DEFT CAST-IN-PLACE AND PRECAST STRUCTURE ENGINEERING, BARGE FLEET MAKE WAY FOR TAPPAN ZEE BRIDGE REPLACEMENT
by Ken Stadden
The 3.1-mile New NY Bridge is the single largest bridge construction project in New York’s history. The cable-stayed, double span structure, connecting Rockland and Westchester counties across the Hudson River, will replace the aging Governor Malcolm Wilson Tappan Zee Bridge by 2018, thanks to an aggressive construction schedule, lightweight design, and extensive use of near-record scale precast concrete components—some north of 300 tons and engineered as stay in place forms for horizontal, reinforced concrete pier members.
|An optimized pier scheme, coupled with a construction plan leveraging massive precast sections—chiefly 300-plus ton pier cap (above, center) and pile cap (above) tubs, all serving as stay in place formwork—were central to Tappan Zee Constructors’ successful design-build proposal. The 750-ton “Hank Hummel” ringer crane is one of two colossal, floating workhorses deployed to make short work of mammoth precast picks.
Helping to justify the massive new project were high projected maintenance costs on the old cantilever bridge, estimated at $3 billion to $4 billion over the next 20 years. Also, current traffic of about 140,000 vehicles per day far exceeds the capacity of the old bridge, which carried fewer than 40,000 daily during its first decade. The bridge was erected from 1952 to 1955 and designed to last 50 years.
Due to more than 10 years of bureaucratic delays and continuing deterioration of the original bridge, an ambitious plan was devised, beginning with a geotechnical investigation in 2013 and reaching a significant milestone as early as the end of 2016—the opening of the westbound span, which will temporarily carry two-way traffic until opening of the second span, scheduled for 2017. Demolition of the old Tappan Zee Bridge will be concurrent with the second span construction, with all work to be completed by early 2018.
WHY TZC PREVAILED
Submitting bids in competition with two other design-build joint ventures, Tappan Zee Constructors, LLC was awarded the contract by New York State Thruway Authority in January 2013. TZC’s aesthetically pleasing design would require less dredging and less concrete than two competing designs, factors contributing to the firm’s low bid of $3.14 billion. The TZC consortium consists of four major partners, American Bridge Co., Fluor Enterprises, Granite Construction Northeast, and Traylor Bros.
The winning design features 419-ft. tall outward-leaning concrete towers placed 1,200 feet apart, supporting twin cable-stayed road decks 139 feet above the water to accommodate ships traversing the Hudson’s main navigation channel. A requirement that the bridge go 100 years without major maintenance affected decisions ranging from concrete mix selection to number of precast components specified.
Approach spans rest on 48- and 72-in. piles driven to bedrock, or in the case of about one-third of them, driven into deep silt 330 feet below the riverbed to act as friction foundations.
Concrete Products examines the cast-in-place and precast concrete aspects of the New NY Bridge in a two-part series. This month’s focus on C-I-P structures, especially TZC’s three barge-based concrete plants, will be followed in September with visits to the Virginia operations of Bayshore Concrete Products and Coastal Precast Systems.
FLOATING PLANT TRIO
|TZC added a third plant in mid-2015, orienting the Erie Strayer equipment on a 54- x 180-ft. vessel. The placing tower for the pumped mixes is at the left. The BHS mixer discharges into an 18-yd. Maxon remixer, which serves as a buffer for the Putzmeister 14000 pump. In addition to the pump, the newest plant is equipped with a Putzmeister MXTB600 placement conveyor with 110-ft. reach and 150-yd./hour capacity.
|TZC has opted for barge-based production for about 75 percent of the concrete and grout to be placed on site as dense residential development on both sides of the Tappan Zee Bridge left no space to permit a temporary plant. Additionally, project principals deemed impractical the movement of mass quantities of concrete to pours along a three-mile wide waterway. Mix production at the placement point augments tight quality control, while reducing delays from rejected loads. Traffic wise, the barge-based plant scheme eliminates upwards of 20,000 loads of ready mixed by conventional delivery. Mixer trucks dispatched from New York and New Jersey plants of Eastern Concrete Materials, however, will shoulder about 70,000 yd. for placement at certain site points.
At the job site based in Tarrytown, N.Y., TZC brought a third floating concrete plant online in June, an Erie Strayer with a BHS twin-shaft mixer. The previous two plants were also equipped with BHS twin-shaft mixers. “Two plants were on the original bid, but we decided three plants would allow us the flexibility to accelerate all aspects of the work,” says TZC Lead Engineer Wayne Dabrowski.
