NEW YORK - Builders of the Freedom Tower poured a bad batch of concrete into the foundation of the skyscraper replacing the World Trade Center and spent the last few weeks removing it after tests showed it wasn't strong enough, officials said Tuesday.
About 50 cubic yards of concrete was jackhammered away from the core foundation of the 1,776-foot tower under construction at ground zero. More than 22,000 cubic yards of concrete have been poured so far, and no other batches have failed strength tests, said Steve Coleman, spokesman for the building's owner, the Port Authority of New York and New Jersey.
The tower, the symbolic replacement to the twin towers destroyed on Sept. 11, is being built with concrete stronger than any other building in New York City.
The foundation should be able to withstand 14,000 pounds per square inch, about three times the strength of concrete in an average home and over five times the strength in a sidewalk. A tower rebuilt north of ground zero two years ago, 7 World Trade Center, was built with concrete at a strength of 12,000 pounds per square inch, officials said.
Experts said many factors could create a poor batch: weather; the temperature at which concrete is poured and hardens; the time it takes for concrete to travel from the plant; and the way it is mixed. The issue is likely to affect the trade center site more as concrete trucks roll in over the next two years to build the memorial to the Sept. 11, 2001, attacks and the concrete for up to five office towers.
The concrete that was removed from the Freedom Tower varied in strength between 6,000 and 10,000 pounds per square inch, Coleman said Tuesday. It was tested once it was poured, on Dec. 18, and separate pieces were removed from the batch and tested successfully a month after the pour, he said.
But contractors working on the site noticed a 6-inch section “did not look right” and appeared to have excessive air pockets trapped in, he said.
The builders removed core samples from the area, tested their strength, and decided in mid-February to take the concrete out, he said.
The work was done over the past several weeks in areas to be north and south of planned elevator shafts, as well as from three other small sections, Coleman said.
Concrete was poured in other parts of the building over the past few months, and the construction schedule for the tower, expected to open in 2011, wasn't affected, Coleman said.
A message left with the main concrete supplier, Quadrozzi Concrete Corp., wasn't returned Tuesday.
The tower, the symbolic replacement to the twin towers destroyed on Sept. 11, is being built with concrete stronger than any other building in New York City.
The foundation should be able to withstand 14,000 pounds per square inch, about three times the strength of concrete in an average home and over five times the strength in a sidewalk. A tower rebuilt north of ground zero two years ago, 7 World Trade Center, was built with concrete at a strength of 12,000 pounds per square inch, officials said.
Experts said many factors could create a poor batch: weather; the temperature at which concrete is poured and hardens; the time it takes for concrete to travel from the plant; and the way it is mixed. The issue is likely to affect the trade center site more as concrete trucks roll in over the next two years to build the memorial to the Sept. 11, 2001, attacks and the concrete for up to five office towers.
The concrete that was removed from the Freedom Tower varied in strength between 6,000 and 10,000 pounds per square inch, Coleman said Tuesday. It was tested once it was poured, on Dec. 18, and separate pieces were removed from the batch and tested successfully a month after the pour, he said.
But contractors working on the site noticed a 6-inch section “did not look right” and appeared to have excessive air pockets trapped in, he said.
The builders removed core samples from the area, tested their strength, and decided in mid-February to take the concrete out, he said.
The work was done over the past several weeks in areas to be north and south of planned elevator shafts, as well as from three other small sections, Coleman said.
Concrete was poured in other parts of the building over the past few months, and the construction schedule for the tower, expected to open in 2011, wasn't affected, Coleman said.
A message left with the main concrete supplier, Quadrozzi Concrete Corp., wasn't returned Tuesday.
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