As its Wisconsin neighbors enjoy a respectable crop, Marathon-based County Concrete Corp. is looking for a high yield of its own to close out 2001. Last month, the company commissioned a precast/prestressed plant — one of the most advanced outside Europe — in Roberts, Wis., about 30 miles east of Minnesota's Twin Cities.

After staff orientation and typical machinery and control adjustments, County Concrete and a Canadian production partner, Technopref Industries, will move the plant into high-output mode to tackle a major precast tunnel segment contract for the $500 million-plus Hiawatha Light Rail Transit Line Project. With a diameter just under 19 ft., the tunnel will run 1.5 miles north and 2 miles south from an underground station at Minneapolis-St. Paul International Airport.

In contrast to conventional light rail cars traveling on welded rails powered by electricity, the facility will accommodate tramcars that glide on air cushions pulled by cables. Designed by HNTB/St. Paul, the tunnel will be the terminus of the 11.6-mile Hiawatha Line, beginning in downtown Minneapolis and becoming Minnesota's first light-rail facility.

Befitting investment

The County/Technolpref precast operation figures to be among the most automated and costly ever built in North America. The rationale behind the investment and plant configuration is not surprising, given these aspects of the Hiawatha Line: 1) precast represents about $18 million of the $120 million tunnel; 2) the segment order covers 12,000 60-degree pieces, and 3,000 pieces each of 60-degree and 45-degree curved key and 15-degree keystone shape; 3) a construction sequence that will require jacking of tunnel rings in tandem with boring operations; and 4) tight project specifications, which call for high performance concrete of low permeability (< 1,000 columbs) and low tolerances on diagonal, width and other segment characteristics affecting ring assembly, jacking and long term structural behavior.

To equip a line for the Hiawatha contract and develop mix designs suited to the job's quality parameters and narrow fabrication window, County Concrete teamed with Technopref Industries. Through its French parent company, the Lafontaine, Quebec, precast specialist has experience in tunnel segment casting and carousel-type plant design conducive to high-volume work involving a limited variety of shapes.

To that end, Technopref incorporates technology in Roberts similar to what the French-British consortium, Transmanche Link, used to cast 221,000 segments for the English Channel tunnel linking France and the U.K. (“Chunnel's French Connection,” Concrete Products, February 1991). Technopref is the North American subsidiary of Demathieu & Bard Group, one of France's largest precast producers specializing in civil work.

While new to carousel-based fabrication, County Concrete itself is no stranger to specialty precast for underground construction. Most recently, the company delivered 400-plus arch panels to line a cavern housing the University of Minnesota library archives (“Notes from the Underground,” Concrete Products, October 1998).

The Roberts casting line consists of 42 segment forms and a carousel plan — running along longitudinal and lateral rails — that routes forms from stations for steel and concrete placement and vibration; through setting, initial curing and stripping; to cleaning and preparation. A carousel is also set up for steel handling and routing, and for product transfer from edge preparation and ready-to-ship stacking station to final curing.

A 2.6-yd. twin shaft mixer, receiving aggregate plus cementitious materials from one to four silos, charges a traveling overhead bucket with high performance material — formulated to meet strength development and permeability specs — that is dispensed in a hopper at the placement station.

“Without the carousel technology, it would be hard to produce segments at the pace and quality level this project requires,” notes County Concrete's Dave Reneson, general manager of the Roberts-based Western Prestress/Precast Division. “The carousel design allows us to produce in an assembly line manner and meet very tight dimensional tolerances.”

For much of the Hiawatha Line production phase, County Concrete will run two shifts during the week. Each shift will target segment output for six tunnel rings — seven segments per ring, each weighing just over 20 tons. The producer is delivering the segments to Bloomington, Minn., general contractor Obayashi/Johnson Bros. Among preparations for the contract, the contractor has deployed a tunnel-boring machine that was delivered from Germany to Duluth, Minn., then trucked with side-by-side tractor-trailers to the south portal site. The current tunnel, station and track work schedule calls for the Hiwatha Line to begin service in 2005.

Complex

Hiawatha Line tunnel segment and hollow core production bays (below left & right) split the new 120- × 575-ft. enclosure (above) — the latest addition to a 100-acre plot County Concrete Corp. began developing in Roberts, Wis., five years ago. Although the site began as a building materials retail yard, it has grown to what is likely the most diversified and best-equipped concrete production complex in North America. The September 2001 start up of the precast/prestressed line followed commissioning of ready mixed, pipe & drainage product and block plants from 1998-2000.

