One World Trade Center - New York, New York

One of the biggest challenges in developing this concrete mixture was meeting the Port Authority of New York/New Jersey�s strict requirement for the replacement of cement.

Soaring to a height of 1,776 feet (540-meter), the 2.6-million-sf (242,000-square-meter) building soon to become the new One World Trade Center in the lower Manhattan district of New York City is a marvel of design and engineering. Construction of the building, previously dubbed the Freedom Tower, began in April 2006 and, when it is completed in 2013, it will be the tallest building in the U.S.

The Challenge

Sustainable design was a central theme to One World Trade Center�s development, with the Port Authority of New York/New Jersey imposing a strict requirement for the replacement of portland cement with recycled materials. In addition, extremely high performance concrete was necessary to meet the compressive strength requirements of the steel and concrete structural columns, which ranged from 14,000 psi (97 MPa) to 12,000 psi (83 MPa) for the lower 40 floors and 10,000 psi (69 MPa) to 8,600 psi (59 MPa) for the upper floors.

The 12,000 psi (83 MPa) concrete phase of the project was the most challenging, with the engineers, owners and contractors all having their own requirements and specifications.

Engineering Requirements:

Compressive strength: 12,000 psi (83 MPa) @ 56 days
Over-design for safety: 1,900 psi (13 MPa)
Modulus of elasticity: 7.0 million psi (48 GPa)
Heat of hydration: Not to exceed 160 �F (70 �C)
Non-air-entrained

Port Authority of New York/New Jersey Requirements:

Quantity of portland cement in the mixture: Less than 400 lb/yd3 (240 kg/m3)

Contractor Requirements:

Slump flow: 24 - 28 inches (610 - 710 mm)
Ability to pump to at least 40 floors
No loss in concrete workability during transit and placement
Aesthetically pleasing

To achieve these concrete properties which, combined, would be a groundbreaking feat, concrete producer Eastern Concrete Materials, Elmwood Park, NJ partnered with admixture supplier BASF Construction Chemicals, Beachwood, OH.

BASF�s Green Sense� Concrete program optimizes the concrete formulation to reduce the ecological footprint, conserve resources while increasing the structural strength. According to BASF�s Eco-Efficiency Analysis, 15,838,267 kg of CO2 were prevented in the construction of the building�s first 40 floors.

The Results

Through BASF�s Green Sense Concrete mixture optimization service, Eastern Concrete Materials was able to proportion an EF Technology� concrete mixture with 71% cement replacement. The mixture replaced portland cement with the recycled materials, non-cementitious fillers and specialized admixtures to exceed all the performance targets specified by the One World Trade Center project stakeholders. This EF Technology mixture was used for the 38,000 yd3 (29,000 m3 ) of concrete needed for the columns through the first 40 floors.

To quantify the environmental impact of sustainable concrete for the structure, an Eco-Efficiency Analysis was conducted, using a methodology validated by NSF International, to compare the specialized EF Technology mixture to a reference mixture.