Q: Has there been a trend in increased use of extra-strong concrete?
A: Yes. The trend has been to use ever-higher strength concrete mix designs, especially over the last 10 to 15 years. Before that, 7000-8000-psi concrete was considered high strength. When ready-mix producers demonstrated the ability to produce high-strength concrete, engineers and architects began designing buildings accordingly.
More specifically, in New York City, high-strength concrete applications first appeared in the early 1990s with designs delivering strengths of 7000 to 8000 psi. Mixes with these strengths are relatively common today, but at the time, they were unique and expensive because they used high levels of cement and microsilica, both of which are costly. Today, we achieve these strengths by using more affordable SCMs, such as fly ash and slag in lieu of cement and micro silica.
It wasn’t until 1999 that 12,000-psi concrete was produced in the New York City area for the Trump World Tower. The cured mix yielded an average strength of about 17,000 psi. In 2000, the project won an award from New York City’s Concrete Industry Board (CIB) for consistent delivery of high-strength concrete. In 2012, One WTC also received an award from the CIB for concrete sustainability and performance.
After the Trump Tower was completed, higher-strength mixes (e.g., 8000-9000-psi) became more common in the city, but 10,000- and 12,000-psi mixes were still uncommon. Beginning in 2005, the high end of the high-strength market started to open up with the projects such as the Beekman Tower, WTC Tower 7, and other WTC projects. However, One WTC, where construction began in 2006, was the first to require 14,000-psi concrete.
Today’s chemistry and mix optimization know-how also ensure high levels of concrete durability, constructability, and sustainability, providing design professionals and contractors with the certain performance they need for the hassle-free specification and use of high-strength mixes.
Q: What makes it so strong?
Credit: Port Authority of New York and New Jersey
Construction of the Port Authority Trans-Hudson (PATH) Transportation Hub is underway, with high-strength concrete supplied by Eastern Concrete Materials, at New York City’s World Trade Center construction site.
A: A unique blend of concrete ingredients, including cementitious materials, blended aggregates (coarse stone and fine sand) and the latest technology in admixtures enable the production of ready-mixed concrete with very low water content, which enables high strength.
One of the key reasons the concrete is high strength is because the more water that is used in making, placing, and finishing concrete, the weaker the concrete becomes. Low water contents, supplemented by chemicals that mimic the workability provided by higher levels of water, produce high-strength concrete.
Q: Is this the trend for future concrete designs?
A: Yes. The future of high-rise construction will be high-strength, more durable, and more sustainable concrete. Most new projects have 10,000–14,000-psi concrete as part of the design, as well as a requirement for sustainable construction driven by the LEED certification that many owners seek or require.
We have been able to apply our knowledge of how concrete ingredients interact and the properties they impart to concrete to develop new mixes that solve other construction problems. A perfect example is Aridus Rapid Drying Concrete. Aridus Concrete is a 7000–8000-psi mix made with a low water-to-cement ratio, a high percentage of supplementary cementitious materials, and a patented admixture.
This product helps prevent unsightly, inconvenient, unhealthy, and costly moisture-related floor covering failures. This problem has become even more prevalent over the last decade as floor covering adhesives have moved from solvent- to water-based to eliminate environmentally unfriendly volatile organic components (VOCs).
The system uses a vapor barrier and specially designed, self-desiccating concrete that consumes free water to control the moisture vapor emission rate (MVER), and internal relative humidity of concrete so manufacturers’ specification-compliant, ASTM-tested floors can be delivered in no more than 45 days after the building is enclosed and conditioned for 48 hours.
Gary C. Graziano is vice president of sales and marketing for Eastern Concrete Materials Inc., a U.S. Concrete Company. E-mail firstname.lastname@example.org. Visit www. us-concrete.com/eastern_concrete.asp.
For more on high-profile concrete projects in New York City, read “One World Trade Center rising on ‘super concrete’,” Real Estate Weekly, 6/1/11