Concrete Pumps with Innovative Placing System Send One World Trade Center Skyward
Pumping progress has been sending One World Trade Center (1WTC) skyward at one floor per week. Topping out at 1,386-feet in spring 2012, the high rise is being constructed with two concrete cores and concrete on metal decks utilizing an innovative placing system. The system was conceived in cooperation with the manufacturer of the pumping equipment and Collavino Construction Company Limited, a US/Canadian based company and American Concrete Pumping Association (ACPA) member that is forming, pumping and finishing all cast-in-place concrete for the project including some of the highest strength mixes ever used. The final structure will have a shrouded mast that will take it to 1,776-feet making it the tallest building in the United States.
Below grade concrete construction at 1 WTC situated on the 16-acre World Trade Center Site in Lower Manhattan began in April 2006. The excavation for the 80-foot deep hole is located over a slab from the original World Trade Center. The slab separates the excavation from an existing subway line. Crews worked around the slab and approximately 300,000 commuters who use the subway system daily. Collavino moved a Schwing S 58 SX to the site in Summer 07 to start mass pours of slabs that vary from 12 to 36-inches thick and core walls and columns as thick as 6.5-feet. Pumping from street level the truck-mounted boom pump reached as much as 175-feet horizontally while managing to meet all pour schedules with its 213 cubic yard per hour capability. The pump’s Super X curved outriggers, with a front spread of 29’ 2”, kept the footprint to a minimum in the crowded downtown New York City setting.
Test pumping of the 14,000 psi mix assured the construction team that the mix could be placed at required volumes. The mix designs includes: 300 lbs. of cement, 70 lbs, of fly ash, 475 pounds of slag, ¾-inch nominal aggregate, 3 percent air and a water/cement ratio of .3 average for the 14,000 psi which was used up to the third floor. Then the concrete strength requirement for the core reduced to 12,000 psi up to the 35th floor, 10,000 psi to 77th, and then 8,000psi to the roof. The amount of cement in the mix met a LEED requirement set by the owners of the project. The 14 000 psi concrete on the elevator shaft had 160° F maximum temperature requirement on the inner wall with a maximum 30°F difference on the outer. That meant pouring ice on it to cool it in winter and summer to control the temperature differential. The chilled concrete also provided some insurance against extended haul times through the two-mile Battery Tunnel which connects the batch plant on Long Island with Manhattan underneath the East River. Most trips average thirty minutes.
Collavino’ 58-meter was supplemented by smaller booms operated by Schwing owners, and ACPA members, Our Rental of Long Island as construction accelerated on the five levels of below grade concrete which culminated in 50,000 cubic yards being pumped by early 2010. “We are on schedule, but in retrospect, I believe we should have started with the form-mounted placing system we designed for the above grade pours right from the start because real estate is hard to come by when pumping from the street in New York,” explained Renzo Collavino, president of the concrete firm. The above grade placing system is a collaboration of Schwing’s staff stretching all the way to the home office in Germany.
“We discussed the placing system with Mr. Schwing, his son and representatives from Schwing America at the Bauma Fair in 2007, “Collavino explained. The system that they devised has provided high production, versatility and through the help of Schwing engineers, a material handling method for more than just the concrete. “This system works well with the New York style of high rise construction, where the steel is placed ahead of the core and decks, allowing us to place concrete on multiple levels simultaneously,” stated Collavino. The placing system rides on two EFCO self-climbing form systems that are used to pour the dual 120-foot by 60-foot cores including cells.
Pump placement is critical for ready-mix access in downtown Manhattan. A site was chosen across the street from the building but still within the site with three pipelines running under the street and entering the building site one floor below grade. Two Schwing SP 8800 stationary pumps with Tier 3 Deutz 590 horsepower diesel engines are positioned under a temporary roof with dual chutes perpendicular to the hoppers. Truck mixers can be positioned on both sides of the pumps on ramps. The pumps feature high and low pressure settings, high pressure Rock Valves with dual shifting cylinders, and provide piston side pressures up to 2920 psi. Maximum output on the rod side is 123 cubic yards per hour at 31 strokes per minute.
