Precast concrete slabs line the floor of a light-rail tunnel running underneath the University of Washington.
A unique mix design and tight quality control power a light-rail extension underneath the University of Washington.
By Mason Nichols
Precast concrete has a long history of solving myriad design and construction issues. Advanced engineering, problem-solving and innovation all are hallmarks of the precast industry, allowing architects, engineers and general contractors to meet the needs of seemingly any project.
Need a bridge capable of withstanding a harsh saltwater environment and the occasional hurricane?
Build it with precast.
What about a specialized air duct system that can help power HVAC at a forward-thinking entertainment arena?
Build it with precast.
Want to pass light-rail commuter trains underneath a university with no disturbance to the sensitive experiments and equipment above?
Build it with precast.
CPP partnered with Helser Industries, a forms manufacturer, to meet the tolerances for each of the panels.
Support and protect
Sound Transit, the public transit agency serving the metropolitan area in Seattle, designated precast as the building material of choice for the Northgate Light-Rail Extension project, which expanded service to customers in north Seattle.
Along with the build-out of tunnels and new stations, roughly three quarters of a mile of the 4.3-mile extension was slated to include work in tunnels passing under research facilities at the University of Washington. Because of the highly sensitive nature of the equipment in these facilities and the type of work being performed above, vibration and electromagnetic interference generated from the trains had to be kept to a minimum. This requirement meant the Sound Transit team had to determine the best means of mitigating vibration while also designing a solution that supports the trains.
Sound Transit turned to Stacy and Witbeck, a construction firm specializing in transit infrastructure, to lead the trackwork portion of the project. In performing its research for the extension together with a variety of other organizations, the Sound Transit team, led by Rail Vehicle Engineering Manager Shankar Rajaram, concluded that precast concrete – manufactured with a unique mix design and installed with specialized rubber pads – was the best solution.
After determining its approach, Stacy and Witbeck teamed with Columbia Precast Products (CPP) of Woodland, Wash., to manufacture the 2,500 precast slabs needed for the work. According to Matt Johnson, project manager at Stacy and Witbeck, extensive planning was needed to meet the project’s technically complex requirements.
“We spent an entire year performing preconstruction work with Columbia Precast and Sound Transit,” he said. “It wasn’t just that the slabs needed to be a very specific, heavy weight to get the density of the concrete right. These pieces also had to be magnetically shielding.”
Columbia Precast Products experimented with its mix design to meet the weight parameters as well as make sure it would flow around the rebar and complex geometry of the forms.
In the mix
Meeting the precise needs of the extension work meant generating a concrete mix suitable for the application. Although CPP typically manufactures its products with self-consolidating concrete – a special mix exhibiting high flowability, reduced segregation and a slew of other benefits – additional components were needed to achieve the heavy weight and magnetic characteristics required for the slabs.
Jason Miles, plant manager at CPP, said his team ultimately decided on two unique elements in the hybrid mix design – hematite and iron ore slag. The hematite, which consisted of heavy, round balls approximately three eighths of an inch in diameter, played the role of coarse aggregate and was used to achieve the high density and magnetic properties specified. The iron ore slag functioned as the fine aggregate. Fiber also was part of the mix.
“This mix design was one of those where we kind of threw everything at it that would stick to get that weight up there,” Miles said. “And with those parameters, we also had to make it flow around all the rebar and the complex geometry of the form, which is why SCC was the most appropriate.”
According to Johnson, a limited set of options was available to the project team. Hematite was the mineral of choice because it typically is used in applications where magnetic shielding is needed, such as within hospital walls. While it can be particularly difficult to procure, the team was able to locate a solid source in Duluth, Minn. After performing the required due diligence and finding a reliable source for the hematite, CPP began developing prototype slabs. The company eventually finalized the mix design, which would be used on 16hz panels as well as two different sizes of 5hz panels. The panels ranged in weight from 11,000 to 22,000 pounds.
