Los Angeles, CA
Eric Owen Moss Architects (EOMA)
Pouring the Midnight Concrete
MATT's Team Tackles an Epic 12-Hour Concrete Pour at the (W)rapper Construction Site
134 people. 560 trucks. 5,591 cubic yards. 8 pumps pouring concrete over 800 tons of reinforcing steel. Just an average Friday night on a MATT jobsite. For the past eight weeks, the crews at the (W)rapper site has been prepping the foundation area for one massive, continuous pour, which due to site logistics could only be carried out overnight. (See the video here.)
Ahead of the mat pour, they placed and tied in 1,600,000 pounds of reinforcing steel, and installed area drains, wall drains and the tower crane base anchors. They also created a temporary dirt access ramp for trucks to drive in and out of the excavated area. Then on Friday, June 7, they began the meticulously coordinated process of the continuous pour. At 7 pm, set up initiated. They closed the neighboring streets at 9 pm, placed the pumps on the street, and got ready to receive the concrete. The first truck reached the site at 11 pm, and the last truck left around 11:45 am on Saturday morning. The finishers, who finish the concrete out post-pour, stayed on until 5:30 pm Saturday evening. Workers platooned in and out to complete shifts of roughly eight hours. The mat foundation will serve as the wear surface slab for the actual footprint of the building
Subcontractor Success: Concrete Experts At Work
Concrete engineers CTLGroup and concrete subcontractor Largo Concrete proved invaluable to the process. The design called for a high strength concrete with 8000 psi compressive strength, and the heat of hydration required a detailed thermal control plan. CTLGroup developed the 40-page plan, and Largo implement the plan.
There are several ways to provide thermal control during a mat pour, one of which involves water moving through coils piped inside the concrete mass using a portable chiller plant to dissipate the heat of hydration. But that option proves costly, so the team came up with an engineered control plan and specialized concrete mix design utilizing lower cement content with a higher fly ash content and higher strength aggregate. The plan then calls for insulating blankets placed over the concrete for roughly 25-28 days after the pour. Why is this important? Correct execution of thermal control ensures the concrete’s integrity over decades.
What’s next for the 17-story steel-frame-exoskeleton structure? Now that the poured concrete has been finished and is resting snugly beneath insulating blankets, the team will continue to take temperature readings to gauge its readiness. When the temperature difference between the surface and middle of the mat is 10 degrees, they can move away from the thermal control plan. In the meantime, they’re busying themselves with spread footings, columns, removing the dirt rampway, waterproofing and progressing other areas of foundation work.