Carrying out protective coating surveys for bridges requires specialist knowledge, good preparation and a flexible approach.
Bridge types, which include beam, box girder, truss, arch, cable stayed and suspension designs, are continuously exposed to the elements and therefore, may have to be treated with tough and long lasting protective coatings. However, any coating needs to be assessed properly to check its condition and, if necessary, a remedial painting programme prepared.
Any effective assessment starts with a pre-survey investigation into the history of the bridge structure to ascertain coating specification, past maintenance and previous survey reports and paint trials. Depending on the size and complexity of the bridge, the survey may also have to be divided into smaller sections to secure a more accurate assessment of the coating condition.
The survey should be as precise as possible and feature a detailed description of the structure. Steel section terminology should also be used correctly. Access systems, weather conditions, including light levels, and the accumulation of dirt, grime or bird guano, should all be taken into account when preparing and executing a survey. It might also be necessary to examine some of the more inaccessible parts of the bridge from a convenient point using binoculars.
The survey should detail the extent of remedial painting required and confirm if the existing coating can be over coated, or if it should be completed removed. This could require a feasibility trial, involving preparing the surface and applying the specified coating system to a trial section. Once dried and cured, this section is then assessed by eye and adhesion tests to determine the suitability of any remedial action.
It’s important to remember that not withstanding the feasibility trials, some bridge paint specifications require a minimum adhesion value of the existing coatings before further coatings can be applied.
Older road bridges in particular, can suffer excessive corrosion brought on by the effects of de-icing road salt used in wintry weather conditions. The salt gains access to the steelwork through expansion joints, damaged concrete or poorly maintained water proofing membranes. In cases of severe corrosion, steel pitting and metal loss , the removal of paint will be required to enable a thorough assessment to be undertaken.
Even weathering steel, designed specifically to eliminate the need for painting, can suffer from corrosion damage if it is continually exposed to damp conditions and may require a protective coating. Cast iron bridges also need to be surveyed to assess for damage and may require a filling material to prevent rust staining around omega (blow) holes.
Comprehensive guidance on coating surveys for bridges at Fitz’s Atlas of Coating Surveys https://fitzsatlas.com/