The three pipelines are encased in a thrust block making a 90-degree turn upward through the center of the building between the North and South Elevator Cores. The pipelines are painted red, white and blue. Anchors are placed in the rising core to attach the pipeline sections. The 5-inch ConForms pipe is ½-inch wall up to the 50th floor where they will downsize to ¼-inch system.
Where the pipelines meet the forming system, one line is designated for the north core and the other for the south core. Each set of forms carries a Schwing 31 XT 5-section separate placing boom that features a telescopic first section providing 15’11” of movement. A patented Auto-Scissors pipeline rotates to provide a continuous hard line during the telescopic process. The booms are mounted on Schwing’s unique octagonal masts, which are bolted to the climbing forms. The boom’s have 87-feet of horizontal reach from the slewing axis and can rotate 550-degrees. The units were adapted as separate placing booms especially for this project but are available on a Schwing truck mounted boom pump called the S 31XT. “Because the steel is always in the way of the pour, the versatility of these booms with 270 degree articulating fourth and fifth sections allows the crews to pour over and around obstacles, “explained John Abbey, Collavino’s supervisor of pumping.
The high strength twin cores of the structure consume 325-tons of steel per 13’6” lift below grade, 240-tons per lift up to the 20th floor, 145-tons per lift up to the 50th floor, 75-tons per lift to 75th floor, and 50-tons per lift to the roof. This massive requirement for reinforcement meant a method of handling the re-bar would have to be devised to keep up with the climbing form system and pumping combination. Collavino personnel asked Schwing engineers if the same octagonal masts used for the separate placing boom mounting could be adapted for Fassi F800XP knuckle crane arms that would assist in handling the steel for the cores. Schwing’s engineers worked with Fassi personnel, and the results have each climbing form system carrying a placing boom and a small crane. One of two tower cranes on the job off loads steel onto the decks and the knuckle booms feed the steel into the forms. “The tower cranes can drop steel when they can and we take it from there with our Fassi cranes,” commented Collavino.
Collavino crew’s are maintaining one week per floor cycles that consist of core pours Monday and Friday (1,000 yards), deck pours on Tuesday (400 yards) and ring slabs (225 yards) which join the deck to the core walls on Wednesday and Thursdays. Stairwells and steps are poured with the system as well. The versatility of the two pumps, two boom, two pipeline system allows the cores to proceed at different rates. It also allows a deck to be poured at the same time a core pour is being accomplished at a different level. And both pumps can be devoted to one pour if necessary. The SP 8800’s high pressure setting is used to pour the high strength mix at the upper levels and the high volume setting is used to pour the 4,000 psi deck mix. “We can be pouring on three floors in one day with the system, “explains Abbey, “And we rely on our pumps because they must perform everyday.”
The visibility of the project is possibly unprecedented because of the history of the site. “Millions of people are watching this building go up and we are surrounded by fifty-story buildings where many of the project stakeholders have offices, “according to Collavino, “If there is a puff of smoke on the project, the phone rings.” As the building progresses to its 105-story ultimate height in spring 2012, it will dominate the lower Manhattan skyline and the view will be excellent from the final concrete structural slab pumped at an elevation of 1,339’-10”-feet above street level. By that time three other high rises will be near completion on the 16 acre World Trade Center site. Tenants of One World Trade will occupy 2.6 million square feet with views of a Memorial Garden and Museum. Estimated cost of the building is $3.1 billion with 46,000 tons of steel and 210,000 cubic yards of concrete used in the construction.
Owner/Developer: Port Authority of New York
Architects: Skidmore, Owings and Merrill LLP, New York, NY
Construction Manager: Tishman Construction New York, NY
Prime Concrete Sub-Contractor: Collavino Construction, Windsor, Canada
Equipment: Schwing S 58 SX truck-mounted boom pump, two SP 8800 stationary concrete pumps, two Schwing SPB 31 5-section separate placing booms, Schwing S 31 XT truck-mounted boom pump.