Dumbarton Bridge Seismic Retrofit - A Design and Teamwork Challenge

For over a year Shimmick Construction has been retrofitting the Dumbarton Bridge to make safety improvements and increase the its capacity to handle forces caused by earthquakes. The work included the reconstruction of bridge joints to allow for more movement of bridge segments without damage when an earthquake occurs. The joints are composed of heavily reinforced concrete covered by massive steel plates. The construction challenge for Shimmick and Graniterock, the concrete supplier of choice, was to get concrete to entirely fill the cavity under the steel plates through small holes drilled in the top of the steel plates. This meant the concrete needed to be highly fluid and flow freely to make its way around the dense steel reinforcing under the steel plates. The Caltrans specification also required that the mix not segregate into rocky pockets and sandy/cement pockets so that the strength and durability of the concrete was consistent throughout the joint.

Segregation is quite common in highly fluid mixtures because the denser materials in the mix naturally want to drop to the bottom, and obstructions such as steel reinforcing tend to restrict rock from passing while allowing the cement paste to flow freely past. To further complicate the project, Caltrans required that the mix achieve high-strength in 24 hours so that the bridge could be re-opened as scheduled. The project required Shimmick and Graniterock to work closely together to meet all of the specifications. Mix design, mix delivery and construction placement methods had to be highly coordinated. Substantial work went into refinement of the mix design, plant batching, mix delivery, form design and method of placement. Numerous trials, several synergy meetings and many consultations were done to prepare. On the day of the placement, Labor Day weekend 2012, the project site was crowded with Caltrans inspectors and media representatives signaling the importance and urgency to get the work done right and in a timely fashion. Graniterock and Shimmick’s performance exceeded Caltrans’ expectations and as a result the Bridge opened 10 hours ahead of schedule.