Recycling at Bay
- Published: Wednesday, 17 August 2011 14:09
- Written by CP Staff
The City of San Francisco’s emphasis on recycling and green building creates conflicting concrete production and delivery variables. On the positive side, many non-building mix specifications (> 2,500 psi) for city agencies mandate 15 percent or higher content of recycled aggregate derived from reclaimers, crushed, returned concrete, or demolished slabs. A city and county street standard calling for an 8-in. concrete base with 4-in. asphalt topping keeps demand at a level where ready mixed producers must secure adequate recycled-aggregate supply.
Conversely, narrow streets, dangerous inclines, choking traffic and confinement of ready mixed plants to a handful of sites in a city of 700,000-plus complicate deliveries and foster a tendency to overcompensate for a job’s mix volume requirements. That spawns a leftover or returned concrete factor contrary to waste minimization goals of green building proponents and ready mixed producers alike. And San Francisco producers, like peers throughout California, attempt job site truck washout at their own peril.
Displaced from an original waterfront site by construction of the San Francisco Giants’ AT&T Park, independent Bode Concrete looked to a new ready mixed plant at the Port of San Francisco Pier 92 for operational efficiency and an underlying green goal: Respond to state and local regulations that nullify common industry waste disposal regimens with a water, aggregate and cement fines recycling plan in a league of its own.
A family-owned producer with San Francisco ties dating to 1915, Bode Concrete relocated to its 3.3-acre property in 2004. It combined central mixed and flanking transit mixed plants, each with back-up charging, and routed all inbound and outbound truck traffic along a wide expanse featuring a slump adjustment, water tank charging/delivery prep and washout station.
A galvanized pipe truss carries processed, storm or municipal water lines to what at peak could be six loaded and six returning mixers, their charge hoppers back-to-back toward station hoses and nozzles. A 4-ft.-wide grill covers a 105-ft. trench, sloping from 30 in. to 12 ft.; it transfers returned aggregate and slurry, along with mixer, pump and dump truck washout, to a high-capacity, screw conveyor-equipped Knelson Concentrator Reclaimer.
That equipment, along with 8,000-gallon (agitated) and 16,000-gallon (settled water) vessels, anchored an initial recycling strategy. It exhibited efficient fine and coarse aggregate capture, but moderate success in slurry management and gray water usage. Reclaiming capacity was limited to 40 yd./day, while handling and processing charges escalated.
Bode Concrete has stepped up recycling with a cement fine capture method that maximizes usage of reclaimed water in a minimal timeframe. Processed water from a morning delivery’s washout can be part of the batch for an afternoon load, thanks to the installation of an Alar Engineering Auto-Vac system.
“We have gone from recycling 20–40 yards to 40–75 yards of returned material per day,” says Bode Concrete Production Manager Rob Fontes. “Before this equipment, we would often hit the specific gravity threshold in the recycled water tank, limiting the reclaimer effectiveness.” The Auto-Vac, he adds, nets water with -1.012 specific gravity, and can be activated when processed water solids exceed a targeted threshold.
Bode Concrete’s space limitations, coupled with the difficulty of permitting below-grade, process-water storage anywhere in California, compelled investment in the Alar technology. It separates cement fines from water by pumping slurry to the base of a cylindrical drum with an outer layer of diatomaceous earth. As the drum rotates, the porous material captures cement particles; thin layers of the two materials are then scrapped off by blade. Water of optimal specific gravity characteristics is piped to the existing 16,000-gallon supply tank.
Bode Concrete runs the largest capacity Auto-Vac Alar has delivered in concrete, with a 6- x 9-ft. drum compared to 6- x 4-ft. sizes in other ready mixed operations, and 3- x 4-ft. sizes common in precast plants. The San Francisco installation can take up to 20,000 gallons of slurry in an eight-hour period down to the specific gravity range suited to a wide variety of mix specs. — Alar Engineering, 708/479-6100; www.alarcorp.com