The cast-in-place, reinforced concrete core that boasts superior safety features and robustness has reached the 72nd floor (as of July 2011), with the steel columns stretching several floors higher. There will be over 80 floors in the final structure.
The developer of One World Trade Center, Larry Silverstein, president of Silverstein Properties Inc, has vowed to build ?the world?s safest skyscraper?. The result will be a building with a core of cast-in-place reinforced concrete and an extra strong ?backbone? running the full height of the building.
On the floors already poured at time of writing, the core walls measure nearly a metre (or three feet) thick or more above ground level and up to twice that below grade, making them almost impervious to fire. Higher up, the cast-in-place, reinforced concrete will slim down to 61cm (or two feet) thick concrete walls around the tower core.
The core of a traditional high rise would have a compressive strength of 8000 to 10,000 pounds per square inch (psi). The objective with the new tower is to increase the compressive strength to 14,000 psi, setting a new standard for New York skyscraper construction. The fact that the engineers have been able to alter the concrete mix proportions, lowering the amount of traditional cement and producing greener, more high strength concrete by blending fly ash, slag cement and silica fume with concrete has been an added bonus.
?Creating stronger concrete is basically a matter of figuring out how to fill the voids,? declared Yoram Eilon, vice president of contractor WSP Cantor Seinuk. This involves selecting the right gradation by combining different aggregates that will interlock together tightly ? but not too tightly ? when combined with different cement materials, and water and other admixtures.
Concrete hardens as a result of a chemical process that produces heat as it cures. The use of night pours and a high level of ice incorporated into the mix helped the contractors keep the consistency right while pouring large quantities of concrete with very low water cement ratios. It also enforced the ?green? concept of the project.
The final mix for Tower One was fine-tuned in the laboratory. Prior to its construction, 0.5 tonnes (or 1200 to 1300 pounds) of cement material was used to produce 12,000 psi concrete in high rise commercial construction projects. As a result of the work done on Tower One, a concrete mix of 0.4 tonnes (900 to 950 pounds) can now be used in other high rise commercial construction projects, producing a greener, economical concrete.
?This product will probably lead to fairly wide use of 15,000 to 16,000 psi concrete,? Yoram Eilon said. ?And that will create new structural opportunities for high rise buildings.?
The One World Trade Center will incorporate many ?green? footprints, including advanced energy conservation technology, next generation glass, reusable water conservation and 40 per cent less carbon emissions per tonne of concrete produced.
?We have many case histories and research studies that show concrete is the best choice for the environment,? said Robert Ledwith, president of the Concrete Alliance Inc, a non-profit consortium of skilled union craftsmen and contractors that seeks to improve public awareness of the superior safety and strength of cast-in-place reinforced concrete. ?And when combined with modern techniques like post-tensioning and cast-in-place pouring, reinforced concrete can also be the most economic choice for a building?s developer and certainly outperforms other alternatives in operating costs over a building?s lifetime.
?I?m confident,? he added, ?that One World Trade Center will be the latest proof of concrete?s overall superiority.?
The new World Trade Center development will be opened for business in late 2013.
Sources: H Margrill & Associates, Real Estate Weekly (US), Aggregate Research