In early 2010, Eastern Concrete Materials, a U.S. Concrete company, began producing high-strength concrete for One World Trade Center (WTC), also known as Freedom Tower. Within three years, the New York City-based producer had supplied 150,000 cubic yards of ready-mix for the tower’s superstructure—with a concrete strength that has never been used on such a scale in building construction. Collavino Construction Co. then pumped this mix as high as 103 stories.
The owner’s drive for a sustainable design was central to the development of the tower and resulted in uncommon durability requirements. This included a challenge to use less than 400 pounds of cement per cubic yard of concrete (less cement than is used in most standard concrete mixes for common applications such as basement slabs, garage floors, and sidewalks) as well as provisions for recycling construction debris and materials, and integration of renewable energy, daylighting, and the reuse of rainwater.
Construction began in 2006 and is scheduled for completion this year. Its supporting columns are made of steel and concrete ranging in strength from 8600 psi to 14,000 psi. Columns on the first 40 floors are made from 12,000-14,000-psi concrete and the upper floors with 8,600–10,000-psi mix designs.
The ready-mixed concrete was pumped by Collavino’s crews to the highest elevation to which concrete has ever been pumped in the Americas. Because the mix design was so workable, pumping was accomplished with a single pump that moved the concrete directly from the ground to the top story, instead of to an intermediate station where it would have been remixed before being transferred to a second pump.
Key members of the Eastern Concrete Materials team responsible for this high-profile and complex job each had more than 20 years of experience in the industry: Paul DeRosa, technical services manager and Constantine Quadrozzi, general manager of sales for the producer’s New York City operations. Together with Gary Graziano, AIA, vice president of sales and marketing, they provide a behind-the-scenes view into working with the highly specialized mix design, and the important role of high-strength concrete.
Q: Why was high-strength concrete chosen for the WTC?
A: High-strength concrete facilitates the design of smaller structural-member cross-sections for columns and walls, leaving more net usable or rentable floor area. This is an especially important benefit in high-rise buildings in high-rent districts, where the supporting members can be very large. High-strength concrete also enables design of safer structures because key supporting members, such as elevator and stair enclosures, often relied upon to resist wind, seismic, and other impact forces, can be designed with an extra measure of strength and safety.
Q. Is the 14,000-psi concrete new?
A: One WTC’s high-strength, environmentally friendly, and workable mix designs are unique, if not revolutionary, concoctions.
The highest strength ready-mixed concrete Eastern Concrete Materials had produced previously was 10,000 psi. Before this project, 14,000-psi ready-mixed concrete had been used but never on this scale or for a project with so many challenging placement, structural, and environmental performance requirements. In fact, the concrete for One WTC actually reached 18,000 psi at 56 days.
One WTC’s owner, the Port Authority of New York and New Jersey, was very interested in sustainable design and required that the high-strength concrete mix designs use no more than 400 pounds of cement per cubic yard, effectively mandating that high levels of supplementary cementitious materials such as fly ash, slag, and silica fume be used in lieu of cement.
The project also had a strict requirement for controlling the heat of hydration, and the workability of the concrete had to be maintained for a two-hour window, a relatively long time for concrete to remain workable. This is especially true for a high-strength concrete that loses its workability because it sets up faster than a low-strength concrete. However, it was necessary due to the over-the-road and up-the-tower time that the concrete spent in transport through New York City and up to the tower’s highest floors.