High-Performance Concrete At World Trade Center
|The iCrete system means that there is less reliance on reinforced steel. In addition, at Freedom Tower, the project was actually able to reduce the total amount of cement used as well.|
In designing the Freedom Tower at One World Trade Center at Ground Zero in New York City, the Port Authority of New York and New Jersey had some pretty tough specifications for their concrete for their $2-billion project, which will be under construction through some time in 2012. Some walls would be nearly 6 feet thick, requiring high strength and a low heat of hydration to insure no thermal cracking. The iCrete Corporation, which produces special mixtures of concrete, rose to the occasion and provided the material for the project's 240,000 cubic yards of concrete.
iCrete is used in the 1,776-foot tower itself, as well as the 186-foot inner safety core supporting the superstructure. Since the first pour at Freedom Tower in October 2007, iCrete mixes have been used at more than 30 construction locations in the Greater New York tri-state area alone. Such projects include Eleven Times Square, Beekman Plaza, The Harrison – all in New York City, and construction of the Revel Hotel and Casino in Atlantic City, NJ.
The formulation for this concrete has earned endorsements from people like architects Frank Gehry, Robert A.M. Stern and David Childs, as well as key contractors working on major construction projects within the New York City area. Its unique qualities allow the concrete to have a strength of 14,000 pounds per square inch (psi), whereas the highest strength of a high-rise in New York City had been 12,000 psi before being used at Freedom Tower.
iCrete – Features And Benefits
“The iCrete System is a revolutionary concrete mix design and production enhancement technology system that significantly improves concrete performance, explains Juan Carlos Terroba, CEO of iCrete, and former chairman of the largest independent concrete producer in Mexico, Productos Cementeros Mexicanos. “Users obtain greater consistency, greater cost efficiencies and a reduced carbon footprint. Exactly how the materials – ingredients comprised of air, cement, crushed aggregate, sand and water – are mixed is the key to iCrete's performance.”
According to Terroba, iCrete mix designs have significant benefit for contractors using it. Because the product has some unique characteristics that allow it to be installed quickly, it ends up saving time, which can translate into labor costs as well as adherence to schedule.
“They can reduce labor time and shorten construction cycles because it is easier to place and to finish,” he says. “They deliver a more consistent product with fewer rejected loads. They can lower the heat of hydration for mass pours.”
For a general contractor there are notable environmental advantages to the product. It helps reduce the amount of cement needed per mix, resulting in less harmful greenhouse gas emissions. The cement industry alone has been responsible for some 5 to 7 percent of global carbon emissions worldwide. The incidence of iCrete in a project can reduce such emissions by 40 percent according to the company. “If the specified contractor is working on a project that is going for LEED certification, using iCrete will contribute to that goal,” says Terroba.
When using the iCrete system, the reformulation of current mix designs and the process of particle packing optimize the use of raw materials. That is certainly important to the ready-mix concrete producer. The system iCrete uses actually optimizes the void space between aggregates. This reduces the amount of cement paste required to bond the aggregates. As a result, the mixture sets quicker and reduces its finishing time. It also affords reduced shrinkage and less creep. All of these unique characteristics reduce labor time. This also means that there is less reliance on reinforced steel. In the Freedom Tower, the project was actually able to reduce the total amount of cement used by selecting the iCrete system.
Because contractors are always concerned about schedule, it's important to consider materials that might not impede their schedule in any way. One wouldn't ordinarily think that the selection of a concrete product could do that; however, the iCrete technology can produce concretes with enhanced characteristics – easier to work, to place, and to finish.
“All of this can lead to labor savings and shorten construction cycles,” says Terroba. “That speaks directly to contractors. With the iCrete system, quality is monitored, a better product is produced, strict building deadlines are met, risks are reduced, and costs are controlled. Construction managers are always happy to hear that.”
iCrete offers some flexibility in its mix design. When owners, engineers and architects come up with requirements for a building, the selection of materials to be used in fulfilling such requirements can be a challenge. As new technology is introduced, there's always a slam-dunk to be had when a product can improve their profit equation, meet the specs, and perhaps even keep the total cost down. Add to that a green component and, well you're really on to something.
“With flexible and capable iCrete mix design, you can confidently redesign the physical footprint of the building – making support columns and slabs thinner and stronger, opening up more usable square footage in the total design of the building,” says Terroba. “That speaks to owners and engineers, but particularly to architects who can factor more freedom into their designs.”
When iCrete was suggested as a material source for New York's Freedom Tower, it was an implied understatement that the materials used must be the best available and must be ready to withstand some of the most rigid requirements of strength and durability. Some argue that the Freedom Tower is New York's most important new development project, as well as the nation's most visible.
“The tower presented serious challenges,” explains Terroba. “The thickness required for the walls, up to 5 feet 10 inches thick, was one constraint. There were also elevated safety concerns, which led to a requirement for concrete that is exceptionally strong even though the highest strength previously used in New York was 12,000 psi. Compare that to houses, which are generally built with concrete at strength levels close to 3,000 psi.”
In order to meet a high-strength requirement for a project such as the Freedom Tower, an enormous amount of cement would have to be used in the mixture. Ideally, the concrete had to be able to maintain a consistent and lower temperature that could allow it to be installed quickly. “High-strength concrete is often achieved by increasing the amount of cement in the mix,” says Terroba. “But that approach would have been cost-prohibitive at Freedom Tower because so much cement would have been required. The goal was to find a concrete that would maintain a consistent and lower temperature, allowing fast installation times.”
The conventional mix designs that the Port Authority and developers of the Freedom Tower were initially considering were unable to meet the strength demand because those mixes lacked the design technology to satisfy the engineering requirements.
Another high-visibility project that iCrete is being used for is the new Revel Casino Resort, currently under construction in Atlantic City, NJ. Revel anticipates that the 20-acre development, which includes a luxury hotel, a world-class casino, a 5,000-seat state-of-the-art theater, and other amenities, is to be completed in 2010. It is purported to be the “world's greenest casino resort.”
“iCrete technology aligned nicely with the developer's goal of creating the world's greenest resort and casino,” says Terroba. “Strength was also a factor in specifying iCrete at Revel, the largest project in New Jersey to use our designs.”