One World Trade Centre
New York, USA
Soaring to a height of 1,776 feet (540-meters), the 242,000m2 skyscraper known as 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 called the Freedom Tower, began in April 2006 and it is now the tallest building in the United States.
Sustainable design was a central theme to One World Trade Centre’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 97 MPa to 83 MPa for the lower 40 floors and 69 MPa to 59 MPa for the upper floors. The 83 MPa concrete phase of the project was the most challenging, with the engineers, owners and contractors all having their own requirements and specifications.
- Compressive strength: 83 MP at 56 days
- Over-design for safety: 13 MPa
- Modulus of elasticity: 48 GPa
- Heat of hydration: Not to exceed 70°C)
Port Authority of New York/New Jersey requirements:
- Quantity of Portland cement in the mixture less than 400lb/yd3 (240 kg/m3)
- 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.
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 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 (EPD) was conducted, using a methodology validated by NSF International, to compare the specialized EF Technology mixture to a reference mixture. The results of this cradle-to-gate analysis are included in Table 1.