Water flowing over weathering steel contains suspended particles of insoluble iron oxide (rust), particularly when the steel is subjected to frequent rainfall during the early months of exposure. As this water runs over concrete piers and abutments, the rust stains and streaks the concrete. Although the rate at which weathering steel releases oxide particles decreases with the length of exposure , the rust particles accumulate on concrete surfaces and the stains get worse.
To investigate whether weathering steel would continue to stain a bridge structure after several years of service, two sections of two piers at Beltway 8 over Ella Boulevard in Houston were coated with a layer of lane-marking spray in May of 1998. In March of 1999, the site was revisited and stain discoloration was observed on both test patches . Samples were taken from each of the paint patches, cutting through all surface coatings to the concrete of the pier. Inspection of these samples showed that each piece had surface layer of staining, a layer of clean white paint, a layer of much darker oxide staining, a layer of clean white pier coating, and finally the concrete itself. This investigation showed that, in a representative weathering steel structure, the staining process continues. When a structure is not properly detailed with drip caps, a quick paint job to improve appearance will not solve the problem: the run-off will continue, the staining will continue, and the need for re-painting will continue. The solution is to design the structure with drip pans and install them as an integral part of the construction phase, or, on existing structures, retrofit them with drip pans. Even if subsequent run-off water is diverted, the stains remain, creating aesthetic and perception problems. However, rust staining can be controlled and should not be a reason to avoid the use of weathering steel.
Any flow of water over weathering steel members can provide corrosion and subsequent staining. The best way to minimize staining is to incorporate permanent design details that divert run-off water away from adjacent vulnerable materials.