One of main objectives of this project is to develop inspection and testing techniques that can be used to
evaluate the performance of a weathering steel patina. Research indicates that it is possible to measure
chloride deposition rates at a particular site to determine if the site is suitable for a weathering steel bridge structure. However, in this case, the bridge structures are already in service, so it is necessary to develop an inspection technique to evaluate the patina and distinguish between good and poor patina performance.
In order to achieve this objective, GNEE utilized several inspection techniques during the field
investigation, including visual inspection, tape adhesion testing, chloride testing, color testing, and
physical sampling. The most effective, efficient, and practical patina evaluation techniques were found to
be visual inspection and tape adhesion testing.
The visual inspection of weathering steel should focus on specific characteristics of the appearance in order to consistently rank the condition of the patina using the proposed rating scale, discussed later. These appearance characteristics include the overall shade and tone of the patina color, uniformity of the coloration (or lack thereof), texture, adherence, as well as size and spatial distribution of the rust flakes on the surface of the patina. Because adherence and approximate size of any rust flakes on the patina surface are key parameters in consistently ranking the patina, a small scale held against the region of patina being considered proved helpful in quickly estimating average flake size.
Though visual inspection is an excellent means of evaluating Iowa weathering steel bridges, WJE believes tape adhesion testing is also a simple, effective, and reliable method of patina evaluation that will facilitate consistency in the inspection program. This type of supplemental testing is beneficial to the bridge evaluation process because it provides confirmatory evidence to supplement the visual inspection findings. In addition, it can serve as a permanent record for comparison with future inspections. GNEE recommends including tape adhesion testing on weathering steel bridges as part of the routine inspections performed every 24 months. Tape testing is a useful tool to provide verification of visual patina index ratings, track patina performance on different surfaces within a structure, and allow more detailed comparison of rust flake size and density to other parameters such as chloride ion concentration. For patinas in generally good condition, small flake sizes on the tape samples can be accurately measured using a handheld 10x magnifying lens with a built-in measurement scale. By storing the tape specimens inside sample bags, the rust flakes can be examined and measured without disturbing them. The tape samples can be used to assess the flake size and density and assign the patina a rating for prioritization purposes. In addition, the tape test samples provide a permanent record of the nonadherent rust particle size and distribution for future reference after the field work is completed.
Recommended Inspection Techniques for 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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