There is no building in the world for which resilience is more important than One World Trade Center. Cast-in-place reinforced concrete, with its structural strength and fireproof nature, was chosen to ensure the safety and durability of this security sensitive structure.
The building's structural system is a hybrid of structural steel and concrete. A concrete core runs the full height of the building and provides a support spine; stairwells and elevator shafts are also encased in cast-in-place, reinforced-concrete. The tower has a concrete base that measures 200 x 200 feet and rises 70 feet above street level. This pedestal is heavily reinforced to protect against explosions.
But despite the extreme priority of resilience in the finished structure, other sustainability measures were not ignored. The Port Authority of New York and New Jersey (the project owner) mandated the use of high-strength concrete containing no more than 400 pounds of cement per cubic yard. To that end, researchers formulated and tested various mixtures of high-strength concrete, successfully reducing the amount of cement and, consequently, the CO2 emissions associated with its manufacture.
High strength concrete uses less water than traditional mixtures, substituting chemicals and aggregates—fly ash, slag cement and silica fume—to maintain workability. One World Trade Center has the highest strength concrete ever poured in New York City: 14,000psi (96Mpa). The high strength mix enabled structural members to be smaller than a traditional mix would allow and also provided extra durability in susceptible areas such as stairwells. According to the American Concrete Institute, the 71 percent cement replacement resulted in an environmental savings that was equivalent to 1,835,494 gallons of gasoline, 29,872 barrels of oil and 1,177,329 half‐liter bottles of water. Strides made by engineers for this project are expected to drive future trends in using high strength concrete for skyscrapers.
These innovations in concrete construction, combined with the material's traditional benefits (ranging from thermal mass to local production) are helping ensure a stronger, safer future at the World Trade Center site.