In buildings, Mass Concrete is frequently used for mat foundations, blast resistant walls, transfer girders or deep beams. However, there are no hard and fast rules to determine if your concrete element qualifies. According to the American Concrete Institute (ACI), “Mass concrete is any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of the cement and attendant volume change to minimize cracking.” Just like icebergs remain colder longer than ice cubes, Mass Concrete cannot dissipate heat as quickly as thinner elements due to the low surface to volume ratio. ACI has several reference documents which explain the requirements for proportioning the concrete mix, proper placement and curing procedures.
When the components of concrete cure, the reaction of the cement and the water releases heat (also called an ‘exothermic reaction’). If the concrete gets too hot, or the temperature difference between the center and the surfaces is too great, the quality of the concrete will suffer. Concrete can develop internal cracks, the cement paste might not adhere to the aggregate and the compressive strength may be reduced. To avoid these problems, it is critical that Mass Concrete be kept cool.
At the new 39 story Virgin Hotel tower, the tower core is founded on a 6.5 foot thick mat approximately 100 feet by 150 feet, columns are founded on spread footings and the perimeter walls are socketed into the site’s rock subgrade. The perimeter walls are a combination of 26” diameter secant pile walls and 18” reinforced concrete foundation walls. Placement of the mat concrete required two separate pours, the first included 350 yards of concrete placed in 10 hours overnight and the second placed 1,500 cubic yards of concrete, or 120 trucks in 15 hours overnight this past August.
To reduce the initial temperature and avoid issues related to an exothermic reaction, the concrete supplier added ice to the mix. Another strategy to cool Mass Concrete is to chill the aggregate. Thermocouples were tied to the reinforcement bars at strategic locations at the bottom, in the middle and near the top of the mat. These transmitted temperature data to the contractors as part of the quality control system. Test cylinders were cast during the mat’s placement, but for obvious reasons, the curing conditions of a 6” diameter cylinder will not match those of the middle of the mat. Thus, it is standard practice to core additional samples from the mat after 28 days and 56 days to confirm the concrete has obtained its specified breaking strength.
The Virgin Hotel, which will occupy the full block between 29th and 30th Streets along the west side of Broadway in NoMad (north of Madison Square), Manhattan, has the foundation level slab 49 feet below the ground level and surrounding sidewalks, with deeper areas at elevator and sump pump pits. There is subway infrastructure at 26 feet below Broadway and the groundwater level is only 13 feet below the curb.
Atop the foundations, the tower portion of the project will be constructed using traditional cast-in-place concrete. But the 5 story podium, which contains retail and amenity spaces, will be constructed of structural steel with metal deck and composite concrete slabs. Completion of the structure is anticipated in Fall 2018.
This project is being developed by the Lam Group with the Virgin Hotels brand; Stantec, formerly VOA Architects is serving as Architect, MEP engineer and Interior Designer; Gilsanz Murray Steficek is providing structural engineering; Geotechnical engineering by Mueser Rutledge and Flintlock Construction is acting as General Contractor.