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Dublin Port – Alexandra Road Bridge Architectural Concrete Façade

The Challenge

The 150m long Alexandra Road Bridge in Dublin Port links the CDL yard and Terminal 4.

Roadstone was the main supplier of ready-mix concrete to the project.

The standout feature of this bridge is the concrete façade, defined by its complex geometry, its slenderness and lighting inserts.

The design of the façade, while striking, posed a particular challenge when considering the placement and compaction of concrete into the complex façade form.

The Solution

Rather than attempting to erect the formwork and cast the concrete in place in a restricted setting it was decided that it would be easier to cast the concrete façade in the site yard on the horizontal and lift the individual elements into place by crane.

This allowed for the concrete to be placed through small openings left at either side of the 5/6m wide formwork. The shape of the façade rendered it impossible to compact the concrete by means of internal vibration.

As external vibrators could not be placed on the underside of the formwork it was also felt that external vibration would not sufficiently compact the concrete to produce a high-quality surface finish and the desired quality definition along edges and at corners/angles.

A self-compacting concrete (SCC) was chosen as being the only concrete type that could fully encapsulate the formwork and achieve the high-quality finishes as specified.

Through careful planning between the design team, contractors and the producer a design mix and placement methodology were agreed upon. An SCC concrete to a slump-flow class of SF2 was seen as the best option to achieve full encapsulation of the formwork and the desired quality of finish.

The concrete was placed through a tremie pipe that was immersed into the formwork. This placement methodology reduced the entrapment of air in the mix due to placement and reduced the volume of blowholes on the cast surfaces resulting in a high-quality finish.

16 individual façade panels were cast on site and form an 84-tonne architectural façade on the east and west of Alexandra Road Bridge.

Takeaways

Careful consideration and selection of the concrete mix type for a given application can result in ease of placement, with reduced potential for health and safety incidents and the potential for cost reductions.

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