County Concrete and partner Technopref Industries turned to two companies for the bulk of equipment. CBE of Tours, France, supplied the segment forms, carousel rails and product-handling assemblies other than overhead cranes. To serve the tunnel segment and hollow core bays, Waukesha, Wis.-based PCE Elematic Inc. furnished a batch plant with containerized, pre-assembled modular components and a 3-yd. EA715/3000 twin shaft mixer, plus a 2.5-yd. EB405 Concrete Shuttle traveling bucket. The manufacturer also equipped the four 4- × 500-ft. bed hollow core line with a EL900E extruder (8-, 10-, 12-, 16- and 20-in. capability) and EL1300A slab saw. The plant marks the first North American installation of PCE's full ELi Plan system. A plant-controls package developed around Oracle database software, ELi Plan has modules for mixing, production equipment, quality control, scheduling and administrative functions.

The Roberts operation is the base for County's Western Prestress/Precast Division, which includes Eau Claire, Wis., and Maple Grove, Minn., plants. The Eastern Division encompasses operations in the Wisconsin cities of Appleton, Green Bay, Madison and Marathon. Based in the latter city, privately held County Concrete Corp. is also a major player in ready mixed.

Fabrication

Steel fabrication for the Hiawatha Line tunnel segments shares space with four beds in the hollow core bay. County Concrete and production partner Technopref Industries have oriented cage handling and transfer around four welding stations and a carousel rack made of steel tubes. Cages are welded from high tensile strength, deformed wire for one of four segment sizes; stacked on the carousel; and transferred to a form prep station (below left) in the adjacent bay.

A rail system creates the main plant's carousel. It accommodates 42 segment forms, netting six seven-piece, 5-ft.-long tunnel rings of 18-ft. 10-in. diameter. After cleaning, release agent application, and cage and hardware insertion, forms are charged and vibrated, then conveyed to an end point (right) for lateral transfer to one of three chambers for setting and initial curing. Forms remain in the chambers for six to eight hours, with temperatures not exceeding 120°F.

The segments' mix design contains silica fume and GGBF slag to lower permeability and promote early strength development. The setting and hardening phase sees segment concrete reach a minimum 2,000 psi compressive strength, sufficient for stripping with a vacuum clamp. The capacity of the clamp is 6,000 lbs., just over the weight of the largest segments. In addition to expediting green-product transfer, the clamp also ensures handling that is virtually stress-free.

Curing

The vacuum clamp delivers segments to a table that turns them upward for transfer to three joint- and edge-finishing stations (above). The first entails adhesive preparation, followed by a seven-minute vulcanizing. After rubber joints are secured, segments continue for application of thin African mahogany edge strips and stacking. The strips alleviate edge stresses and potential damage from concrete-on-concrete conditions finished rings will encounter during jacking and future settling of the tunnel structure. Also bearing some of those stresses are bolts that connect segments into individual rings and to each other.

A Kraft vapor generator serves the setting tunnel and 10 kiln cells (above right), each with 12 segment positions. Over a five-day curing phase, segments — stacked seven high for ease of handling and rapid bolting at the site into single rings — begin with 110°F exposure that gradually decreases through automatic settings. Tight curing methods, coupled with pozzolanic agent-rich mixes, yield finished concrete whose compressive strength exceeds an 8,500 psi minimum written into the project specs. The vapor unit is equipped to log and store temperature data throughout the setting and curing phases for every one of the Hiawatha Line tunnel's 21,000 segments.

Although precast is typically touted in construction for its reduced sensitivity to weather, County Concrete and Technopref Industries officials acknowledge that certain scheduling and climate matters unique to their tunnel contract factored into the plant's design. The tunnel construction sequence will see jacking of rings immediately following a tunnel boring machine. As an underground site, weather will not play much of a role in the tunnel formation, yet it will dictate segment fabrication and staging at Roberts.

The first four to five months of full production will span the coldest part of the Upper Midwest's calendar. During that window, when temperatures might easily fall below zero, more than one-third of the project's segments will be subject to outdoor storage. At any point during construction, the pace of ring placement could spike and leave segment inventory at or near a minimum level. That factor, along with limited indoor storage space, prompted County and Technopref to adopt a curing scheme that would enable product to be turned in as little as five days. Throughout most of the 14-month production, County Concrete will operate with two shifts Monday through Friday.

The contract required County Concrete to build a three-ring test segment, demonstrating that product tolerances — including 1/16-in.-diagonal and 1/32-in.-skew — have been met. The bottom ring will remain for random testing.

Thinking short and long term, County Concrete opted for a 450- × 350-ft. storage and staging area, paved to specifications common in superflat floor work. The pavement will help curtail handling-related damage by minimizing segment movement. Maintaining stability of the stacks will in turn expedite ring assembly. While County officials are not anticipating extraordinary construction delays, the area is sized to store up to 600 segment stacks.