In the state of Washington, cracking in a concrete pontoon designed for the world's longest floating bridge is forcing engineers to rethink their strategy for the first phase of building the pontoons.
As part of a $4.65-billion state Route 520 bridge replacement program east of Seattle, Omaha-based Kiewit Corp. and its subsidiary General Construction Co. are working in Aberdeen, Wash., on a $367.3-million project to build 33 of the largest pontoons ever constructed in the state. The pontoons are part of a total of the 77 needed for the overall replacement of the current Seattle-to-Medina floating bridge across Lake Washington.
During post-tensioning of an 11,000-ton longitudinal pontoon measuring 360 ft long, 75 ft wide and more than 28 ft tall, concrete crumbled in the top and bottom slabs and in areas adjacent to the post-tensioning, says Dave Ziegler, principal engineer for the project with the Washington State Dept. of Transportation.
This pontoon was the first to undergo post-tensioning, which was done to ensure it didn't crack under the pressure of the 105,000 vehicles a day planned for the future 7,710-ft-long, six-lane bridge. Once the deterioration was found on May 11, work stopped on two other identical pontoons. "What we're doing is preemptive modifications to the other two pontoons that have not yet been post-tensioned," Ziegler says. "We will make modifications so that they are ready [before post-tensioning starts]."
The damaged pontoon was de-tensioned at its 60 separate strand points. Crews will remove the damaged concrete, which is believed to be at each end of the pontoon where the curved tendons had straightened during the tightening. "It is not a complete removal," Ziegler says, "but enough to get access to all post-tensioning ducts." Kiewit-General has declined to comment.
Next, the team plans to add necessary rebar to support the concrete, Ziegler says. The other pontoons—the two identical ones already built and one that measures 240 ft long, 75 ft wide and 35 ft tall—will be modified to improve concrete performance.
"Crews will remove concrete to add rebar to support the tendons. This fix will be part of the new pontoon design going forward," Ziegler adds. "It is not a significant amount of rebar, but it needs to be in the right place."
Delay and Repairs
As crews excavate the damaged pontoon, they will evaluate concrete for specific damage and determine if something about the pontoon design or installation methods was at fault. That answer will determine who will pay for the four- to six-week delay and repairs—Kiewit-General or WSDOT.
WSDOT already has evaluated and tested the custom 7,500-psi concrete mix. Under a $2.8-million contract in 2010, Aberdeen, Wash.-based Quigg Bros. Inc. built a 1/6-size test pontoon (ENR 6/14/10 p. 21). To reduce cracking, engineers developed a heating system cast into the keel slab to ensure it cooled at the same rate as the walls, says Scott Ireland, WSDOT project engineer. The pontoon mix also contains a healthy dose of fly ash and microsilica to ensure concrete density.