SEATTLE – Float-out of the first six concrete pontoons for the new State Route 520 floating bridge will be delayed to allow crews time to make repairs and modifications after damaged concrete was found in one of the pontoons.
Work is planned on four of the six concrete pontoons under construction in Aberdeen and is preliminarily expected to take four to six weeks to complete. As a result, the completed pontoons are not expected to be towed out of the casting basin until July or August, depending on favorable tides.
The Washington State Department of Transportation is working with contractor Kiewit-General to analyze the cause of the damage and how the repair work might affect the construction schedule for the new floating bridge. Other SR 520 floating bridge construction will continue on pontoons in Tacoma, on anchors in Kenmore and bridge supports on the east side of Lake Washington.
“We have a plan, and by making these repairs and modifications before the pontoons leave Aberdeen, we can assure that the new SR 520 floating bridge will safely carry traffic for 75 years or more,” said Julie Meredith, WSDOT SR 520 program director.
The damage was discovered May 11 after construction crews completed post-tensioning on a longitudinal pontoon measuring 360 feet long, 75 feet wide and more than 28 feet tall. Post-tensioning is a process whereby steel tendons are stretched through the top and bottom slabs of a pontoon to better strengthen the concrete box against the forces expected on Lake Washington once the bridge is complete and carrying traffic.
Inspections of the first pontoon after post-tensioning showed damaged concrete where there was insufficient steel rebar reinforcement to hold the tendons in place. Crews plan to detension the steel tendons, remove concrete and add steel rebar reinforcement needed to contain the steel tendons. New concrete will be poured in the damaged area and allowed to cure before the steel tendons are retensioned.
Three other pontoons – two longitudinal and one cross pontoon that measures 240 feet long, 75 feet wide and about 35 feet tall – face the same issue though no post-tensioning work has yet occurred. Those pontoons will undergo preemptive modifications before steel tendons are installed and stretched into position, said Dave Ziegler, principal engineer for WSDOT’s pontoon project.
WSDOT and contractor KG are focused on making the modifications carefully and swiftly. It’s too early to say how much the repairs will cost or how costs will be assigned.
“As repair and modifications are made in coming weeks, it will be clearer when we will float out this first batch of pontoons,” Meredith said.
The Pontoon Construction Project builds 33 pontoons of the 77 necessary for the new SR 520 floating bridge. Information about the project is at www.wsdot.wa.gov/Projects/SR520/Pontoons.htm.