Location: Chicago, Illinois
Award: Lower Carbon Concrete
Owner: Hines, the Kennedy Family, AFL-CIO Building Investment Trust Contractor: Walsh Construction Architect: Pelli Clark Pelli Engineer: Magnusson Klemencic Associates Concrete: Prairie Materials Slag Cement: St Marys Cement
At a height of 850 ft (260 m), the Salesforce Tower Chicago, Chicago, IL, USA, makes a substantial appearance on the northwest corner of the skyline, particularly when looking southeast along the Kennedy Expressway. Incorporating new concrete technologies, the structure uses lightweight concrete decks supported by steel pans connected to a central core specified for 6600 x 106 ksi (45,505 MPa) modulus of elasticity. Furthermore, Salesforce was the first high-rise in Chicago to specify third party verified environmental product declarations (EPD) for all concrete designs to certify carbon impact. The combination of slag cement replacement and portland-limestone cement allowed for a substantial reduction of carbon emissions from industry baselines.
Slag cement was used for numerous applications for the structure. For example, to manage the heat of hydration, slag cement was used in the large core structure and massive base elements, as well as caissons, grade beams, and mat foundation. Similarly, slag cement was used for long-term strength gain with reduced carbon impact. For the lightweight decks, slag cement was used to reduce carbon impact and for the concrete’s finishability and consistency of appearance.
Slag cement use allowed for cement reduction along with long-term strength development for mixtures in excess of 10,000 psi (70 MPa), good mixture consistency at high slump to achieve pumping requirements for the height of the building, increased paste density to achieve maintenance of effort requirements with local aggregates, and exposed concrete finishes due to mixture aesthetic consistency.
% Slag Cement Replacement
19-30% (project average 19%)
% Portland Cement
% Portland Limestone Cement
50-100% (project average 73%)
% Other SCM (if applicable)
Fly Ash from 10-27%, Silica Fume 2-4% (project averages: ash 6%, SF 2%)
1700 lbs 3/8” pea stone + 1420 lbs sand