The concrete mix
The entire bridge rests on only 2 piers, but they required a lot of concrete—200 yard pours, to be exact, meaning lots of heat, lots of expanding and shrinking as the concrete set, and potentially lots of cracking. The concrete chemists decided to use a greater than usual proportion of ground-granulated blast-furnace slag (GGBFS) for these massive junctions, called icebreakers. GGBFS doesn’t produce as much heat as portland cement, so the junctions’ mix was made up of 60% slag and 40% cement—a first for the Willamette River Bridge project. When properly proportioned, slag also leads to reduced permeability and better durability than portland cement alone.
This mixture gave the concrete the needed strength while minimizing the excess heat. To make sure they had the right mix, the concrete testers moved the typical acceptance strength from 28 days to 56 days. They were aiming for 4500 psi, but on 2 separate tests, the concrete cured to 6250 psi, so they were pleased with the result.
Prime contractor Hamilton Construction Co., Springfield, Ore., cast the concrete arches in place at the worksite. They are approximately 400 feet long, in contrast to the usual 100-foot beams. The point where the arches touch down and the first 23 feet of the arch ribs themselves posed the greatest challenge: Because of the extensive steel reinforcement needed in these areas, the crews could not vibrate the concrete to remove air pockets and fill voids that could have compromised the integrity of the arches.
To solve this challenge, Knife River created a concrete capable of flowing almost like water that self-consolidated without vibration, filling every nook, cranny, and void within the form. While many additives were used to achieve this feat, the most important were a viscosity modifier and high-range water reducer (superplasticizer).
Knife River upgraded the long-term performance of the bridge (durability) with the mix used on the bridge’s deck. It is made of high-performance concrete, which includes the fiber Novomesh 950. When thoroughly integrated and placed, this fiber helps the concrete resist stress, significantly reducing the risk of cracking. The fibers are evenly distributed throughout the concrete to form a secondary matrix of support.
Because the deck is the most vulnerable part of a bridge, susceptible to the most wear and tear, we expect the fiber reinforcement will enable it to better handle the forces of compression and tension, and we’ll continue to monitor its performance for possible use in future projects.
Construction on the Willamette River Bridge began in summer 2009, and the project is currently ahead of schedule. Traffic in both directions now runs on the new southbound bridge, and the new northbound bridge is scheduled for completion in fall 2013. Landscaping, park improvements, and final paving should be finished by summer 2014.
Karl Wieseke is the ODOT project manager for the I-5 Willamette River Bridge.