Rendering of the new Willamette River Bridge

Wednesday, March 21, 2012

Crews dish up a menu of specialized concrete mixes

From ODOT-
The concrete suppliers and specialists at the Willamette River Bridge are mixing up the standard components of concrete to deal with the complexity of the project. They designed 17 mixes to build the recently completed southbound structure. The arches, the drilled shafts, the columns and the deck all needed concrete specific to each function.

The mix for the bridge deck is made of high-performance concrete, which includes a special fiber additive. This fiber helps the concrete resist stresses placed on it, significantly reducing the risk of cracking. The fibers are evenly distributed throughout the concrete to form a secondary matrix of support.

Even though the drilled shafts forming the piers supporting the bridge were surrounded by rock, there was a chance that the concrete could leak into the river before it hardened. To ensure this would’t happen, our team created a mix for the piers that includes a special anti-washout additive to keep the concrete contained within the rock socket.

The two footings on the banks of the river require a lot of concrete — nearly 500 cubic yards each. The more massive the concrete structure, the more heat is generated while it cures. The more heat, the more the concrete expands and then shrinks as it sets, which can create cracks. So we designed a special mix for the riverbank piers to minimize heating and reduce cracking.

The point where the arches meet in the river and the first 23 feet of the arch ribs themselves required a concrete mix capable of flowing almost like water, then self-consolidating without vibration, filling every nook and cranny within the form.

The special concrete designed, mixed and placed resulted in a strong, beautiful southbound bridge. We’ll repeat our success as we build the new northbound bridge.

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