Pumping concrete is a fast, flexible, and efficient placement method which has grown in use along with the capabilities of pumping equipment and the skills of pumping contractors. Some recent projects demonstrate new pumping techniques and applications.
One World Trade Center rises at ground zero
Thanks to an innovative placing system, the concrete cores and floor slabs of One World Trade Center (1WTC) are going up at the rate of one floor per week. The main structure will top out at 1386 feet in spring 2012. With the eventual addition of a shrouded mast, the high-rise will reach 1776 feet, making it the tallest building in the United States.
Excavation for 1WTC was done above a slab from the original World Trade Center, which separates the excavation from an existing subway line serving about 300,000 commuters daily. Since summer 2007, Collavino Construction Co., New York City, has used a series of Schwing concrete pumps for mass pours of slabs that vary from 12 to 36 inches thick, and core walls and columns up to 61/2 feet thick.
The project includes some of the highest-strength concrete mixes ever used, including a 14,000-psi mix specified for the elevator cores up to the third floor. The core concrete compressive strength requirement is reduced to 12,000 psi up to the 35th floor, then to 10,000 psi up to the 77th, and then 8000 psi to the roof. The mixes used a combination of cement and supplementary cementitious materials (both fly ash and slag) to help meet a LEED requirement set by the project owners. Still, the 14,000-psi concrete on the elevator shaft developed so much heat of hydration that it needed to be chilled in both summer and winter to keep its maximum temperature and the temperature differential from inner to outer wall within acceptable limits. Chilling the concrete also provided some insurance during the 30-minute trips from the batch plant on Long Island to the jobsite so that the concrete never exceeded the maximum temperature for the mass placements.
Collavino and Schwing devised a material handling technique for both the concrete and reinforcing steel. In the New York style of high-rise construction, steel is placed ahead of the core and decks, making it possible to place concrete on multiple levels simultaneously. The placing system rides on two EFCO self-climbing form systems that are being used to pour the two 120x60-foot cores.
Two stationary pumps positioned across the street from the building supply ready-mixed concrete through three pipelines that run under the street and enter the site one floor belowgrade. Truck mixers deliver concrete to the pumps from ramps positioned on both sides. The three pipelines, encased in a thrust block, make a 90-degree turn upward through the center of the building between the north and south elevator cores. The third pipe serves as a backup in case one of the others becomes clogged or jammed.
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 separate placing boom with a telescopic first section. The booms are mounted on octagonal masts bolted to the climbing forms. With 87 feet of horizontal reach from the slewing axis, the booms can rotate 550 degrees, which allows the crews to pour over and around obstacles.
Each climbing form system carries both the placing boom and a small knuckle crane attached to the masts to help handle reinforcing steel for the cores. One of two tower cranes on the job offloads steel onto the decks, and the knuckle booms feed the steel into the forms.
Collavino crews are placing one floor per week, pouring cores Mondays and Fridays, decks on Tuesdays, and ring slabs (which join the deck to the core walls) on Wednesdays and Thursdays. Stairwells and steps are poured with the system as well. With the two-pump, two-boom, two-pipeline system, cores can proceed at different rates, or a deck can be poured while a core pour is occurring on a different level.
Special train lays slab track
A new railway tunnel, opened last year in the city of Malmö, Sweden, provides enhanced travel options between Malmö and Copenhagen, Denmark. Slab track was laid in the two 3.73-mile tunnel sections. Slab track is a type of railway line in which the ballast is replaced with a solid track structure made from concrete or asphalt. For trains traveling at speeds of more than 125 mph, slab track offers better stability and lower maintenance costs than conventional track. It also resists deformation and weathering better, so track position problems, and associated speed restrictions, rarely occur. The ties are embedded in the concrete using flexible inserts that absorb the vibration caused by high-speed trains.
The track installation required the utmost precision. Ties freshly embedded in concrete must not be subjected to vibration, as even slight variations in the alignment of the tracks can make the railway unsuitable for high-speed travel.
Railway engineering specialists Rhomberg Bahntechnik GmbH, based in Bregenz, Austria, were engaged to construct the slab track in the Malmö city tunnel. The objective was to install almost 71/2 miles of concrete track bed with minimal vibration, placing concrete at a rate of about 50 to 65 feet per hour.
Rhomberg Bahntechnik worked in collaboration with Putzmeister’s Austrian company, Hans Eibinger GmbH, and its concrete project division in Aichtal, to produce a tunnel-concreting train based on Rhomberg’s patented installation solutions.
