9/11 goes down in history as the most shattering episode ever. 

The loss of nearly 3000 people on this day and site deserved a befitting memorial. 
A memorial that reflected not just the sorrow but also the resilience and the strength of the human spirit. 
 

The Challenge

• Construction of the 9/11 memorial tested not just the strength of the human spirit, but also the power and flexibility of construction equipment.
   
• Once a concrete jungle the 8 ac (3 ha) memorial site is today very much a part of the busy and congested area of the city.
   
• Rebuilding of Ground Zero encompassed 9 major projects, each different from one other.
   
• The construction involved multifaceted processes, variety of equipment, harsh and varied kinds of mixes, difficult to reach pours - all on an extremely congested job site and that too within a stipulated time frame

Work begins on the challenging 9/11 memorial

The Solution

Nine major projects encompass the rebuilding of Ground Zero, which is a historic construction feat so complex and diverse it is difficult to comprehend the gargantuan size, the multifaceted construction processes, the countless number of contractors and equipment, and the resilient human spirit involved in transforming the site from rubble to reality over the past decade. The 9/11 Memorial is one sentimental part of the big picture.

Before the attacks, a typical big city concrete jungle existed at the site the 9/11 Memorial occupies today. Now, this eight-acre location is evolving into a living space of trees, flowers and grass that surrounds two enormous pools, with the largest man-made waterfalls in the country. he reflecting pools, each nearly one-acre in size, are symbolically set within the footprints of the Twin Towers. Beneath the plaza is a 70-foot below-grade structure containing a museum, train station, mechanical center and other underground facilities. Today, although concrete is still significant, it is less noticeable.

The plaza is scheduled to open this year on the tenth anniversary of the September 11 attacks, and the museum is to open in 2012. To get to these completion points has required the coordination of numerous logistics while working with multiple contractors on the busy site; however in the spirit of cooperation, a common goal is clearly in sight.

Reflecting Absence

The 9/11 Memorial’s design, “Reflecting Absence” by architect Michael Arad and landscape architect Peter Walker, was selected from a competition that included more than 5,200 entries from 63 nations. To build it, Bovis Lend Lease of New York, New York is the construction manager; and Navillus Contracting Inc. (Navillus) of New York, New York, is the concrete contractor awarded the base contract with work that began March 2009. Precision Concrete Pumping, Inc. (Precision) of Albany, New York, is placing the concrete, utilizing every Putzmeister truck-mounted concrete boom pump model in their fleet. This ranges from the compact 20Z-Meter, to the pumping company’s largest, the 63Z-Meter.

The majority of the work, however, has been performed by the larger BSF 47Z.16H and BSF 63Z.16H boom pump models, with horizontal reaches of 138-feet, one-inch and 190-feet, seven-inches respectively. These longer-reaching booms are needed to extend over numerous obstacles on the highly congested construction site that often prevents close setup to the pours. Plus, the two models are equipped with Putzmeister’s One-Sided Support (OSS) system, which reduces the outrigger span on the non-working side of the unit for setup in a smaller footprint. This enables concrete to be placed as scheduled when precious space must be shared with other equipment and materials on site. In addition, with only a handful of spots for pumps to even set up on the large, confined site, additional delivery line must also be used on various pours.

Get Down

The two reflecting pools, although the most symbolic, used only 2500 cubic yards of concrete each for the floors, walls and parapets, a relatively small amount of the 49,000 total cubic yards needed for the 9/11 Memorial. Bill Douglas, Precision’s boom pump operator on-site, says, “Although pumping the iconic pools was fairly straightforward, most of the remaining work below grade was not built in the traditional approach from the bottom up. That’s because the complexity of many tasks and their unusual deadlines prevented following the typical order of steps in the construction process.”

Douglas adds, “To access big pours below street level often required extending the boom into a fairly small hole and snaking it over, under and in-between obstacles. You can’t do that type of maneuvering without the special configuration of the five-section Z-booms on our larger pumps, and of course, a radio to communicate with the crew below.”

Navillus, specializing in high-end commercial work since 1987, cited other challenges. Navillus project manager Kevin Smith says, “A significant portion of our work was immediately adjacent or above the operating PATH train tracks, and this work had to be carried out on service outages that normally involved work from Friday midnight until Sunday evening.”

Tough Job, Tougher Concrete

According to Smith, “most of the concrete on the 9/11 Memorial is for the job’s structural slabs, heavy concrete shear walls and concrete walls. There is a tremendous amount of mechanical space on the Memorial to support the fountains and provide support to other on-site buildings.”

