Rebar gets improved corrosion protection

New, more cost-effective materials are coming onstream to give added cathodic protection to steel in concrete.

Metallisation, the thermal spray equipment company, has been using a recently developed alloy of aluminium, zinc and indium in a small number of applications.

This material is more active than the traditional zinc application and is claimed not to need an impressed current to provide adequate levels of corrosion protection. Metallised zinc cathodic protection systems are usually operated in impressed current mode.

The spray coating, a high purity zinc alloy, is connected to one pole of a DC power supply and the electrical circuit is completed between the rebar and the zinc by the presence of moisture in the concrete.

The action of the corrosion cell causes the zinc to corrode in preference to the steel rebars, thereby protecting them from corrosion.

The new alloy has been trialled by Aeroports de Paris at Charles de Gaulle (Roissy) Airport. After two years of monitoring, the system has been successful in treating corrosion of the concrete panels that carry the viaducts.

The prime causes of corrosion in concrete include salt, chloride and de-icing treatments. The salt seeps into the concrete and erodes the steel reinforcing bar (rebar) causing cracks and spalling in the concrete and eventually the potential for failure of the structure.

[Contract Journal, 25 October 2006, p17]