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The Bay Bridge: Special Job, Special Equipment

The Bay Bridge: Special Job, Special Equipment

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    At a cost of more than $5 billion, the bridge's new eastern span is scheduled to be complete in 2013. The span saves commuters time driving from San Francisco east to Oakland and beyond.

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    The plant supplying concrete for the San Francisco - Oakland Bay Bridge project sits on 1½acres at the Port of Oakland. Cemex is supplying 100,000 cubic yards of concrete for the new eastern span.

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    One of the three delivery barges delivers concrete to the project. They are named Micki, Margo, and Kathy, after the names of the wives of the Cemex management team.

Barges were also not available locally because of the volume of marine construction underway in San Francisco Bay. Finally, we found suitable 33x150-foot barges on the Columbia River in Washington state. Although they were fitted for ocean voyage, it took two months before a weather window allowed us to transport them to San Francisco Bay. One of the barges was caught in a storm and heavily damaged.

At the San Francisco Dry Dock, the drum frames, deck superstructure, and control room construction took place and the drums were mounted. We named the barges Micki, Margo, and Kathy, after the wives of the Cemex management team. They were then transported to the Cemex Oakland ready-mix plant dock where construction was completed.

20-yard drums

The key features of the barges are the 20-cubic-yard mixer drums—to our knowledge, the largest in the world at the time—mounted on a custom frame manufactured by McNeilus. The high-capacity drums were made by adding an 11-foot section in the middle of a standard truck drum. Custom controls allow remote operation of drums and chutes. Dedicated diesel engines for each mixer provide hydraulic power. Twin drums are mounted on the centerline of each barge, hopper to hopper, allowing both mixers to be loaded and discharged from the same point.

A control room and catwalk sit above the center of the drum hoppers to allow easy access for the barge operator to control the speed and direction of the drums, start and stop the conveyors, dose and dispense admixtures, and direct the flow of concrete from the mixer drums.

Three pump systems handle high-pressure washout water, high-volume fresh water, and high-volume slurry. W.R. Grace provided four 500-gallon admixture tanks and a CalTrans-approved admixture dispensing system for each barge. The barge operator can control the admixture system from the catwalk and monitor the dispenser sight gauges with a camera.

A tower was built in the forward starboard of each barge, where a hydraulic swing chute allows discharge into the contractor's receiving hopper or onto the waste discharge conveyor. The waste discharge conveyor can be extended over the port side of the barge to offload returned concrete into a loader bucket on the dock at the batch plant.

The barges are loaded at the plant from the portside to allow handling waste without requiring tugs. Drum cleanout is put into a portable tank, which is picked up by a truck crane at the end of each pour and dumped in the waste area. This crane is also used to load supplies, admixtures, and parts. A 12-inch-high steel kickboard area around the entire perimeter deck of each barge was designed to contain any spills.

All waste is transported offsite daily, and secondary containment is required under all conveyors and storage areas. All water used in processing the concrete is trucked in, captured, and removed nightly.

Concrete cooling

CalTrans required KFM to reduce the potential for thermal cracking. To stay within the 47° F to 73° F called for in the thermal control plan, we utilized three methods of cooling.

Rain bird sprinklers provided evaporative cooling, lowering the temperature of the coarse stockpiles by five or more degrees. Continuously monitoring the gravel moisture controls the water content and maintains the specified water/cement ratio.