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Construction Close-up: On-Site Batch Plant Churns Out Volumes of Concrete
More brawny than graceful, a complex of conveyer belts, tanks and massive cylinders churns noisily. Hillocks of sand and gravel disappear into the maw, to reemerge as grey form-fitting slurry needed to construct an engineering marvel
To withstand the full fury of a major seismic event, the concrete must be perfect. To ensure that the finished concrete will be uniformly strong, the composition is continuously monitored and adjusted. As the temperature and humidity change so do the ingredients, with water, sand, chemical binders and cement all variables.
Though tiny compared to the bridge's monumental scale, a combination of just-in-time delivery and immediate distribution, mean the plant produces far more than seems possible from an initial glance.
The Batch Plant is owned and operated by California Readymix, Inc., a nine-employee subsidiary of RMC Pacific Materials. RMC Pacific Materials is a division of RMC Group Plc, one of the largest makers of ready mixed in the world. The London-based firm has nearly 32,000 employees and operations in 25 nations.
Tandem tractor-trailers deliver 20 loads of gravel and sand to the site daily, in a relay race of just-in-time production. After a carefully monitored mixing process, the concrete is pumped through a sluice-like system of conveyor belts and onto one of three custom built barges that began life as a sketch on the back of an envelope. The barges, Kathy, Margo and Micki, named in honor of the wives of three executives, deliver the goods.
Once mixed, a chemical reaction begins in the concrete, generating heat as it begins hardening, much like papier-mâché. To slow that process—assuring the concrete is poured at the optimum consistency—ice, and water, is folded into the mixture.
Keeping it cool is key. Engineers set up an ice-making machine, akin to a super-sized hotel hallway ice-maker to help with this. Ensconced essentially in the back of an 18-wheeler, the ice plant is capable of producing up to 100 tons of ice a day. Once aboard a barge for the roughly 30-minute journey to the work site, the concrete is further cooled with liquid nitrogen.
The plant produces between 6,500 and 8,000 cubic yards of concrete per month. A cubic yard is an area three feet long by three feet wide by three feet deep—about the volume of a standard washing machine.
The largest single pour of material coming out of the batch plant is 706 cubic yards for a pair of pilings. Under the existing contract the batch plant will produce 183,120 cubic yards of concrete—just for the "Skyway" segment alone. That's enough concrete to build an inch deep, four-foot wide sidewalk 2,809 miles, practically the distance between San Francisco and the East Coast.
The batch plant production figures excludes what will be needed for the cable-stayed suspension portion of the bridge. A contract for that section hasn't been awarded, so the concrete for that part of the span may be produced elsewhere.
Another 145,000 cubic yards of concrete for the actual roadway is being produced seperately at the Stockton "Pre-cast Yard."
HOW IT'S DONE:
The raw materials are carefully mixed in a gigantic enclosed mixing bowl—similar to the rotating tank on a cement mixer truck. Two large cylindrical containers, one filled with portland cement, the other with fly ash and slag (a steel production byproduct), stand above the central mixer, connected by a network of pipes. Water is fed in through ducts. Ice, rock and sand arrive via conveyor belts.
Maintaining the mixture to the exact specifications is vital. Slight shifts in temperature and humidity require constant tweaking of the mix. From a control board in a trailer, Harold Arnaudon, Batch Plant / Ice Plant Operator, keeps a watchful eye on video monitors, pressing buttons to add chemicals, managing the chemical reaction, giving workers at the pouring site the proper amount of time to pour before the concrete hardens.
The massive mixing bowl homogenizes the final product, feeding conveyors once a barge moves into position. Barges ferry the wet concrete to the job site, docking with a pier. Massive pumps spring to life, driving the entire load through a pipe and into place in less than 20 minutes. All the while rising and falling waves make the barge and the pipe bob up and down, echoing the Bay itself.
As the concrete advances lava-like into a pour, the curing process is already under way. It will take just three hours for the fresh concrete to support a man's weight and 14 days to design strength.
Caltrans inspectors and quality control technicians from California Ready Mix and KFM monitor the composition of the mix and the entire process of the pour, assuring that the future Bay Bridge now taking shape will stand tall in the next earthquake and serve Northern California well into the 22nd century.