NEW YORK (AP)  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. 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 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 is built 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. 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 poured Dec. 18 and passed initial strength tests, he said. But contractors working on the site noticed a 6-inch section appeared to have excessive air pockets, he said. After further tests, the builders decided in mid-February to take the concrete out, he said. The construction schedule for the tower, expected to open in 2012, wasn't affected, Coleman said. A message left with the main concrete supplier, Quadrozzi Concrete Corp., wasn't returned. Tony Arnold, president of iCrete, the company that developed the high-strength technology for the tower, said the small amount concrete must have been mixed improperly. "We have had no batches that have not achieved the strength," he said. Removing below-strength concrete that has already been poured is rare in the industry, said Greg Vickers, managing director of operations for the National Ready Mixed Concrete Association. Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more