14.07.2015 : Bruce Chappell

There’s a fraction too little friction

The art of the asset manager is to try and align the timing of failure mechanisms to have compelling evidence for renewal expenditure.

Have you ever driven over the centre lanes of the Auckland Harbour Bridge (AHB) and felt uncomfortable with the narrow lanes and the smooth road surface?

The centre four lanes are separated by a movable lane barrier that is moved to allow more lanes for traffic in the direction of peak flows.  The lanes are very narrow and the asphalt surface is very smooth.  In wet weather, drivers need full concentration to keep safe side distances with adjacent vehicles or the barrier, and a good following distance to enable them to stop safely should vehicles ahead brake or make sudden manoeuvres.

Due to weight restrictions, the asphalt surfacing on AHB is only 25 mm thick and is comprised of a mix of fine stones.  This surfacing is very smooth and not ideal in wet weather.  Coupled with this, over time, the action of vehicle tyres will polish the stones at the mix surface, making it harder for tyres to grip the surface when braking.

As part of the Auckland Motorway Alliance, Beca was tasked with maintaining and operating the Auckland Motorway network.  To replace the existing asphalt surfacing with a new asphalt mix that has adequate skid resistance would have been a huge cost, and has the potential to cause significant disruption to road users.

Our solution
We decided to trial fine milling technology as a means of creating a textured surface that mitigates spray in wet weather, and creates freshly milled faces on the existing asphalt stones for vehicle tyres to grip on when braking.

We imported a fine milling drum from Australia for trialling this technology.  The drum fits on existing milling machines that usually have much coarser milling drums.  The fine milling drum has milling teeth much closer together enabling a cut depth of as little as 1 to 5 mm to be achieved.  The milling drum and resulting surface texture is shown in the following images:

The resulting pavement asphalt surfacing is significantly coarser than the existing surface and provides excellent skid resistance in dry and wet weather.

Failure mechanisms that trigger asset renewal may occur at times during an asset’s life.  The art of the asset manager is to try and align the timing of failure mechanisms to have compelling evidence for renewal expenditure.

In this case we have delayed the replacement of the asphalt surfacing by introducing a new technique to correct one failure mechanism. The result of which has restored a safe asset level of service and extended the asset life until the next failure mechanism occurs.

About the Author

Bruce Chappell

Technical Director - Civil Engineering

Bruce is a Technical Fellow specialising in pavements. He’s spent the last seven years embedded in the Auckland Motorway Alliance (AMA) where he leads the asset management team. His recent experience has highlighted the need for maintenance and operation safety and whole of life cost issues to be represented during the planning, design and procurement stages of capital works projects, to help get better outcomes for all stakeholders and customers.

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