Each slab was installed in concert with a series of custom-built rubber pads that were approximately 12 inches in diameter by 7 1/2 inches tall.
Quality, quality, quality
Several factors presented challenges to the project team throughout the manufacturing process. When CPP secured the work with Stacy and Witbeck, they were in the process of building a new plant.
“We had to accelerate the schedule of our build-out to satisfy the Northgate extension timeline,” Miles said. “We were actually doing test batches in the plant before the roof was complete. That’s how tight the schedule was.”
Additionally, because of the magnetic properties of the materials used in the mix design, the CPP team encountered issues with some of its equipment during production.
“We had microwave probes for moisture in our aggregate bins and mixer,” Miles said. “So, microwaves and all that metal – the hematite and iron ore slag – didn’t cooperate very well. We were getting some strange numbers and had to change out those probes.”
With the high weight of each panel and the massive number of products that needed to be made, the work also caused significant wear and tear at the CPP production facility. This eventually necessitated the construction of new roadways at the plant to make the manufacturing process more efficient.
During the entire process, quality control was of particular concern, as officials from Sound Transit sought to ensure that all the panels produced for the work met the tight tolerances of the project. This resulted in four QC teams collaborating on the work – one from CPP, one third-party agency hired by CPP, one from Stacy and Witbeck and one from Sound Transit.
“Having all the third-party inspectors was actually an asset, because they could help us achieve the tolerances,” Miles said. “We’re used to dealing with 1/4-inch to 1/8-inch tolerances, but on the 16hz panels, we had to build them to within 1/32 inch. We really had to trust in our formwork.”
To help achieve this, CPP partnered with Helser Industries, a forms manufacturer based in Tualatin, Ore. According to Miles, Helser’s work was exemplary, with the forms being so well made that they did most of the work in helping meet the tolerances for the CPP team.
It all comes together
For the system to work as intended, each slab was installed in concert with a series of custom-built rubber pads that were approximately 12 inches in diameter by 7 1/2 inches tall.
“The rubber is really where you get your vibration mitigation, but you need to have the massive weight of the slab on it,” Johnson said. “Each slab sits on four support pads, and there’s additional compression achieved via rubber pads installed on the sides.”
Because of the way they were installed on top of the rubber system, the precast pieces are referred to as “floating” slabs. In addition to the pads and precast, the track also benefitted from the use of ultra-straight rails, which Sound Transit, Stacy and Witbeck, and the rest of the team sourced from Europe due to the material’s limited usage in the U.S. at the time. The ultra-straight rail, which was just as crucial to the job as the precast floating slabs, also aids with vibrations and potential interference as trains pass through the 3/4-mile section of the extension.
According to Johnson, this was the first 5hz precast concrete floating slab system in the United States. Beyond simply meeting the needs of the project, Johnson said precast brought significant advantages to the work.
“By going with a precast solution, we could perform all the complex design work, sourcing and production planning – even how to handle our quality control – ahead of time,” he said. “If we were trying to do this on the fly as we were building the job, it just wouldn’t have been possible.”
Miles pointed to other scheduling advantages brought about by using precast.
“We could manufacture all the pieces for this project while other aspects of the work were still taking place,” he said. “This sped up the schedule at an immense rate.”
Quiet, please
Public transportation systems generally are not known for their lack of noise and vibration. However, thanks to a bit of ingenuity and the use of precast concrete, Sound Transit developed a system capable of getting Seattle travelers where they need to be while simultaneously keeping complex research equipment safe at the University of Washington.
To date, extensive testing performed by 40 monitors positioned at 300-feet intervals in the project area shows that the precast rail system is meeting all criteria for quiet operations specified, making the Northgate extension a success for all involved.
“Columbia Precast was prepared and committed to making sure this project went well for us and the owner,” Johnson said. “This wasn’t simple, but everyone did their job and it worked out well for everyone.”
Mason Nichols is a Grand Rapids, Mich.-based writer and editor who has covered the precast concrete industry for nearly a decade.
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