Cathodic Protection of Concrete Bridges
The cost to apply metal sprayed coatings to large concrete structures is not insignificant, particularly when many structures are difficult to access, such as bridges.
However, the long-term benefits can make the process extremely commercially attractive. If performed correctly and depending on the coating applied, the process can offer corrosion protection for up to 20 years before the next significant maintenance is required. The protection offered can greatly prolong the life of the structure and also prevent costly accidents from cracked sections falling from the structure. Once applied, the coating requires minimal maintenance. If required for aesthetic purposes, Metal Sprayed coatings can also be painted or powder coated.
Al-Zn-In Application of San Luis Pass Bridge, USA
Image Attribution: Larry D. Moore, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
A recently developed alloy of aluminium, zinc and indium has been used in a small number of applications. This material is more active than zinc and it is claimed to not require an impressed current to provide adequate levels of corrosion protection. A significant application of the Al-Zn-In alloy in the US is the San Luis Pass Bridge near Galveston, Texas. More than 30,000m² of concrete beams and caps are protected with this alloy, installed using Metallisation ARC 700 units by Corrosion Restoration Technologies of Jupiter, Florida.
Cape Creek Bridge, Oregon (USA)
Oregon Department of Transportation (ODOT) demonstrates another success story for cathodic protection on concrete. In a bid to reduce the high costs of bridge reconstruction, ODOT has applied a system of metallised zinc anodes and impressed current cathodic protection. This process has been used to protect its Cape Creek Bridge from corrosion and subsequent reconstruction. The bridge is exposed to a coastal environment and is subject to attack by chloride from the salty air.
Prior to the cathodic protection project on the bridge, it had suffered substantial concrete spalling on its columns and under deck. By selecting to protect the bridge in this way ODOT saved over $13 million by not having to reconstruct the bridge. The cost of cathodic protection is quite expensive. This is due to the requirement of a movable work platform, which is enclosed to contain the abrasive blasting and zinc spraying residues. These measures are critical when spraying zinc to protect the environment. However, when compared to the cost of reconstructing a bridge the size of Cape Creek Bridge – the savings are phenomenal.
Dave Wixson, Metallisation distributor in the US says: “Cathodic protection is a cost effective way to stop rebar corrosion in existing structurally sound structures. Rebars in dry alkaline concrete are protected by a passive ferric oxide film, however, when the rebar is hit with 250 ppm chloride solution, generally from salt, the protection breaks down. The protective ferric oxide film is converted to red rust and corrosion begins.
Concrete thickness >4cm (>1.5 in), will prevent chloride penetration. For exposed rebar and thin concrete, where there is chloride concentration in excess of about 250 ppm, rebar corrosion will be initiated with the red rust spalling adjacent concrete. Protecting the rebar with a barrier using an impressed or passive cathodic protection system, counters the corrosion.”
Many thanks to Palmer Consulting of France, TMS of the USA and Corrosion Restoration Technologies Inc (now part of Structural Group, Inc.) of the USA for information supplied.
