Steel fiber concrete cuts need for rebar
- A new mix allowed crews to build shear wall link beams with 40 percent less rebar at The Martin apartments in Belltown.
By LUCY BODILLY
Pouring concrete into congested rebar is difficult: Concrete is hard to place and vibrating around the tangle of rebar to remove the voids makes it even harder.
Structural engineer Cary Kopczynski, senior principal and CEO of Cary Kopczynski & Co. in Bellevue, found a solution. At his latest project, The Martin, his firm specified steel fiber concrete in the shear wall link beams, a place that is especially congested with rebar.
The Martin is a 24-story apartment building under construction at Fifth and Lenora in Seattle’s Belltown neighborhood. When it opens this fall, the project will have 188 units over four levels of parking for 125 vehicles and 4,000 square feet of retail.
Originally planned as condominiums in 2005, the LEED silver project was shelved after it received its master use permit in 2007. Construction began in February 2012.
The shear wall link beams play a critical structural role in connecting the wall piers, particularly during an earthquake.
“Our objective in using fiber was to reduce the quantity of rebar, making the beams easier to construct, and replace the lost strength (due to the reduction in rebar) with steel fiber mixed into the concrete,” Kopczynski said.
He first heard about testing of the steel fibers while working on a national committee that reviews building codes. Also on the committee was a University of Michigan professor who was studying the steel fibers.
“The goal is to introduce modifications to the International Building Code that will make the design approach used for The Martin mainstream,” Kopczynski said.
The recession and construction downturn stalled use of steel fiber concrete, adding to the hurdles the new technology faced. The first hurdle was to convince building owner Vulcan Real Estate to try the product.
“There is a wide range of clients we have proposed it to, and we have spent quite a bit of time telling people about it. They were quite receptive,” Kopczynski said.
Next on the list was the Seattle Department of Planning and Development, which required two peer reviews before it would accept the new technology.
Todd Williams, senior project manager with general contractor Exxel Pacific of Bellingham, said they didn’t start using the concrete until they reached the 12th floor. “We were waiting for the peer reviews to be completed and the city required several mock ups,” he said.
The concrete was a specialized high-strength mix with 3/8-inch pea gravel aggregate, a self-consolidating chemical admixture and the steel fibers. The specified strength was 10,000 psi. “It cost about $800 per yard,” Williams said.
The technology will eventually save both time and materials, because the steel fibers replace the tedious process of placing and tying rebar.
“We reduced rebar in the beams by about 40 percent and made them easier to build, which saved significant labor. Because this was the first time this had been done, however, the net savings ended up being a wash,” Kopczynski said.
The biggest challenge in the early pours was creating enough slump to allow the concrete to be easily placed. “Each cubic yard of concrete has 200 pounds of steel fibers added,” Williams said. “It made it much harder to pour.”
Ultimately, chemical admixtures that are used in self-consolidating concrete were tried and provided enough slump — about 8 inches — to allow easy placement and vibration.
The contractor also had to buy the fibers, which are a proprietary product made in Belgium.
“The city specified that we had to use the same fibers that the University of Michigan used in its testing. They only tested the fibers from Belgium, so those are the ones we had to use, which added to the cost,” Williams said.
CKC is specifying steel fiber concrete in future projects.
“I’ve worked with Cary on many projects. I hope to see the steel fibers again because they create a better product,” Williams said.