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PAINTING OF BRIDGES

PROTECTIVE PAINTING OF STEEL GIRDERS On Indian Railways a major part of expenditure on maintenance of steel girders is spent on painting of steel girders. In reality, experience gained during the past 100 years is that proper protective painting along with greasing of bearings are the only factors for economical service life of steel girders. The steel rolled prior to 1975 has more corrosion resistance than the steel rolled after 1975. This is noticed in present railway girders. The new girders provided during doubling of lines have more corrosion problems than old girders. Corrosion is more where steel comes in contact with wood and water stagnation in pockets formed on account of structural form. For this reason plate girder has less corrosion problem than open web girder. Steel girder bridges situated in the following locations are prone to severe corrosion:

  • Where humidity in atmosphere is more than 80%
  • Coastal areas
  • Bridges across creeks
  • Bridges surrounded by chemical industry
  • Bridges near metropolitan city on account of public nuisance.

In addition to above, type of traffic carried on bridges i.e. salt, fish, manure and chemicals also cause severe corrosion. Maintenance engineer has to adopt proper painting scheme during maintenance as stipulated in IRS-B1-2001 depending upon the location of bridges.

Metallising

Metallising is one of the superior methods of corrosion protection. Metallising means coating by sprayed aluminium or zinc. However, only aluminium coating has been covered in the Indian Railways' manuals. The aluminium has to be coated to average 150 micron thickness is to be provided as given in the Appendix VI of IRS-B 1-2001 or as per para 218 of IRBM. In steel girder following members are to be provided with the above protection:

  • Floor system (cross girders and rail bearers) of open web through girders in all cases
  • Top flange plates of plate girders and under slung girders in all cases.
  • Top bracing system of open web through spans (i.e. top laterals, sways, portal bracing etc) in electrified section, as it is difficult to get power block for frequent oil painting.
  • Entire Steel girders, subjected to salt spray such as in the close vicinity of the sea and/or over creeks, as well as in industrial, suburban or coastal areas which are prone to severe corrosive environment. (IRBM mentions only salt spray but the logic is clear that any structure which cannot be protected by oil paints shall be protected by metallising or epoxy paints. Even other vulnerable locations such as the bottom flange and bracing of the Foot Over Bridges, sleeper seat etc shall also be metallised to ensure the steel structure retains its cross section and serves its purpose properly)

A film by Prof V B Sood, IRICEN covering different aspects of metallising is available here...

Metallising protection will be more effective and economical if carried out in workshop during fabrication of new girders. Metallised components shall be handled properly during loading, unloading, while transporting from workshop to site and erection at site. Life of metallising if done in workshop with sand/grit blasting and touching up of cover aluminium oil paint whenever required will easily be 25 to 30 years. The metallising may also be done in field taking due care to protect the passengers and other rail users from the problems caused by sand blasting in field.The metallising shall be over coated by oil painting as per schedule given below:

  • One coat of etch primer to IS 5666 - 1970: This coat is required to do some etching of the surface so that the surface gets rough and further paint layers are
  • One coat of zinc chromate primer to IS 104 - 1979: This coat is the primer layer.
  • Two coats of aluminium paint to IS 2339 – 1963: These coats are the top coats. If the metallising is done in workshop and the article/ girder is placed at site later on, out of the two coats of aluminium paint, one shall be applied in the workshop and the other shall be applied in field after touching up portions damaged during the erection/launching process. This will also ensure good, new look to the structure after the erection/ launching work is complete.

Epoxy painting

Epoxy paints have good adhesion and anti-abrasion power. Epoxy based paints can be used in lieu of metallising in the field for existing girders.Scheme of epoxy based paint

  • Surface preparation-
  • Remove oil/grease from metal surface by using petroleum hydro-carbon solvent to IS: 1745.
  • Prepare the surface by sand or grit blasting.
  • Epoxy painting-
  • Primer coat: Apply by brush two coats of epoxy zinc phosphate primer to RDSO specification No. M&C/PCN-102/86, to 60 microns minimum thickness (DFT)
  • Intermediate coat: Apply by brush one coat of micaceous iron oxide paint to RDSO specification No. M&C/PCN-103/86 to 100 microns thickness (DFT).
  • Finishing coat: Apply by brush two coats of polyurethane aluminium finishing to RDSO specification No. M&C/PCN-110/ 88 for coastal locations or polyurethane red oxide to RDSO specification No. M&C/PCN-109/88 for other locations to 40 microns thickness (DFT).

Use of epoxy painting system for railway girders has been introduced recently. Real benefit of epoxy painting can be achieved with longer life of paint than frequent oil painting which is more costly for maintenance of steel girders on existing bridges. Life of epoxy painting is 12 to 15 years if correct epoxy scheme is selected.

Oil Painting

For locations where corrosion is not severe following painting scheme should be applied

Primer coat: * One coat of ready mixed paint zinc chromate primer to IS : 104 - 1979, followed by one coat of ready mixed paint red oxide zinc chrome primer to IS : 2074 - 1992

Finishing coat:

  • Two finishing coats of red oxide paint to IS-123

For locations where corrosion is severe- To avoid frequent painting metallising or epoxy painting for existing girders is recommended but when there are no facilities available for the same, protective coating by oil painting with following painting scheme should be applied.

