Antebellum landmarks may be sacred in Charleston, S.C., but the recently opened Cooper River Bridge (officially known as the Arthur P. Ravenel Bridge) has changed at least part of the city's skyline in a big way. As North America's longest cable-stayed bridge, it was designed to withstand an earthquake more powerful than the 1886 Charleston earthquake, estimated at 7.3 on the Richter scale. Construction of the bridge required 330,000 cubic yards of concrete, enough to fill an entire NFL stadium.
The Cooper River Bridge was built through the cooperative efforts of a number of different companies. The general contractor was Palmetto Bridge Constructors (PBC). The O.L. Thompson Construction Company of Charleston handled the road construction. Wando Concrete Company, a subsidiary of O.L. Thompson, produced the concrete.
David Hand, vice president of O.L. Thompson Construction, and a partner in Wando Concrete Company, said that when approaching the building of the new Cooper River Bridge they had two levels of problems: first the quality (or behavior) of the concrete and then the quantity of concrete. Wando Concrete used information from the admixture suppliers to come up with mix designs that would provide what PBC wanted. Because most of the requested concrete mixes had diametrically opposed goals, such as being plastic for a long period of time then setting very quickly, it required extensive research and testing. “I don't think we found a middle-of-the-road answer for this question,” said Hand. “But we did end up finding some things that, though they didn't exactly meet the extremes at either end, still filled the needs of what PBC wanted. This was done by studying many different chemicals, mixing processes, delivery schedules, and the supply movements to get everything to work.”
Once the mix was figured out, Wando looked at how to generate concrete in large enough batches that could be handled effectively to reach the forms before it hardened. The concrete used in the bridge's titanic diamond-shaped towers and eight-lane deck had to be mixed, driven by seventeen mixing trucks to river barges, placed on the barges, transported to the construction site, and finally raised up to the forms and placed. This all took between 45 minutes and two hours from the initial mixing at O.L. Thompson's downtown Charleston location to the final placement. Once placed in the bridge, though, each 14-foot lift had to set before subsequent lifts could be placed.
Over 400 drilled shaft footings were constructed in the riverbed, some reaching nearly 200 feet deep. Steel tube casings, some 10 to 12 feet in diameter, were driven into the earth to the specified depth. A drill was then used to excavate inside the casing. The drilling continued below the 100-foot casing into the stiff clay below. A huge rebar cage was placed inside the drilled space before up to 500 cubic yards of concrete were placed.
Concrete used for the drilled shaft footings had to stay liquid for 9 to 10 hours. “This is unheard of in concrete production,” said Hand. “That concrete had to be liquid from the bottom to the top, hardening as one big mass. It couldn't harden from the bottom up.” This was accomplished using hydration inhibitors that actually put the concrete to sleep by coating the cement particles to prevent hydration. “Then, after about nine hours or more, the concrete would ‘wake up' and start acting like regular concrete again,” said Hand. “A Grace product called Recover accomplished this. It was initially developed for projects in cities where a place to clean ready-mix trucks may not always be readily available, thus it was developed for logistical purposes more than anything else. We used it in this application as an active ingredient, not as a clean-up material. Grace called us early on wondering what we were doing with all this stuff. They normally sell it in 100-gallon containers—we were buying it by the tanker load.”