Wade Watson, Tidewater Skanska
Palmetto Bridge Constructors, a joint venture of Tidewater Skanska and Flatiron Constructors, Inc., was awarded the $531 million contract for the construction of the Cooper River Bridge in July 2001. Construction began in earnest in January 2002.
The project is located near the mouth of the harbor, close to the Atlantic Ocean, and crosses the shipping channel to the port of Charleston — the fourth busiest container port in the country. This location produces an extremely corrosive environment and tidal currents that are often severe with an average tide range of 6 ft (1.8 m). This clearly presented challenges for construction of the spans over water.
Marine transportation of concrete was accomplished with a system of custom-made 125 cu yd (96 cu m) hoppers mounted on barges. Two “traveling hopper” barges received concrete shore-side and then moved to the placement location assisted by tugboats. A third “holding hopper” and a 180-ft (55-m) pump truck were mounted on a barge stationed at the placement location. Once in position, the contents of the traveling hoppers were transferred, via a high-speed conveyor, into the holding hopper. The pump was fed directly from a small conveyor at the bottom of the holding hopper.
Not only did the project’s design dictate concrete performance requirements, construction means and methods, as well as placement limitations, added additional performance needs. Virtually each element of the structure, depending on its access (i.e., marine, trestle, or land), had its own particular placement requirements.
Marine drilled shafts required a tremie mix with high slump and small aggregate size. Additionally, this concrete had to remain plastic during the entire transport and placement cycle, often requiring 18 hours or more. This was accomplished with admixtures such as hydration stabilizers and water reducers.
Footings for the main span towers required a continuous placement of approximately 5000 cu yd (3800 cu m) each. The mix needed an initial long life for transportation and then had to begin to set to reduce form pressures on the 20-ft (6-m) high formwork.
The diamond-shaped main span towers rose to a height of about 575 ft (175 m) and required a 7000 psi (48 MPa) compressive strength concrete with long plastic life for marine transportation. However, once the placement was completed, the schedule demanded a strength of 2500 psi (17 MPa) in 12 hours, so the next construction cycle could begin.
Bridge decks were typically pumped from the previously placed decks. This required pumping long distances over newly placed sections as well as a staggered placement sequence. The placements of 700 to 1000 cu yd (535 to 760 cu m) for the 160-ft (49-m) wide deck required a pumpable mix that maintained plastic performance during the entire 6 to 8 hour placement time. Even a normally “routine” placement such as slip forming the barrier walls necessitated a zero slump mix, 200 ft (61 m) over the river.
Specified concrete compressive strengths on the project ranged from 3000 to 8000 psi (21 to 55 MPa). Due to the congestion of the reinforcement associated with seismic design, a high slump small aggregate size mix was often used.
The heat of hydration was also a major concern due to the specifications for mass concrete, which stated a maximum concrete temperature at placement of 80°F (27°C), a maximum concrete temperature of 160°F (71°C) during curing, and a 35°F (19°C) maximum differential temperature between the core and outside surface. Where placements were smaller, mix designs were optimized to reduce the heat of hydration using the lowest possible concrete temperature at placement. Exterior insulation was used on the formwork to control temperature gradients. Still, most placements required a closed-loop internal cooling system.
The South Carolina Department of Transportation (SCDOT) standard specifications had to be modified and supplemented to address the new mix designs. This included not only extensive, but also intimate involvement with design. Over-strength concrete can be as much of a problem as under-strength in seismic design. The design-build approach, and a partnering owner, allowed us to use the best technology available to create concrete mixes that would meet the design and placement requirements.
All concrete was purchased as ready-mixed concrete from a local supplier, who set up a facility dedicated to the project. The concrete was procured on a performance-based specification that met design and construction requirements. This took a close working relationship between the supplier, the contractor, the designer, the SCDOT, admixture suppliers, and inspection personnel.