Primer coat: * One coat of ready mixed paint zinc chrome priming to IS 104 - followed by one coat of zinc chrome red oxide priming to IS 2074.

Finishing coat: Two coats of aluminium paint to IS : 2339 - 1963

Note: 1. Surface preparation of the steel to receive the paint should be as prescribed in item 217 para 1 of Indian Railway Bridge Manual 1998. 2. The Red Lead paint vide IS: 102 has been removed from IRBM vide correction slip no 12 as the IS code has been withdrawn by BIS the paint is not available in market.

The metallised girders can also be regularly maintained by painting with conventional oil based paints. The paints will protect the metallising layer and the metallising layer will protect the steel girder. This will increase the overall life of the costly metallising layer. On the other hand epoxy paints are difficult to overcoat and these have to be blasted away and repainted with epoxy paint after the service life is over.


Precautions in painting

Surface preparation is key to the performance of painting system.
Aims of Surface preparation

  • To clean the surface off any rust, moisture, loose/perished paints etc.
  • To give sufficient roughness to the surface to be painted for better adhesion of the paint
  1. Scraping to be done where the existing paint film is deteriorated
  2. Only cleaning to be done where the existing paint is satisfactory
  3. Scrapping area shall be decided in advance of start of painting and advised to contractor
  4. Scraping: Remove all rust, loose/perished paint, any other surface contaminants. Tightly adhering paint can remain. Emery paper, wire brush, scrappers, power tools etc shall be used for the same. Flame cleaning using oxy-acetylene flame can also be done(not to be done on plates with thickness<10mm). The surface may be washed with luke warm water having 2% detergent. Surface preparation can be checked in accordance withe St2 in IS:5909. In this case, Primer as well as top coats to be applied
  5. Cleaning: Remove the external impurities clinging to the painted surface. Since the paint condition is good, it shall not be removed, but the surface roughness is to be improved. The cleaning shall be done using wire brush, cloth rags, water etc. In this case, only top coats of paint are to be applied
  6. Surface preparation is key to the painting scheme as a whole and upto 80% contribution is given by the surface preparation.

Aim of painting

  • The paint barrier must be complete, uninterrupted and strongly adhering to surface to provides effective protection.
  • Not to be done during rainy weather (Relative Humidity shall be less than 90%)
  • Not to be done in dusty winds/ foggy/ misty conditions.
  • Rust/ dust etc shall not be included in the paint film

Quality of paint film shall be good and uniform

  • Good quality brush with minimum width 5 cm shall be used
  • Brush shall be held at 45 deg to the surface
  • Light strokes
  • One coat shall mean two layers, second one cross wise
  • Uniform and even painted surface shall be obtained
  • Can be sprayed or rollers can also be used
  • Rags, cotton waste, cloth etc shall not be used
  • Paints from approved suppliers in RDSO list shall only be used. Paints shall conform to relevant IS codes for which test certificates and original bills may be taken and markings on drum shall be seen.
  • Paint film thickness shall be checked. No thicknesses have been specified in bridge manual as the thickness depends on the type of surface, temperature, humidity and method of application as also brand of paint being used. Therefore, trial shall be conducted for the actual surface and the brand of paint and the thickness obtained in trial shall be monitored during work.
  • Initial and final readings shall be taken with elcometer to check the thicknesses. Paint consumption shall also be monitored to get the idea of quality. For this, drums may not be returned till the work is complete
  • Outside testing may be got done if doubt is there on quality or source of paint
  • Turpentine and kerosene must not be added as thinners. Only approved quality thinners from the same manufacturer may be added if the need is felt at site (due to very high temperature etc)

Paints must be stored properly when brought to site.

  • Paints must be used within shelf life. The shelf life is as follows:
  • Paint red oxide – 1 year
  • Aluminium When paste & oil not mixed – 1 year
  • Aluminium When mixed – 4 months

Time Between coats: Too much time shall not be there between coats as the surface is lekely to become contaminated and/or corroded due to excess time lag. The time lag as per bridge manual shall not exceed:

  • Surface prep and Primer – 24 hours
  • Surface prep and first finishing coat – 48 hours
  • Primer coat and first finishing coat – 7 days
  • First finishing coat and 2nd finishing coat –7 days
  • Paint layer shall be applied only after sufficient drying of the earlier paint layer
  • Even for prepared surface, there may be contamination and the surface shall be continuously cleaned during painting
  • If due to some reason, the painting cannot be taken up immediately after the surface preparation

Temporary coating of linseed oil shall be applied thinly and uniformly. 1/3 litres per 10 sqm shall be sufficient

Patch Painting

  • If only small area is corroded when inspection is done, it is better to take it up for painting instead of waiting for the entire girder to get due for painting. This work of painting only only the affected area is called patch painting. This is normally got done with departmental resources using oil paints and departmental labour. Sometimes, the specialized paints such as epoxies get disturbed in patches. These shall be painted using the paints recommended for over coating/ repairs by the manufacturer.

Sleeper Seat Painting

  • Sleeper seat is an area subject to severe corrosion due to accumulation of water, due to large number of surfaces mating and due to abrasion between the members under train loads. This is required to be painted at more frequency (once in 2.5 years was prescribed on western railway as against once in 5 years for the complete girder). Alternately, the sleeper seat shall be painted with metallising or epoxy paint which have better life.
 
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