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Cathodic education

Cathodic protection - A high grade cathodic protection system is being installed to keep traffic running on the M40, reports Adrian Greeman.

There are few construction projects where the local residents complain after the construction crews have gone away. But for repair work on the M40 Loudwater Viaduct, near Beaconsfield, they did just that.

'We blocked off a road that runs along the valley under the motorway viaduct and so for some weeks in the spring and summer the locals had some unusual peace and quiet, ' explains John Gardner, general manager with UK Highways, the company that has been managing and maintaining the M40 since 1996. UK Highways has a 30 year PFI concession.

Traffic diversion was needed while concrete repair crews were working to install cathodic protection on 22 transverse beams of the 500m long viaduct, which carries of the dual three lane M40 across the River Wye valley.

There are twin decks of 12 spans each, supported on concrete piers up to 20m high. The hefty transverse beams have shown signs of deterioration with 'concrete blown off the surface in places and corroded reinforcement below' says Gardner.

Investigation by specialist consultant Sandberg, and later Hyder, showed that in the main, the damage is not yet critical or even deep. But it could become so, especially as rainwater runoff loaded with de-icing salt drips onto the beams in winter as it has been doing since the viaduct was built in 1968.

'There is not a road joint made that does not leak a little, ' says Hyder project engineer Paul Burton.

Hyder is civils consultant for the project, and has mapped and designed the basic concrete repairs for the job. These are needed before the cathodic protection system is added.

'There is a joint above each cross head where the longitudinal steel plate beams of the deck spans are simply supported, ' Burton explains. A concrete road deck finishes the composite structure.

Infiltration of chlorides into the concrete beams has been increased over decades by an odd detail, a small upstand around the top face of each beam which has caused pooling of water. The detail was originally intended to prevent water spilling onto the vertical faces of the beams but has created a problem with accelerated penetration of water through the top surface, despite a thin asphalt layer.

UK Highways felt measures were needed, as it is responsible for maintenance of the viaduct for another 20 years and because it must hand back the structure to the Highways Agency in 2026 with some residual life left.

The company decided to install cathodic protection. The system works by passing a very small electrical current through the reinforcement within concrete to neutralise the effects of deicing salts.

Creating the electrical circuit can be achieved in various ways, explains John Drewett, director of Concrete Repairs which is installing the system. At Loudwater, specialist consultant Capsis recommended to UK Highways that it go for the 'Rolls Royce' choice of a titanium anode embedded into the concrete, lasting for around 50 years.

It was initially thought titanium rods could be mortared into holes drilled at intervals, but the density of the 50mm diameter rebar at the top of each beam ruled this out.

Instead a special mesh is being wrapped around the beams and held in place with spray concrete.

The mesh, imported from America, has a special oxide coating which prevents some unwanted electro-chemical reactions occurring.

Design of the protection system was carried out by Concrete Repairs as part of its £1M contract. It worked to a performance standard specied by Hyder.

Installing the mesh involves setting up a scaffolding cage around the beam, which is then cleaned with high pressure water jetting, leaving a scabbled concrete surface.

The jetting is also used to break out damaged concrete in areas mapped by Hyder. These are hand mortared for repair.

Holes are drilled to allow the necessary wiring to be welded to the reinforcement and then the mesh is applied.

Spacer batons hold this above the scabbled surface and it is spray concreted into position along with its own wiring. A single titanium band is also hand installed around the bearings, where it is too difcult to install the mesh.

'We end up with a 'stucco' nish, ' says Drewett. Trowelling to achieve a smooth nish could disturb the set of the spray concrete around the mesh, he explains, possibly causing delamination and threatening the integrity of the electrical connection with the concrete.

Much of this work is now virtually complete and has gone well, Drewett says. The only major worry was during the intense heat of July when it was feared that spray concrete work might be suspended by drought restrictions.

Adjacent beams in each deck are coupled via a junction box and wired to ground level where a mains connection is used to create the permanent 5V to 10V direct current into the circuits.

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