Webcor Concrete has succeeded in completing a three-day, floor-to-floor construction cycle for pouring the floors at the 63-story high rise Rincon tower (at Infinity Tower they are on a fourday cycle, due to work hour restrictions). This is the fastest cycle ever accomplished on the west coast, where seismic design requirements, mostly with the placement of reinforcing steel, typically leads to much longer cycles. “We compared our buildings to projects with two-day cycles in New York City and found that in our walls there is over three times the amount of reinforcement,” says Webcor’s director of construction Chris Plue.
Achieving this speed requires a lot of advance planning and every means possible to streamline construction operations. “We knew before we started the Rincon project that a three-day per floor cycle was possible, so it has been our goal all along,” said Webcor concrete project manager Jack Harrington. “When the project began, we were doing a five-day cycle which evolved into a four-day cycle, and then finally trimmed to a three-day cycle.”
On both towers, the key has been to form the building’s central elevator core with self-climbing forms – Peri forms at Infinity and EFCO forms at Rincon. “The core walls are constructed three levels ahead of the floor slabs, allowing the deck and wall crews to operate independently,” says Plue. Concrete is pumped up the center of the elevator core from street level with a high-rise pump that feeds a placing boom mounted on the self-climbing formwork. The concrete is then pumped to the core wall and floor slab three levels below. “One of the nuances,” says Plue, “is that one might look at the core wall being three floors ahead of the decks and wonder why not run the core five or 10 floors ahead, but if we did that we would not be able to use the placing boom to pour the floors.”
But probably the greatest factor pushing the speed along is the interaction between team members. “The rebar contractor, the concrete contractor, the structural engineer and the ready mix supplier must work in partnership on the constructability of the elements, many months in advance,” says Plue. Another key is the mockups they build before construction begins. “We build full-size mock-ups of the critical elements in advance, which allow us to get as many of the kinks out of the installation as we can. What may seem like a small change in the reinforcing configuration can pay big dividends in the field and we’ve never done a mock-up that we didn’t think was worth every penny. This is one of the keys to getting the rebar to go in as fast as possible.”
Webcor workers tie as much of the reinforcing steel as possible on the ground then it is hoisted into place by crane. The Patent deck forms are also flown by crane and ProShore forms are used for the floors inside the core. “Luckily all these different formwork systems operate rather independently,” says Plue, “so there aren’t any problems fitting them together.”
To reach the three-day (or fourday) cycle took several floors to work out the process, but Webcor expects to maintain this speed to the tops of both towers. “We are on the constant lookout for process improvement,” says Webcor’s senior vice president Ross Edwards Jr. “Every job has a new mix of designers, subcontractors, and tradesmen, so there is another opportunity to make the process better and we're very lucky to work with some of the best in the industry.”