One major consideration in planning the tunnel-concreting train was that the freshly embedded tie shoes could bear only a certain amount of load. Beginning with the setting rate of the concrete and the travel speed of the concreting train, the engineers calculated that a distance of between 820 and 1300 feet had to be maintained between the concrete paving machine and the pump unit. The entire pipeline would have to be made mobile. The pump unit consisted of an electrically powered concrete pump and two concrete mixing drums--the pump supplied the system with fresh concrete while the mixing drums provided enough concrete to maintain a continuous concrete placement.
The concreting train consisted of several railroad cars, which transported all of the components necessary to carry out a smooth and continuous procedure. This included space for cleaning and maintenance work and an emergency power generator.
A Putzmeister stationary concrete pump was positioned on the third car, along with a small conveyor belt for feeding the pump from the mixers. In order to prevent the pump from drifting during the pumping procedure, the rail car’s frame was filled with concrete and the support legs of the pump screwed directly to it. The pump was operated using a cable-connected remote control.
The concrete was transported from the pump to the concrete paver up to 1300 feet away via a steel delivery line supported on special rail cars. The maximum permitted concrete pressure of the delivery line was 1885 psi. The delivery line diameter was 4.9 inches and its wall thickness was 0.28 inch. Because the slab track in the Malmö city tunnel also has a long curve with a radius of about 2500 feet, the line was adapted so it could run on a patented alignment system made by Rhomberg Bahntechnik.
The paving machine, located at the end of the delivery line, consisted of an upstream work platform and a hopper. Three chutes directed concrete to the track bed, where it was presmoothed using skimming boxes, then consolidated using vibrators.
Particular attention was paid to disposal and cleaning procedures in order to meet Sweden’s strict environmental regulations. To that end, a media separating device was installed in the concrete delivery line to ensure that residual concrete and washing water wouldn’t mix, so that the residual concrete in the line could be used in the track bed. The media separation system consisted of a combination of sponge balls, soaked cement bags, and a washout pig.
Pumped cellular concrete eases loft conversion
First, a visionary developer recognized the potential value of renovating an unused factory complex for combined living and work space. Then, a forward-looking contractor brought an innovative concrete pumping application to help make the project possible.
The former typewriter factory has five interconnected buildings located along the Erie Canal in North Tonawanda, N.Y., near Buffalo. Now known as Remington Lofts, the project incorporates 81 one- and two-bedroom loft units available for rent, as well as boat docks, a high-end restaurant, a yoga-based wellness center, and a hair design school and salon. Its Buffalo-based developer, The Kissling Interests LLC, expects the project to attract young entrepreneurs and to help revitalize the area.
Converting the open factory space into residences required extensive leveling of the concrete floor slabs across the complex, but the existing structure lacked the capacity to support the needed volume of normal-weight concrete. General contractor/project manager R&P Oakhill Development LLC, Buffalo, brought in Geotechniques LLC, a Lancaster, N.Y.-based contractor, to come up with a solution. Geotechniques owner Mick Honeck recommended using cellular, or foamed, concrete for most of the new roof and floor slabs, topped with a thin layer of standard concrete for a strong and durable surface.
Geotechniques placed just under 1700 cubic yards of cellular concrete on the project. Ready-mix trucks arrived onsite carrying only 2 to 3 cubic yards of ready-mixed concrete. The contractor then used a Mearlcrete foam generator and BASF’s Rheocell 30 foaming agent onsite to generate a closed-cell foam that then was added to the concrete in the truck. When mixed with the foam, the ready-mix would expand to 9 or 10 cubic yards of cellular concrete. After mixing, the truck would discharge the material into the pump for placement.
“We used a Blastcrete 3-inch Hydraulic Squeeze Pump,” says Geotechniques engineering liaison Kristin Rotella. “It works through peristaltic action that lets us pump the material to the necessary heights but is gentle enough not to destroy the bubble structure. On the Remington Lofts job, we were able to place 12 to 20 cubic yards of cellular concrete per hour at slab thicknesses ranging from 2 to 12 inches, depending on the specific location,” Rotella says.
In addition to building construction and rehabilitation projects, Geotechniques has pumped cellular concrete for a wide range of applications, including sliplined pipe grouting, underground fills and closures, void fills, runway overrun areas, soil replacement, and tunnel backfill.
Rotella also described a recent project in which the firm pumped cellular concrete to encase a vial of radioactive waste material within a steel box. The cellular concrete was intended to keep the vial from shifting around inside the box during shipment, without adding significantly to the shipping weight.