These types of structures must be analyzed with different techniques that keep them always protected, due to the importance of the work they provide.
by Juan Manuel Álvarez*
This article referring to the inspection of corrosion and coatings on bridges in principle is aimed at metal bridges or that in their construction contain some element with this material.
Bridges are structures that are apparently at rest until their use, such as the passage of vehicles, pedestrians or fluids, through pipes or ducts. This use generates loads, stresses, vibrations, plastic deformations that are protected for the control of corrosion by organic coatings such as paints; these coatings must withstand this kind of substrate behaviors and overcome them to remain as a film protecting from the reaction with the environment in which this structure is located.
In addition to the above, it should be borne in mind that the bridges will have an anchor with some type of electrolyte present that can affect the coating, causing the formation of corrosion products. Thus, we could cite bridges that are anchored in natural terrain with some degree of acidity, in modified land anchored in concrete with some presence of alkalis, anchored in fresh water, anchored in seawater, and other anchors that have some interference in the process of formation of corrosion products.
Based on the two points of view mentioned in the two previous paragraphs, coatings will have a requirement that is explained as follows:
From the point of view of its mechanical strength, it suggests a good product and a good application process for the formation of continuous and uniform film that results in the protection of the bridge substrate. With regard to a good product, these are products that from their technical sheet indicate that they pass demanding tests of conical or cylindrical mandrel in fairly small failure dimensions, impact tests, abrasion resistance tests, and other tests demanded by the behavior of the substrate or the external environment of the bridge, either by its use or by the environment to which it is exposed.
The second point of view has to do with the chemical resistance of the coating, which is related to the correct selection of the product, the correct storage, the preparation of the mixture, the correct environment at the time of applying the paint, the drying and curing parameters well met within the parameters recommended by the paint manufacturer, and other tasks related to the conservation of the properties object of the selection of the product, which must be appropriated through the process to achieve the continuous and uniform film that will allow the protection of the substrate.
The check-up
With regard to the design of bridges, the following should be taken into account for protection and inspection purposes:
1. Anchor points.
2. The geometric configuration to be protected.
3. Buried structural surfaces.
4. Partially submerged structural surfaces.
5. Structural surfaces completely submerged.
6. The upper areas on high-rise bridges.
7. Splash areas.
For the selection and correct application of the paint, the previous points or recommendations must be taken into account, because they are linked to the permanence or duration of the coating fulfilling the protection work. At the anchor points, there are usually apozamiento zones that due to their exposure time generate a reaction that tends to affect the coating, allowing the beginning of the formation of corrosion products by the reaction with the surrounding environment.
As for the geometry to be protected, the variation of surfaces present in cables or guayas, areas of reinforcement points (plates), bolt heads, indentations in joints or overlaps, and others, hinder or make more demanding the detail during the application of the paint. Buried structural surfaces are those areas of the bridge that are exposed to a different environment, such as the soil where it is buried, which is why particular care must be taken with the finish of the coating and the continuity of the film of the same.
Partially submerged structural surfaces are related to those areas of the bridge anchor supports where the level of the river or sea fluctuates generating temporary environmental changes (change in reactivity by oxygenation, for example) that demand a lot from the coating (abrasions or impact by particles in the fluid).
Fully submerged structural surfaces require the coating to be compatible and chemically resistant to the medium where it is continuously submerged, which is why the scheme or coating system varies with respect to the rest of the structure to be protected from the bridge. The upper areas in high-rise bridges are areas that have particularities, such as being exposed to higher wind speed, greater condensation of humidity, lower surface temperatures, greater exposure to ultraviolet light, etc.
The splash zones are those where the anchor supports of the bridge that by effect of the mobility and blow of the fluid generate recurrent projections towards areas of the anchor support, demanding chemical and mechanical resistance to the coating, but with the aggravating factor that the speed of aging and affectation of the coating is greater than in other areas where the reactive environment is constant but, where the paint has been properly selected and applied to protect the substrate.
In relation to corrosion inspection in bridges; the points, areas and zones listed above correspond to the checklist to be met for inspection purposes by monitoring the condition of the coating and possible formation of corrosion products. What will vary is the frequency of inspection and the method or technique of inspection with respect to the points, areas or zones to be inspected.
In principle, inspections should be carried out by direct visual method and selecting the inspection aid equipment according to the access to the points, areas or areas of inspection. Continuity inspection with micro-orifice detection equipment is recommended. Dry and/or wet adhesion tests will be recommended according to the environment, coating scheme and access.
The inspection frequencies shall be determined by the probability of coating failure and the aggressiveness of the environment to which the bridge is exposed at points, areas or areas. These checklists for monitoring inspection will be carried out in monthly, semi-annual or annual periods, mainly.
For further expansion, applicability, concerns or queries on this subject, I can gladly advise you through our editor.
As always I wish you a good aging and many successes in your work.
* Juan Manuel Álvarez Rodríguez. Designer for Corrosion Control, Certified Coating Inspector, Corrosion Technician and Marine Coating Inspector (NACE International, the Corrosion Society) A.S.T.M. member of the committees of corrosion, paints and coatings, fuels, general aviation, adhesives, nondestructive testing and composites. Certified Instructor ICAO (International Civil Aviation Organization). Inspector in Non-Destructive Testing. Instructor, lecturer and columnist in Paints and Corrosion Specialist in Metal Structures and Composite Materials. Advisory Services, Inspections, Training and Contracts for the Control and Prevention of Corrosion.
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