Corrosion protection
Durability is an important issue to consider when designing and detailing steel bridges. Various protection systems are currently available, and paint technology is advancing at a rapid pace.
Conventional Painting Systems
Conventional painting systems are based on the use of blast primers, primers, undercoats and finishes. Systems involve the application of up to four coats to produce total dry film thickness of 200–300 microns, following appropriate surface preparation. Selection of a suitable paint system from the alternatives is determined by the environment and accessibility. Details of the alternative systems are described in Highways Agency Specification 1900 Series and Departmental Standard BD35, and Railtrack Line Specification RT/CE/S/039: Specification RT98 and Line Code of Practice RT/CE/C/002.
High-Build Painting Systems
High-build paint systems developed for the offshore industry are being introduced to bridge construction. The two main types are:
Elastomeric Urethane Coatings
These can be applied to film thicknesses of 1000 microns in a single coat. Specialist equipment is required and controlled shop conditions are needed to achieve a satisfactory coating. The life to first maintenance is anticipated to be well in excess of 30 years.
Glass Flake Epoxy Coatings
These are capable of similar coating thicknesses to the elastomeric urethane types and produce extremely tough, abrasion resistant, durable coatings.
Weather Resistant Steels
Weather resistant steel is a low alloy steel that forms an adherent, protective oxide film of ‘patina’ that, in a suitable environment, seals the surface and inhibits corrosion. Such steels are specified to EN 10025-5 and have similar properties to conventional grade S355 steels to EN 10025-2. The most commonly used grade for bridgeworks is S355J2W+N.
Advantages of Weathering Steel:
- Elimination of painting at time of construction which more than offsets additional thickness allowances.
- Reduced maintenance cost – repainting eliminated.
- Traffic delays during inspection and maintenance are minimised.
- Reduced need for access for maintenance work. These savings can lead to reduced whole life costs for the bridge.
Restrictions on Use:
The use of weathering steels is outlined in Highways Agency standard BD7. This standard provides guidance on suitable environments, corrosion allowances and headroom limitations. The design of weather resistant steel bridges should also consider the effects of corrosion products, formed during weathering, on adjacent materials.
Bridge Enclosure Systems
The concept of enclosing the structural steelwork on composite bridges is based on the fact that clean steel does not corrode significantly if environmental contaminants are absent. The advantages of enclosure systems include:
- Corrosion rates drastically reduced.
- Enclosure provides safe permanent access for future inspection and maintenance.
- Traffic disruption below the deck is minimised during future inspection and maintenance.
- Any paint or debris from maintenance work is contained within the enclosure.
- Maintenance work may be carried out in control conditions during sociable hours.
Despite the numerous advantages, there are relatively few examples of bridge enclosures in the