The latest plant’s barge measures 54- x 180-ft; the initial two 54- x 150-ft. All three plants can feed 80 yd. of concrete per hour to a Putzmeister 14000 pump serving a placing tower with articulating boom and 5-in. hose. Operators at the new plant benefit from an additional placement option: a Putzmeister MXTB600 conveyor with 110-ft. reach and 150 yd./hour capacity. A small Schwing pump capable of moving 30 to 40 yd./hour is used for grout pours. Typical daily output from the new plant is 200-plus yards.
An overview of the Erie Strayer floating plant begins with a small excavator chained to one corner, moving rock and sand from adjacent supply barges into hoppers. Load cells weigh the aggregates, slag cement is added from a 550-bbl onboard silo, and screw augers move cement and fly ash from a robustly braced 300-bbl split silo, built with TZC design input by Shumaker Industries. A 48-in. inclined belt moves aggregates up into a weigh hopper while admixtures enter the hopper through as many as six hoses. Water comes from a 25,000-gal. Pearson water tank, heated and fitted with a Pearson chiller.
Though capable of shorter turnaround, the twin-shaft mixer runs 90-second cycles per NYS Thruway requirements, dropping its 6-yd. load into an 18-yd. Maxon Industries remixer, which acts as a buffer to feed the concrete pump’s 1/2-yd. hopper. The addition of the placement conveyor enables crews to limit the number of hose swings over the water, an environmental benefit, and increase output.
A 480-volt, 800-kW Caterpillar generator and 125-kW backup generator provide electricity; the concrete pump has its own diesel power pack to generate hydraulic pressure. During a pour the barge is anchored by two spuds—posts hydraulically lowered to the riverbed. In addition to supply barges, crane barges are sometimes placed adjacent to the plant for stability. Finally, crew quarters and second-story batch room occupy a central location on the barge.
Mixing & Testing
The concrete and grout schedule at the New NY Bridge operation spans pile, pile and pier cap tub, and pier pours. A sprawling deck of 6,000 precast panels, 40- x 12-ft., will consume the largest volume of grout.
Work thus far has involved primarily two concrete mixes, notes Dabrowski, both able to be placed above and below the splash zone. “They’ve been designed to be robust enough to be used nearly anywhere on the job,” he says. The large Putzmeister pump moves up to 80 yards per hour of what’s described as a “sticky mix,” which tests with a one-inch slump before chemicals for workability, freeze/thaw, permeability and compressive strength are added. “All mixes break at 8,500-10,000 psi due to everything we have in the mix,” says Dabrowski, well above the 5,000 to 6,000 psi required.
The main mix uses #67 (3/4 in.) stone, with #8 (3/8 in.) stone specified for closure pour mixes such as those between tub and piles. In addition to chilled water in the mix, cooling lines are employed at the pour site to reduce the heat produced by high slag and cement content.
Each floating concrete plant has its own lab, housing two quality assurance and two quality control specialists, plus NYS Thruway oversight inspector. Slump measurement, air tests, and cylinder preparation are performed every 50 batches. Plants are certified by the New York State Department of Transportation, which also does annual and random quality audits. Tolerances for concrete are ± 1 percent for cement, and ± 2 percent for aggregates. Altogether, “Several hundred people are involved in QA, QC and random testing,” says Mogheera Nagra, a project engineer working for GPI and assigned to the TZC project. “Concrete, rebar, all materials are tested.”
Dabrowski reports that waste concrete comprises less than 0.5 percent of production. Forty 6-yd. capacity, ¼-in. poly lined waste pans are used to collect leftover and rejected loads. TZC has two waste barges for hardened concrete, and two waste scows requiring excavation. Waste concrete is barged to a yard in Tompkins Cove, 12 miles up the Hudson River.
Project status, milestones
TZC is about halfway to project completion as of August 2015. A notice to proceed was issued in January 2013; dredging and pile installation began later that year. In 2014, main span substructure and approach substructure were started; in 2015, piers are rising and caps are being installed. Main towers will follow, then bridge decks will be installed. By late 2016, traffic will be diverted to the westbound span and demolition of the old bridge will begin. In late 2017 the second, eastbound bridge should be completed less than five years from the design-build project’s start. Finishing work will continue into 2018, with final physical completion scheduled for April 2018.
