The choreography of a concrete pour
This week, at the south entrance to the bored tunnel, Seattle Tunnel Partners is pouring concrete for a section of the future southbound highway. On one hand, the pouring of concrete (also known as a "concrete placement”) is nothing extraordinary – it’s a common occurrence on a project that will use enough concrete to build nine football stadiums. But their frequency belies the complex choreography that goes into executing each pour successfully. Since concrete plays such an important role in the project, we are taking a closer look at how one of these concrete pours is choreographed in the field.
Setting the stage
Working with concrete isn’t easy. It is hard work and it is heavy work. If you’ve ever mixed a batch for a home project, you know that it has to be mixed with the right consistency, and as soon as it gets wet there’s a timer counting down until it hardens. You want your tools, your plan and your location all prepped and ready so you don’t end up with concrete in a shape you don’t need (or hardened solid while still in your wheelbarrow).
Caption: Workers smoothing fresh concrete that forms the future southbound SR 99 roadway at the south entrance to the tunnel.
Preparing for a pour first requires setting rebar and building strong, rigid formwork. The concrete will be poured over and around the lattice or matt of steel rebar, giving the final structure (in our case, usually a road deck or wall) its required strength. Metal and wood formwork is the mold that gives the poured concrete the desired shape. Freshly-poured concrete is a strange substance, both sticky and also wet and somewhat fluid; formwork must be strong to contain the weight of concrete, waterproof so it does not leak, and coated with oil so the concrete doesn’t stick to the sides.
Caption: The worker at the center of this image is using a stinger to settle the freshly poured concrete. The hose descending into the concrete sends vibration waves that eliminate pockets of air and rock, ensuring the concrete is well mixed. Air pockets or rock pockets reduce the structural integrity and strength of the concrete once it hardens.
Completing the pour
Once crews begin pouring concrete into the formwork, they have a limited time before it starts to harden, so everyone has a job to do. Those roles include:
- Operating the concrete pump or (whatever means is used to deliver the concrete)
- Directing the poured concrete to fill in the formwork
- Using stingers to remove air bubbles
- Spreading and smoothing the concrete using automatic and hand tools to ensure even distribution
- Placing curing blankets or compounds along with spraying water to prevent the surface from drying out and hardening too soon (i.e. curing of the concrete)
After the concrete is smoothed even, it cures and hardens until it reaches the designed strength.
The raw video footage from June 2015 posted below captures a crew pouring concrete for a section of roadway at the north portal. You can see how everyone involved has to stay coordinated to ensure they pour the right amount of concrete and quickly smooth it using a stinger, roller screed (the automatic rolling pin) and hand tools. This pour created what will be the southbound entrance to the tunnel.
Building the new SR 99 tunnel will involve hundreds of concrete pours both big and small, each helping to construct and support a vital piece of Seattle’s transportation infrastructure. Those walls and roadbeds might look ordinary, but as you just saw, they are quite complex to create – something to think about the next time you’re rolling along the highway.