Weathering steel is a material that provides a great potential advantage to TxDOT for use in bridges in terms of improved durability and lower construction and maintenance costs. However, there are disparate opinions within TxDOT about how well the material performs. Some contend that it does well in most environments, and others contend that it does not perform at all, but there is little scientific basis for either opinion. The purpose of this research is to determine in what environments, if any, weathering steel performs well, based on field evaluations of the TxDOT’s weathering steel bridges, and to provide recommendations for achieving good performance in these bridges.
In the presence of moisture steel and oxygen generally combine to form rust. On most carbon steels, the rust forms a loose crystalline structure, allowing more water and air through to attack deeper into the steel, forming even more rust and weakening the base metal. Weathering steel, however, in its bare, mature state, has a unique, coating occasionally referred to as a “patina.” The coating is more properly referred to as a ‘protective oxide film,’ which is about the same thickness as a heavy coat of paint. This protective oxide film adheres tightly to weathering steel in fine, dense grains that are relatively impervious to further atmospheric corrosion, thereby sealing the base metal from the air and further corrosion. The protective oxide film has different colors than the rust on other carbon steels, ranging from a dark reddishbrown to purple gray, depending on the age of the structure, the pollutants in the air, local weather conditions, or the location of the steel within the structure. The appearance, texture, maturity, and anticipated utility of a protective oxide film depends on several factors. The primary factors are age, degree of exposure, and environment
· It takes time for the oxide film to change from a rusty red-orange to a dark, rich, purplebrown color. The moderately rough texture becomes more distinct as the coating matures. This weathering process continues over an extended time, depending upon other factors.
· The degree of exposure has a strong influence on the weathering process. Steel exposed to rain, sun, and wind weathers more quickly than steel in a sheltered location. On sheltered surfaces, the oxide tends to be rougher, less dense and less uniform.
· Frequent wet-dry cycles caused by rainfall and/or dew, followed by wind and sun, are important factors affecting protective oxide film formation. In moderate industrial environments, weathering steel usually matures most rapidly and achieves the darkest tone. In rural locations, the protective oxide film develops more slowly and generally has a lighter tone