However, as with other forms of construction, there are certain environments which can lead to durability problems. The performance of weathering steel in such extreme environments will not be satisfactory, and these should be avoided: Marine environment Exposure to high concentrations of chloride ions, originating from sea water spray, salt fogs or coastal airborne salts, is detrimental. The hygroscopic nature of salt adversely affects the 'patina' as it maintains a continuously damp environment on the metal surface. In general, weathering steel should not be used for bridges within 2km of coastal waters, unless it can be established that chloride levels do not exceed the salinity classification of S2 (cl < 300mg/m2 /day) to ISO 9223. De-icing salt The use of de-icing salt on roads both over and under weathering steel bridges may lead to problems in extreme cases. These include leaking expansion joints where salt laden run-off flows directly over the steel, and salt spray from roads under wide bridges where 'tunnellike' conditions are created. However, salt spray is unlikely to be a problem for typical weathering steel composite overbridges even at standard headrooms of 5.3m, which are now permitted in BD7/01. 'Tunnel-like' conditions are produced by a combination of a narrow depressed road with minimum shoulders between vertical retaining walls, and a wide bridge with minimum headroom and full height abutments. Such situations may be encountered at urban / suburban grade separations. The extreme geometry prevents Limitations on use Weathering steel bridges are generally suitable for use in most locations. Continuously wet / damp conditions Alternate wet / dry cycles are required for the adherent 'patina' to form. Where this cannot occur, due to continuously wet or damp conditions, a corrosion rate similar to that of conventional carbon steel may be expected. Examples include weathering steel elements immersed in water, buried in soil or covered by vegetation. Design standard BD7/01 requires a minimum headroom of 2.5m for crossings over water to avoid such damp conditions. Atmospheric pollution Weathering steel should not be used in atmospheres where high concentrations of corrosive chemicals or industrial fumes, specifically SO2, are present. Such environments with a pollution classification above P3 (SO2 > 250µg/m3 ) to ISO 9223 should rule out the use of weathering steels. However, this is an extreme level, which is rarely encountered.
Environment category Typical environment
C1 Interior environments only.
C2 Atmospheres with low levels of pollution. Mostly rural areas.
C3 Urban and Industrial atmospheres with moderate sulphur dioxide pollution. Coastal areas with low salinity.
C4 Industrial areas and coastal areas with moderate salinity.
C5-I Industrial areas with high humidity and aggressive atmospheres.
C5-M Coastal and offshore areas with high salinity.