PLC Provides Foundation for 46-Story Tower

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Concrete construction, with its strength and durability, provides safe housing for the long term. In Florida, the appeal is particularly strong for reinforced concrete towers due to their ability to endure coastal exposures, hurricanes, and other severe-weather conditions while sustaining minimal damage. Naturally, such impressive resilience relies heavily on a sturdy foundation.  As such, an 8-foot-thick foundation provides the required support for the 400 Central tower. 

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The 400 Central tower foundation represents a best-in-class approach to sustainability, but it's based on technology and materials that are readily implementable for any size or scope of concrete construction. Its mass foundation is built with Cemex’s eco-friendly concrete mixture, a combination of 37% portland-limestone cement (PLC meeting ASTM C595 Type IL) with two supplementary cementitious materials (SCMs): 16% fly ash and 47% slag cement. Known as a ternary blend because it contains three cementitious materials, the high SCM-content concrete was needed to help mitigate heat development in the mass placement and minimize expansion and cracking as the concrete cools. 
 

The SCMs could be accommodated in the mat foundation at 400 Central because the specification called for a 56-day strength rather than the more typical 28-day requirement. The extra 28 days allows the concrete to develop its 8,000-psi strength with a lower cement content, qualifying it as “high strength” per the American Concrete Institute Committee on High-Strength Concrete (ACI 363). Additionally, higher SCM replacements yield less heat generation within the internal matrix of the concrete mass protecting it from thermal stress and the potential for internal cracking.
 

Ultimately, this greener concrete mixture with its lower-carbon PLC and SCMs replacing traditional portland cement is in line with current trends, whereby all types of construction must address sustainability. Embodied carbon is being examined by designers and government personnel at all levels, and environmental footprints inform consumers’ decisions, too.

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The Concrete Difference

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Large mat foundations like this are typically performed in one continuous placement. Starting around 9 p.m. on a spring night in 2023 and continuing for the next 16 hours, more than 600 truckloads were used to deliver a steady stream of concrete totaling 5,700 cubic yards to the project. (Notably, this is the second-largest private single pour in St. Petersburg, second only to St. Pete Pier, which was also built using PLC.) 

 

Even though the mat foundation concrete mix for 400 Central had a low water-cementitious materials ratio (w/cm) of 0.30, it needed to be quite workable (7- to 9-inch slump) because it was being placed via pump. Cemex used two admixtures to achieve the pumpability: a normal water-reducing admixture along with a super-plasticizer. Pump placements require consistent fresh properties to ensure smooth operations. Finishers used both hand finishing and floating techniques to complete the placement and applied an evaporation retarder to the surface to help manage operations in the 77-degree Fahrenheit weather.

 

The concrete mix for a project like 400 Central had to check a lot of boxes: low heat development, workable, consistent, and high strength. The fact that it was even more sustainable than traditional concrete mixes is a bonus. Because the project team of dedicated construction professionals could deliver high-end results that a project like 400 Central demands, Florida can provide world-class housing and continue to be a popular destination for people from all over the world. 

 

The mild climate and beautiful beaches allow for a lifestyle that can be enjoyed by almost everyone: young people, active families, and retirees. Real estate remains at a premium in Florida, where estimates suggest a continued large influx of new residents every week. Urban development requires taller construction to maximize land use and keep people close to the amenities that city living offers, whether that’s on their own rooftop or outside their front door. As sustainability goals continue to drive decisions for building project materials, 400 Central serves as an example of how lower-carbon portland limestone cement will grow in importance. 

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Project Team:​

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This project was a collaboration between:​

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• Red Apple Real Estate

• Suffolk Construction

• Baker Concrete

• Arquitectonica

• WSP

• Cemex