There is many different types of bridges and it is not possible to give a general outline of specifications.
However this part deals with large bridges mainly constructed as box girders.
Furthermore there is several different areas such as below boxes, inside the boxes and in atmospheric exposure and also pillars immersed in water. It is also important to consider both
steel and concrete materials. In the following specification considerations are presented for the main part of the bridge structures.
- Atmospheric exposure steel (Pylons and box girders). Steel pylons are becoming more popular because it gives more design freedom and reduces construction time. Steel pylons are bolted together and therefore it is important that the zinc primers has a friction certificate. Apart from that paint specification is the same as for the box girder. Inside the box girder, film thickness can be reduced normally to one coat of mastic epoxy, especially if dehumidifiers are in use. If dehumidifiers are permanent, the internal surfaces can be left in the shop primer only. For the underside of the box girders also film thickness can be reduced since those surfaces are not exposed to sunshine, but they can develop condense.
Apart from above mentioned Paint specifications can be selected on the basis of ISO 12944. JGC has developed a paint selector for bridges based on ISO 12944. For access to a demo of such a system press here http://www.svjo.dk/software2019/lcc/index.htmlISO 12944 has three durability scales, below 5 years, 5 15 years and above 15 years. For many bridges up to 30 years are required. JGC has experience to specify up to 30 years and also results that point in that direction. The trend in topcoats for longer durability is towards polysiloxanes and fluopolymers
- Atmospheric exposure concrete. Pylons, ramps etc can be treated the same way as concrete for houses or highway bridges meaning first an epoxy concrete sealer to close pores and prepare the surface for adhesion of the next coat then a couple of epoxy coatings followed by topcoats. For reinforced concrete in a polluted area with CO2 it is necessary to consider anti-carbonation coatings because otherwise CO2 can penetrate the concrete and accelerate corrosion of the reinforcing bars with catastrophic failures as a result.
- Immersed areas. Bridges over seawater or over freshwater rivers might have pillars immersed in water. for the immersed part, ISO 12944 can be used or specs used for offshore pillars or general immersion specifications. The real problem with immersed pillars are the splash zone. Splash zone specifications from offshore can be used here.
An interesting question raised by several customers is how to estimate the life-cycle cost of a paint system for corrosion protection of steel bridges. Svend Johnsen has worked on developing life cycle calculation software, which are based on a simulation engine he invented 7 years ago. A description of this software was reported to NACE conference in Kyoto 2013 “How to specify coatings for lifetime protection of large steel structures”. This paper can be downloaded here.