system. Excellent partnering with the owner in a design-build approach allowed the contractor intimate involvement to address new mix designs and implement the best technology to get the job done.
The concrete supplier was challenged to produce over 320,000 yd., including high strength, high early-strength, and low permeability mixes, plus concretes with extended set or transportation times, and tight initial-temperature control. While permeability requirements and economy dictated the use of cementitious materials containing 400 lb./yd. of cement and 300 lb./yd. of fly ash for substructures, strength requirements were met using a high cementitious-materials content and low water-cementitious materials ratios. Heat of hydration concerns were addressed by the producer through immersion and chilling of coarse aggregate in large pits filled with near-freezing water. Additionally, cement imported from Greece to supplement local supply (delivered relatively hot due to high demand during that period) was allowed a 14-day cooling period during shipment. Again, the design-build process and close collaboration among project principals provided the flexibility to implement techniques and optimize mix designs to construct the best bridge possible.
PROJECT PRINCIPALS
Owner: South Carolina Department of Transportation
Design engineer: Parsons Brinckerhoff Quade & Douglas, Inc.
Contractor: Palmetto Bridge Constructors, a joint venture of Tidewater Skanska and Flatiron Constructors, Inc.
Concrete supplier: Wando Concrete, LLC