Concrete producer Ferrara Brothers Building Materials in Brooklyn, New York, is supplying nine different concrete mixes. These range from 4000 psi to 12,000 psi, with both normal and lightweight mixes, corrosion-inhibiting mixes, high-strength mixes, and mixes specifically designed for toppings. Although varied, most have one thing in common ? they are harsh.

“The majority of concrete we pumped on this project was extremely tough. However, we operated our Putzmeister pumps at maximum pressure and pumped them without a problem,” says Douglas. “For more reach, we even added 650 feet of delivery line to the end hose of a 63Z-Meter and pumped an extremely tough mix through 850 total feet of pipe and delivery line. That speaks volumes about equipment performance.”

New Life Grows

To symbolize life and rebirth, more than 400 trees will be planted on the plaza, vividly changing colors each autumn on the anniversary of 9/11. Navillus, with a separate contract for plaza site work, is responsible for all structural slabs and drainage structures, as well as building masonry walls to partition off corridors for electrical service maintenance, drainage and irrigation of the trees.

To help the trees survive and thrive, a special soil has been developed with an unusual blend of lightweight aggregate, special sand and worm castings—a fertilizer produced naturally by earthworms. Trucks dump the soil into piles on the job site, and a Putzmeister Telebelt® TB 130 conveyor immediately relocates it to designated areas. The conveyor must work around job site obstacles while telescoping its main conveyor belt out as far as 39 m horizontally to place the grainy soil into special forms, and do so quickly with pinpoint accuracy.

The TB 130 is from the fleet of Our Rental Corp. (Our Rental) of West Babylon, New York. Nick Avella at Our Rental says, “Without the Telebelt’s speed and versatility to backfill the dirt exactly where needed, the task would have required an unimaginable amount of time and labor to place the 25,000 cubic yards of dirt that will ultimately fill the plaza.”

However, the conveyor, just as unique as the structural soil itself, easily places the rocky mix as the machine is designed to convey numerous aggregates from sand to four-inch rock, as well as concrete from zero- to 12-inch slump. And when it comes to placing difficult concrete, Our Rental and its conveyor are no strangers to Ground Zero.

Avella says, “A 16,000-psi concrete with two-inch stone, a 0.28 water-cement ratio and a low 350# cement content proved impossible to pump by concrete pumps for the footings and foundation of the Freedom Tower. However, the belt conveyor could handle the job and conveyed 20,000 cubic yardsof the unpumpable mix.”

“With the conveyor, crews didn’t have to worry about plug-ups,” adds Avella. “Plus, the unit placed stone on this same project. It was so versatile that it was the last machine out of the hole.”

Signature on History

“Ground Zero is so much more than anyone can comprehend without seeing it firsthand,” states Avella. “Almost every imaginable piece and brand of concrete and material placing equipment has had a role in its construction, with the Memorial just one of many projects. During the past ten years, we’ve handled a significant portion of the concrete and material placement at the site; and like others involved, we’re proud to be a part of its historic rebuilding process.”

As the Twin Towers were massive structures, so too is the 9/11 Memorial with the names of every person who died in the 1993 and 2001 attacks being inscribed in bronze panels edging the pools, while 52,000 gallons of water per minute cascade down its sides. This conveys the enormity of the sorrow the world felt that September day. It also conveys the exceptionally huge undertaking by all construction crews at Ground Zero who are putting their signature on history.

A boom pump from Precision Concrete Pumping and a telescopic belt conveyer from Our Rental Corp. work alongside each other at the 9/11 Memorial.

A Telebelt® TB 130 belt conveyor efficiently places a specially-fabricated soil for planting over 400 trees on the plaza.

Working on an incredibly congested job site, the belt conveyor offers a time- and labor-saving way to place 25,000 cubic yards of soil that will ultimately fill the plaza.

Concrete Takeaways

• The name of each person, who died in the attacks are inscribed into the bronze panels placed by the Memorial pools.
   
• The memorial cost $700 million to build, the money was put together mostly through public and private donations.
   
• The memorial design, created by Architect Michael Arad and Landscape Architect Peter Walker, was selected from amongst more than 5200 entries from almost 63 nations.

Project Partners

Construction Manager: Bovis Lend Lease  ̶  New York, New York
Concrete Contractor: Navillus Contracting, Inc.  ̶  New York, New York
Ready Mix Producer: Ferrara Brothers Building Materials  ̶  Brooklyn, New York
Concrete Pumping Contractor: Precision Concrete Pumping, Inc.  ̶  Albany, New York 
Material Placing Contractor: Our Rental Corp.  ̶  West Babylon, New York
Equipment: Putzmeister truck-mounted concrete boom pumps ranging from a 20Z- to a63Z-Meter, Putzmeister Telebelt® TB 130 truck-mounted telescopic belt conveyor