Sustainable Concrete

More than a fifth of the way into the 21st century, it is hard to imagine a time when smart phones were not ubiquitous.  But in the late 1990s, these now-indispensable devices and their precursors, personal digital assistants, were relatively rare, in use mainly by early adopters and other technophiles.  For professionals in the building industry, it might also be difficult to recall that sustainable design and construction was once a new concept.  In 1998, the U.S. Green Building Council began evaluating 19 pilot projects in their nascent Leadership in Energy and Environmental Design program.  And yet today, LEED has reached its fourth iteration, with more than 100,000 certified buildings.  Severud Associates has contributed to the trend towards green building practices with almost 25 years of developing sustainable concrete design.

A significant portion of this work began in 1999 with The Durst Organization’s NYCyberCenter.  With the tech industry booming, the developer proposed a 420,000 sq. ft. telecommunications and data center in the middle half of a city block on Manhattan’s west side.  The electronic equipment would need massive quantities of electricity.  Rather than rely on the local utility, the building would house its own 47-megawatt power generation station.  The structure necessary to support the resulting loads—data storage, turbines, cooling towers, and other mechanical equipment—was substantial and for blast resistance, reinforced concrete was the logical choice.  The volume would be unusually high so the concrete would have to be as green as possible to minimize the building’s environmental impact, another goal of the project.

Concrete’s primary impact on climate change is the manufacturing of cement, which per ton produces about a ton of carbon dioxide.  Therefore, the most effective means to reduce concrete’s impact is to reduce the amount of cement used.  The concrete industry had already embraced the use of fly ash as a supplementary cementitious material, but only at a rate of about 15 percent.  Durst commissioned Severud to perform an investigation of other cementitious replacement materials and higher rates of replacement.  The study focused on ground, granulated blast furnace slag (GGBFS), a waste product from steel mills that has cement-like properties, and included mix designs with replacement rates between 15 and 45 percent.

Cylinders were cast, cured, and tested and petrographic analyses were performed.  The data showed that any addition of GGBFS increased the strength and durability of concrete.  At higher concentrations, the slag increased set time noticeably, which might be undesirable where finishing is critical—the top surface of a floor slab, for example—and could be beneficial in warm weather.  The engineers concluded that the 45 percent GGBFS mix design would be ideal for mass concrete structures such as bulk fill, mats, spread footings, and heavy columns and slabs.  In other words, perfect for the NYCyberCenter.

Unfortunately, due to post-9/11 security concerns and changes in the real estate and tech markets, plans for the center were put on hold.  Shortly thereafter, in 2002, Durst retained Severud Associates to design the Helena, a 33-story residential building at the east end of the same city block (and now known as Helena 57 West).  Although not as massive as the NYCyberCenter, it would be an entirely reinforced concrete structure and a good test project for mix designs with 45 percent cement replacement and strengths of 4,000 and 6,000 ksi.

The foundation contractor, whose scope included footings, basement walls, and slabs on grade used for parking, had no reservations.  But the superstructure contractor—who would have to finish 600,000 sq. ft. of floor slabs, possibly in freezing weather—was concerned about set time and workability.  Arrangements were made to place a mock-up panel at grade and as a back-up, mix designs without slag were submitted and approved.  However, finishing of the mock-up went well and over many months of construction, there were very few problems; the alternate mixes were not needed.  In fact, by end of project, the concrete supplier was so impressed by the results—including LEED Gold certification for the building—that they updated their standard mix designs and storage facilities to incorporate only blast furnace slag.

By 2003, when The Durst Organization hired Severud to design One Bryant Park, its 51-story flagship office tower, concrete with high rates of GGBFS substitution had become the industry standard.  The entire 2.4 million sq. ft. building, including a mat, spread footings, basements walls, slabs, and hybrid steel-concrete shear walls, used more than 86,000 cu. yd. of concrete.  The substitution of slag for 45 percent of the cement reduced carbon emissions by 16,000 tons, diverted 17,000 tons of waste—the slag—from landfills, and contributed to the building’s Platinum LEED certification, the first by an office building.

Since 2014, Severud has been lead structural engineer for a study by Building Product Ecosystems into the use of ground recycled glass.  Like fly ash and slag, glass can be diverted from waste and recycling streams, processed, and used as a replacement for cement in concrete.  Ground glass pozzolan (GGP) requires less energy for processing, contains no heavy metals or other potential contaminants, and is more often a local material (fly ash from coal burning plants and slag from steel mills are potentially distant sources).

As part of an 18-month investigation, with funding from Durst, concrete mixes were designed for strengths ranging from 6,000 psi to 12,000 psi.  All mixes contained a constant 50 percent Portland cement while the remaining 50 percent consisted of GGP and GGBFS in varying proportions.  The mixes were adjusted based on test results until they produced reliable results meeting all specifications.  Finally, mock-ups of concrete components using GGP and GGBFS substitutions were placed.  Mixes using up to 50 percent GGP had no placement, curing, or ultimate strength issues.  The study also led to acceptance by NYC Buildings and led to development of a national standard, ASTM C 1866.

Halletts Point is a brownfield redevelopment by The Durst Organization on a waterfront site in Queens.  Its uses include residential, retail, and public open space.  Severud provided structural design of the first three phases, which consist of residential towers of 22 and 17 stories (Building 10), 26 stories (Building 20), and 31 stories (Building 30) springing from three-story retail podiums; there is also underground parking.  Their height and configuration require up to 12,000 psi concrete for shear walls and columns and 9,000 psi concrete for flat plates and slabs.  For the buildings’ foundations, 6,000 psi concrete will contain 50 percent ground glass pozzolan (no slag) and 9,000 psi concrete will contain a mix of GGP (35 percent) and GGBFS (15 percent) as replacement for cement.  Building 10 was successfully completed in 2018 and achieved LEED Gold certification.  Buildings 20 and 30 are under construction and targeting LEED Gold certification when finished.

Throughout its nearly 95-year history, Severud Associates has been at the forefront of the structural engineering profession and continues to use best practices for innovation, economy, and sustainability in their designs and engaged service to their clients.

Halletts Point, Building 10; photo courtesy of Dattner Architects