Stretching almost 1.5 miles, the state Route 520 Bridge is the longest floating bridge in the world. It carries state Route 520 across Lake Washington between Seattle and Medina. The Washington State Department of Transportation (WSDOT) developed a plan to build 77 new pontoons for the bridge.

Pontoon construction began in 2011 in a new casting basin in Aberdeen, Wash., where 33 pontoons were built. Twenty-one of these were longitudinal pontoons, which are the biggest ones used for the new floating bridge, making up the spine and primary support. They are each 360 feet long and weigh about 11,000 tons. In 2012, construction of 44 of the smaller, supplemental pontoons began in an existing casting basin in Tacoma, Wash. Engineers also added an entirely new layer to the design, elevating the 10.5-inch-thick roadway deck 10 feet above the pontoons using concrete columns.

WSDOT performed a $2.8 million test to find the ideal mixture of concrete for the pontoons. Engineers chose one using fly ash and microsilicia to reduce microcracking. After adjusting the concrete to reduce its temperature rise, but still have an ultra-low permeability, high strength, and high flowability, engineers at CTLGroup developed a thermal control protocol for the walls and slab to prevent restraint cracking at the base of the walls. The protocol involved active heating and cooling of the concrete. The protocol was tested in smaller walls, and ultimately was tested in full-scale pontoon wall sections that measured 1 to 1.5 feet thick, 28 feet high, and 120 feet long.

The original bridge was dedicated in 1963 and was nearing the end of its useful service life. Construction for the $764 million new bridge began in 2012. The new six-lane floating bridge features high occupancy vehicle/transit lanes, a bike and pedestrian trail, and viewpoints over the lake.

“We’re proud of our contributions to this significant structure, including our engineering assistance in the fabrication of the 77 pontoons which make up the substructure for this record-setting bridge,” said CTLGroup Senior Principal Engineer and Project Manager John Gajda.