Lancaster, Pa.-based Ken Stadden specializes in business-to-business marketing communications. He prepared the New NY Bridge report on assignment from BHS-Sonthofen Inc., with assistance from Mixing Division Manager Stuart Bentley. In next month’s second and concluding part, he will examine the critical role of Bayshore Concrete Products and Coastal Precast in TZC Constructors’ timely delivery of a Tappan Zee Bridge successor. A follow up report will focus on Unistress Corp. and its record precast deck panel contract.
NEW NY BRIDGE IRON, CONCRETE SCHEDULES
Equipment needed to build a 3.1-mile bridge:
- 48 cranes (35 floating, including Left Coast Lifter/I Lift NY super crane);
- Three floating ready mixed plants;
- 215,000 yd. of concrete from barge plants; 70,000 yd. from two land-based ready mixed plants;
- Over 100 barges, counting maintenance, supply and cranes;
- 15 boats, 25-, 36- or 40-ft. for ferrying crews; larger vessels have twin Cat or Cummins diesels;
- 7 million gallons of potable water via barge from Haverstraw, N.Y., municipal supply;
- Tarrytown, N.Y., field office for about TZC engineering staff and crews; and,
- Six Bobcats in sand and rock barges.
Raw materials are staged at suppliers’ operations, and immediately relayed to the three TZC plant via five supply barges. Suppliers are Tilcon, providing rock from Clinton Point; Roanoke Sand & Gravel, sand from Long Island pit; and, Lafarge North America, cement and Newcem slag cement. Coarse and fine aggregate barges bear enough material for 1,800 yd. of concrete. Barges transferring cement and slag tankers, water and BASF admixtures are loaded for the equivalent of 1,000 yd.
BHS TWIN-SHAFT mixers exhibit mettle from the Hudson River to the Chesapeake Bay
Seven BHS twin-shaft mixers are producing the lion’s share of the estimated 210,000 yd. of concrete for New NY Bridge cast-in-place structures, as well as tens of thousands of additional cubic yards for the bridge’s main precast structural elements, primarily tubs for pile caps and pier caps.
TZC’s three floating ready mixed plants on the Hudson River each have a BHS DKXS 4.5 6-yd. mixer, two running within an enclosure (the new plant shown here will get an enclosure before winter). “Our previous experience was with drum mixers, so we relied on research and reputation” when selecting a twin-shaft mixer, says TZC Lead Engineer Wayne Dabrowski. “Our mix design needed that shear for uniformity, and BHS came highly recommended. So far, mixing has been consistent, and is the main reason we’ve had success with this project.”
BHS DKXS 4.5 twin-shaft mixers are at the heart of TZC’s three floating batch plants on the Hudson River. BHS engineers underscore the location of the outboard-mounted V-belt drives to facilitate replacement. The main shaft bearings are likewise mounted outboard, away from the shaft seal, to curtail contamination. A hexagonal shaft allows mixer arms to be spaced 60 degrees apart for rapid mixing at low rpm.
At its Cape Charles, Va., location, Bayshore Concrete Products Corp. has been split batching with two BHS DKXS 4.5 6-yd. models for seven years, noting minimal maintenance requirements. “Paddle replacement has been routine, and one mixer was relined for the first time since 2008,” says Bayshore Plant Manager Chuck Hook.
At Coastal Precast Systems in Chesapeake, Va., a second BHS DKXS 3.0 4-yd. mixer has joined an initial model. “The first was a routine upgrade over a previous twin-shaft mixer,” explains Paul Ogorchock, owner and president. “The second is for Tappan Zee and other projects, and will allow us to do split batching.” For the Tappan Zee and other projects, “the BHS mixers help us reach that 5,000 psi strength in 24 hours,” says Ogorchock.
BHS-Sonthofen GmbH is based in Sonthofen, Germany, with subsidiaries in the U.S., China, India and Russia. A member of the Concrete Plant Manufacturers Bureau, BHS is credited with inventing the twin-shaft mixer in 1888, and is a dominant player in the worldwide market with a line of heavy-duty models. Unique features of BHS mixers include hexagonal shafts which place mixing paddles 60 degrees apart, outside-mounted V-belts for easier replacement, worm-drive gearbox rather than planetary gears, and shaft bearings mounted in separate bearing boxes to protect them from seal leakage. “The worm drive is mounted directly on the shaft using a torque arm, providing float that absorbs shock,” notes Stuart Bentley, Mixing Division manager of Charlotte, N.C.-based BHS-Sonthofen Inc. Driven at only 19 rpm, he adds, the closely-spaced paddles promote intense mixing that is 95 percent complete in as little as 30 seconds.