It is important to identify any of the specific problems to which such bridges are occasionally prone as early as possible to allow appropriate remedial measures to be taken. Bridge inspections Visual inspections of weathering steel bridges should be carried out by suitably experienced inspectors at least every 2 years. The surface condition of the 'patina' is a good indicator of performance. An adherent fine grained rust 'patina' indicates that corrosion is progressing at an acceptable rate, whereas coarse laminated rust layers and flaking suggests unacceptable performance. Other signs to look for, and areas to investigate during visual inspection include:
• Leaking expansion joints.
• Accumulation of dirt or debris.
• Moisture retention due to overgrown vegetation.
• Faulty drainage systems.
• Condition of sealants at concrete / steel interfaces.
• Excessive corrosion products at bolted joints (‘pack-out’).
If any serious problems are highlighted by the visual inspection, the cause should be traced and the problem rectified as soon as possible.
Monitoring of steel thickness
The corrosion rate of weathering steel bridges should be monitored every 6 years, by measuring the remaining steel thickness at clearly identified critical points on the structure. These points should be defined on the as-built drawings or in the bridge maintenance manual, along with original (reference) thickness measurements taken at the end of the construction period.
If after a period of say 18 years, the predicted loss of section over the life of the structure exceeds the original allowance, then remedial measures may need to be taken. The 18 year period is suggested as initially the corrosion rate is high while the 'patina' forms, before slowing down to a more characteristic rate. Refer to figure 1 on page 5. Thickness measurements should be taken using specialist portable ultrasonic equipment, that will not damage the protective rust 'patina'.
Surfaces contaminated with dirt or debris should, where practical, be periodically cleaned by low pressure water washing, taking care not to disrupt the protective 'patina'. Such cleaning should also be carried out annually, at the end of the de-icing period, if in practice it is found that chlorides are adversely affecting the stability of the rust 'patina', and causing corrosion of the substrate. Overhanging vegetation causing continuous dampness should be removed, and drainage systems should be regularly cleared. Any leaks should be traced to their source, and the drainage systems or joints responsible should be repaired or replaced. Finally, if there is evidence of 'pack-out' of crevices at bolted joints, then the edges of the joint should be sealed with an appropriate sealant.
Ensure satisfactory performance of weathering steel
Carbon Steel, Corten Steel, Stainless steel
Rust, Bare, Powder Coated, Polished, Black Heat-resisting Paint, Iron Oxide
Diameter : 320mm--1200mm
Thickness : 1.2mm--3.0mm
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Outside : Plywood box
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