Corrosion protection of steel and composite steel bridges
The are some 1,200 steel and composite steel bridges in the federal trunk road network with a overall steel surface area of more than 15 million square metres to be protected against corrosion. Added to this are other steel constructions, e.g. road sign bridges, sheet pile walls, and components such as rails, noise barriers, passive protection facilities, carriageway expansion joints and bearings.
Valley bridge St. Kilian Sankt Kilian
The corrosion protection of steel constructions and steel components is primarily ensured by corrosion protection systems composed of a zinc coat and/or coatings (organic layers). Only occasionally is so-called weatherproof structural steel used to build structures, this steel unfortunately frequently rejected mainly due to its rusty appearance, even though it does not require any further corrosion protection and is thus extremely long-lasting.
Standards and policies
The planning of corrosion protection and execution of corrosion protection measures is based on an extensive collection of provisions consisting of DIN standards and supplementary sets of regulations. The eight-part DIN EN ISO 12944 standard lays down basis principles regarding corrosion protection in layers. These consist of zinc coating in accordance with the DIN EN ISO 1461 (batch galvanising) and DIN EN
ISO 2063 (zinc-spraying) standards.
The supplementary sets of regulations on the subject of corrosion protection for the area of federal transport routes are:
- Additional Technical Terms of Contract and Guidelines for Civil Engineering Works (ZTV-ING), Part 4 Steel Structures, Composite Steel Structures, Section 3 Corrosion Protection of Steel Structures, (ZTV-ING 4-3).
- Technical Terms of Delivery and Technical Test Regulations for Coating Materials for the Corrosion Protection of Steel Structures, 2002 edition (TL/TP-KOR-Steel Structures), available from Verkehrsblatt Publications, Dortmund.
- Guidelines for Corrosion Protection of Steel Ropes and Cables in Bridge Building, 1983 edition, (RKS-steel ropes) henceforth ZTV-ING Part 4-4 Steel Ropes and Cables, ZTV-ING Part 4-5 Corrosion Protection of Steel Ropes and Cables and TL/TP-KOR-Steel Ropes and Cables.
- Guidelines for Maintaining Corrosion Protection of Steel Structures (RI-ERH-KOR).
The BASt undertakes the updating of these sets of regulations based on the continuing development of the state of the art, professional expertise and its own investigations.
Corrosion protection coating
The corrosion protection coating is essentially produced in two operations: In the first step, the steel surface is prepared (cleaned and roughened) by the removal of rust, old coating materials and contaminants. In the second, the corrosion protection material is applied to the prepared steel surface - generally by airless-spraying and in exceptional cases using rolls or by painting.
Preparing the surface
The preparation of the steel surface ensues by what is known as dry compressed air blasting - generally using disposable blasting abrasive material, although non-disposable blasting material is used in exceptional cases. When non-disposable blasting abrasive material is used, then significantly less waste material (blasting waste) is produced in comparison with the disposable blasting abrasive material method. However, the concentration of contaminants and hazardous materials in the blasting waste is markedly higher. The steel surface treated using non-disposable blasting abrasive material generally requires subsequent treatment with high-pressure water jets.
Housed areas undergoing surface treatment on a bridge
Corrosion protection system
The choice of corrosion protection system depends on a number of factors, including corrosive stress (exempli gratia effect of road salting, impact of chippings, dampness from structural components in contact with the ground), the planned service life of the component and associated considerations on the corrosion protection maintenance schedules (duration of protection), size of the structural component, approachability to the surfaces or accessibility.
ZTV-ING 4-3 contains examples of suitable corrosion protection systems for structures and structural components, which take account of these factors. The composition and characteristics of coating materials are described in the TL/TP-KOR Steel Structure guidelines in so-called TL sheets, from which these corrosion protection systems are produced. At present there exist a total of 14 TL sheets. Coating materials complying with the requirements laid down in the TL/TP-KOR Steel Structures guidelines are listed in a "Compilation of certified coating materials complying with the TL/TP-KOR Steel Structures guidelines" maintained by the BASt. The inclusion in this compilation ensues on presentation of appropriate testing certificates as well as proof of external monitoring. The coating materials taken from this compilation are usually used on structures and structural components along federal transport routes.
Thus, for example, 2-component materials (Sheet 87, Sheet 94, Sheet 97) with an epoxide resin base and zinc-dust-pigmented primer, one or two micaceous iron oxide intermediate coats and a final light-resistant 2K polyurethane coat are mostly used as corrosion protection on large objects with a large surface area requiring a major overall effort. The coatings used on our bridges in accordance with Sheet 87 have an excellent track record extending over 30 years. When properly executed, such corrosion protection coatings have a minimum protection period of 25 years and are regarded as the benchmark for comparable investigations of new coating systems. The coating materials in accordance with Sheet 94 represent a further development of Sheet 87 in the direction of less use of solvents. Sheet 97 has been especially developed for use in low temperatures and faster treatment in the factory than Sheet 87.
These epoxide resin and polyurethane coating materials are all extremely resistant and therefore also suitable for use on the undersides of orthotropic road bridge decks, which can be stressed with temperatures up to 150°C on installing the hot mastic asphalt layers.
The requirement for good execution is personal working skills in corrosion protection and is therefore regulated in ZTV-ING 4-3. Verification of the qualification must be provided. The gang leader in his function as on-site management has to be constantly at the building site while the work is in progress. Since 2011, he is required to possess a so-called KOR certification from the Training Council of the German Corrosion Protection Association e.V. as a qualification certificate.
Batch galvanising has proven its worth over the years as corrosion protection for protection facilities. Subject to long-term experience, however, it has been possible to use strip galvanised crash-barrier beams in recent years as an alternative to batch galvanised crash-barrier beams. The low sulphur dioxide concentration in the air over Germany is regarded as one of the reasons for the suitability of this method. Hardly any visual or measurable changes have been revealed from the routes investigated by the BASt in 10 years.