Maintenance Of Corten Steel Bridges

Maintenance of Corten Steel Bridges

After routine inspections are performed, areas where the protective patina is not performing as expected may be identified. Many of the potential problems related to weathering steel relate to extended periods of wetness caused by dirt and debris on the surfaces and/or contamination from chlorides caused by

inadequate management of water. Highway bridge structures are known for collecting contaminants, such

as chlorides, dirt, debris, bird nests, and other foreign matter, on horizontal surfaces. Therefore,

weathering steel should not be perceived as a maintenance-free material. Often, the areas of excessive

corrosion are easily remediated by addressing the source of the problem including:

 Controlling roadway drainage by:

o Diverting water away from low spots and horizontal surfaces

o Cleaning troughs of joints

o Resealing deck joints

o Sealing leaking cracks in the deck

o Maintaining scuppers

 Periodically removing dirt, debris, and other deposits that hold moisture by:

o Hosing off the bridge

o Scraping off loose sheets of rust

 Maintaining screens over access holes

 Removing vegetation that prevents drying

 Painting areas of excessive corrosion when necessary

Although bridge washing is a recommended maintenance action included in the FHWA Technical

Advisory, there are two differing opinions regarding the effectiveness of washing. The research of the

early 1980’s suggested washing on a regular basis to remove chlorides before they are trapped by

additional dirt, debris, and soot. The FHWA sponsored forum in 1988 discussed both sides of the washing

debate. On one hand, many agencies considered washing as an effective method to remove chloride

contaminants. On the other hand, it was stated that simple washing does not provide enough flow over a

steel surface to remove salt contaminant build-up. In other words, salts will migrate to the interface

between the oxide layer and the underlying steel, and washing is not able to significantly reduce these

trapped chlorides.

Evidence shows that frequent natural washing, such as regular rains on a fascia girder, will help keep the

chloride levels low on weathering steel surfaces. Annual high-pressure washing of the interior surfaces

will remove surface contaminants, but it will not be effective in removing the chlorides within or below

the oxide layer. Similarly, multiple high-volume washings that may help to remove chlorides in the oxide

layer are not cost-effective.

Although high-pressure washing is not very effective in removing chlorides embedded in the patina, a FHWA study in the mid 1990’s found some evidence that periodic water washing may help to reduce the corrosion rate. At a minimum, the various studies agree that high-pressure washing will help to remove dirt, debris, loose rust flakes, and surface chloride contaminants that are often responsible for causing extended periods of wetness. This type of washing is also effective in removing much of the poultice layer of dirt and chlorides from horizontal steel surfaces, which often produces a sponge-like condition that may hinder the development of a protective patina.

The weathering steel research performed since the early 1980’s identifies painting as an effective remedial measure to address areas of excessive corrosion or protect areas where the patina has not formed. The literature review identified several studies related to proper surface preparation techniques to remove the chloride contaminants from the steel surfaces and pits. Limited research relating to ruststabilizing treatments was also found; however, these treatments were often used as a sacrificial coating applied before the bridge is erected to help accelerate the development of the stable oxide layer. In some cases, rust-stabilizing treatments were applied to in-service bridges, but surface preparations similar to that required for painting were needed. The high cost of preparing and painting in-service bridges offsets the initial cost benefits of the weathering steel. Although painting can be an extremely effective remediation tool for poorly performing weathering steel, this type of remediation is beyond the focus of this report.