Aggregate from two weigh batchers feeds into the Liebherr specially designed mixing area that is comprised of a pan mixer with an upgraded rotary agitator to produce shotcrete for the Dulles Metrorail project.
Aggregate from two weigh batchers feeds into the Liebherr specially designed mixing area that is comprised of a pan mixer with an upgraded rotary agitator to produce shotcrete for the Dulles Metrorail project.

For more than a decade, community leaders have studied how to solve the traffic congestion that has stifled growth west of Dulles Airport in northern Virginia. The culmination of all the studies and reports is the Dulles Corridor Metrorail project.

Construction began in March 2009, with help from a $900 million federal grant. The project just outside the nation's capital then received $77.2 million in stimulus funds to speed up the timeline with a targeted completion in 2013. And sped it has. When completed, the $5.25 billion project will extend the existing Metrorail system by 23 miles.

In October, the high-profile project reached a milestone as excavation began on a 2400-foot tunnel. This is the most complex engineering and construction challenge of the project. The structure is a soft-ground tunnel. Its position in the soft soil is consolidated with concrete reinforcement.

In late fall, the work was under the busy Tysons Corner area of the Washington, D.C. suburbs. Crews were excavating on the inbound and outbound subway tunnel. Construction requires attention to detail and must strictly adhere to a schedule. Also, the project's center line lies under several large office buildings, including one vital to our nation's security.

Thus, engineers are concerned about any unexpected subsidence, requiring the contractor to immediately support any span they open. In the beginning, crews excavated about 40 feet each shift. They encase the tunnel's walls with steel beams for structural support. A shotcrete crew then sprays a steel fiber-reinforced concrete mix onto the steel beams and fills in the holes around it to make the tunnel more stable. The cycle is repeated every 40 feet.

As work approaches the station area, the tunnel's width will widen. When the project began, the shotcrete crew required about 60 yards of concrete. This will eventually increase to more than 120 yards per shift.

On such a demanding job, the contractor recognized that a proper shotcrete mix was important for the tunnel's stability. The mix must attain high strength quickly. It also must be workable but stable enough to avoid any undue rebound. Finally, it must be consistent to reduce any unnecessary nozzle adjustments for fast application.

Along with proper workability and strength gain was the problem of production. The project is located in one of the nation's busiest centers of commerce, so available space for plant set-up was limited. And while the planned volumes of concrete mix seemed very reasonable, the production facility had to be sized to handle a rapid increase.

Size constraints

The contractor opted for a portable plant with a small footprint that featured a pan mixer. They opted for a Liebherr plant, which features a layout that can maintain a high output in a small area. Its compact design was a particular advantage since it would not draw too much attention from passers-by.

Concrete plants of this design have been used for several other demanding jobsites around the world including dams, roads and bridges, and other large roller-compacted pours. The unit combines the simplicity of dry-batch plant designs with the superior mixing quality of a pan mixer.

The contractor purchased the pan mixer with an upgraded rotary agitator, which mixes quicker than a standard ring pan mixer. The steel fiber-reinforced shotcrete concrete is a very sticky, fluid mix. To mix it, the contractor needed a high-performing mixer.

The mixer is mounted on a mobile trailer, which the contractor easily moves to and from the jobsite without much foundation work. A weigh batching system similar to dry-mix operations feeds raw materials. A standard recipe requires only 30 seconds of mixing time.

Mixed concrete is discharged onto a high-speed conveyor belt, which feeds a holding hopper, or loads directly into the mixer. The mobile mixer is a self-contained unit and an essential option for mobile concrete plants. “There were no complaints once the structure was erected,” explained LeRoy Blue, the plant's operator.

The hydraulic mixer gate opens at a wide angle to minimize discharge times. A special wave pattern of wear tiles minimizes the risk of trapped stones and wash-out of the joint lines between them. This design reduces wear and tear.

Workability is key to the project's success. Engineers chose spread as their criteria rather than slump. After the concrete is mixed, quality control technicians pump out a predetermined sample volume to measure its spread, which must not be wider than 550-650 mm.

When the material flows in this range, it indicates that the mix is properly adjusted with the right amount of moisture to avoid excess rebound. The mix is pumped into the truck when technicians determine the spread is okay.

Initial reports on the mixer's performance have been positive. “The mixer does a very good job blending the mix,” says Blue.

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