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PAINTING OF STEEL BRIDGES

PAINTING OF STEEL BRIDGES. Maintenance of Steel superstructures. Why? Deterioration due to age Rusting of steel Oxidation of steel in presence of water and air Rust is powdery and increases in volume Rust is hygroscopic, promotes more corrosion. Corrosion. References.

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PAINTING OF STEEL BRIDGES

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  1. PAINTING OF STEEL BRIDGES

  2. Maintenance of Steel superstructures • Why? • Deterioration due to age • Rusting of steel • Oxidation of steel in presence of water and air • Rust is powdery and increases in volume • Rust is hygroscopic, promotes more corrosion

  3. Corrosion

  4. References • For existing structures: • IRBM 1998 para 217, 218 • For new structures: • IRS B1 2001 para 39

  5. Atmospheric corrosion • High humidity level ( > 65 %) • The corrosion velocity is doubled for every 10oC increase in temperature. • Air pollution • SO2in air • The sulphur dioxide creates H2SO3 which isoxidized to sulphuric acid (H2SO4) which increases the corrosion velocity. • Similar effect of salts which come in from sea in coastal areas

  6. Painting • Steel girders are more prone for corrosion • Where humidity > 60% • In coastal areas • Bridge across creek • Bridge situated in industrial environment • Girders subjected to • Droppings from traffic • Public nuisance

  7. Planning of painting • Frequency 1 yr to 6 yr as prescribed by the Chief Engineer • Depending on the severity of corrosive environment

  8. Types of Painting • Patch painting • Scraping and painting • Cleaning and painting • Metallising • Epoxy painting • The first three are using ordinary paints

  9. Planning of painting • If a small portion of the girder found corroded during inspection, but girder not due for painting, Patch painting to be done

  10. Planning for Painting

  11. Surface preparation • 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 • The paint barrier must be complete, uninterrupted and strongly adhering to surface to provides effective protection

  12. Scraping and Cleaning Areas to be decided Scraping Cleaning

  13. Planning of painting • Scraping to be done where the existing paint film is deteriorated • Only cleaning to be done where the existing paint is satisfactory • Scrapping area shall be decided in advance of start of painting and advised to contractor

  14. What to do in sleeper seat? • Hidden parts under sleeper to be exposed by shifting • Sleepers shifted and sleeper seat painted. After drying of paint, sleeper shifted back • Damages fittings and affects track parameters • Sleepers seat painting to be done by the SE (P-WAY) (at reduced frequency, once in 2.5 years*) * W Rly Practice • Metallising/ epoxy paints shall be used for better life of paint in sleeper seat

  15. Surface preparation • Scraping • Remove all rust, loose/perished paint, any other surface contaminants • Tightly adhering paint can remain • Use emery paper, wire brush, scrappers • Use power tools • Flame cleaning – oxy-acetylene flame (not to be done on plates with thickness<10mm) • Luke warm water – 2% detergent • Surface shall be as per St2 in IS:5909. • Primer as well as top coats to be applied

  16. Surface preparation • Cleaning • Remove the external impurities clinging to the painted surface. • Paint condition is good, so not to be removed • Surface roughness is to be improved • Use wire brush, cloth rags, water etc • Only top coats to be applied

  17. Precautions during painting • Surface preparation key to the performance of the paint system • Upto 80% contribution • Painting not to be done during rainy weather and in dusty winds/ foggy/ misty conditions • Relative Humidity shall be less than 90% • Moisture/ dust etc shall not be included in the paint film

  18. Precautions during painting: paint quality • Paints from approved suppliers in RDSO list shall only be used • Paints shall conform to relevant IS codes • Test certificates and original bills may be taken • Markings on drum shall be seen • Paints must be stored properly Contd..

  19. Precautions during painting: paint quality • 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)

  20. Paint to be used within shelf life • Paint red oxide – 1 year • Aluminium • When paste & oil not mixed – 1 year • When mixed – 4 months • Other paints • As per manufacturer recommendations

  21. Precautions during painting: painting • Paint film thickness shall be checked?? • No thicknesses specified • Varies with type of surface, paint manufacturer, method of application, temperature/ humidity at time of painting etc • Trial shall be conducted for the surface and the brand of paint for monitoring during work • Initial and final readings shall be taken with elcometer • Paint consumption shall also be monitored to get the idea of quality • Drums may be kept by rly till work is completed

  22. Application of Paint • Good quality brush with minimum width 5 cm • 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

  23. Time lag between stages of painting • Maximum time • Surface prep (scraping) and Primer – 24 hours • Surface prep (cleaning) and first finishing coat – 48 hours • Primer coat and 1st finishing coat • 1st finishing and 2nd finishing coat • Paint layer shall be applied only after sufficient drying of the earlier paint layer • Even for prepared surface, there may be contamination • Must be cleaned during painting }7 days

  24. Temporary coating • 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

  25. Painting Scheme: No Severe corrosion • Priming coat • One coat of Zinc chromate priming to IS 104 followed by red oxide zinc chrome priming to IS 2074 OR • Two coat of Zinc chromate red oxide primer to IRS P31 • Finishing coat • Two cover coats of red oxide paint to IS 123

  26. Painting Scheme: Severe corrosion • Priming coat • One coat of ready mixed paint zinc chromate priming to IS:104 followed by one coat of red oxide zinc chrome priming to IS: 2074 • Finishing • Two coats of red oxide paint to IS: 123 or aluminium paint to IS 2339

  27. Lead paints • Red Lead paints are not available in market • These were replaced by Zinc Chrome priming to IS:104 followed by Zinc Chrome Red Oxide priming to IS:2074 in IRBM vide correction slip no 12 • Even Red Oxide paint to IS:123 is difficult to get in market

  28. Painting During Fabrication • Para 20.1 of IRS B1: All permanent contact surfaces shall be removed of paint and mill scale, cleaned, dried and immediately a layer of Zinc Chrome Red Oxide Priming to IS:2074* shall be applied *Correction vide CS no 2 to IRS B1 (As Red Lead paints not available in market) • Also Para 39.3 of IRS B1

  29. Painting During Fabrication • Same painting schedule as already shown, depending on the severity of corrosion at location where girder will be inserted

  30. Superior paints

  31. Superior Paints • Metallising • Epoxy Painting • Should be done for girders exposed to corrosive environment • Flooring system ( cross girder and rail bearer) in open web girders • Steel girders in industrial, suburban or coastal areas • Also may be done for • Top flange plate of plate girder and underslung girder • Top bracing system in open web girders in electrified areas

  32. Cross girder

  33. Bridge in corrosive environment

  34. Bracings in electrified area

  35. Top flange of plate girder

  36. Metallising Spraying of molten Aluminium on cleaned surface

  37. Metallising • Surface preparation • Remove Oil/ grease from metal surface using petroleum hydrocarbon solvent to IS:1745 • Cleaning by blasting the surface with chilled iron grit to G24 (grit blasting) or with sand (sand blasting)

  38. Surface preparation • Sand/ grit Blasting • Best type of surface preparation • Use air to blast clean the surface with • sand between mesh sizes 12 to 20 (40% retained on size 20) or • chilled shots to G24 as per BS: 2451 • Surface to be clean of all layers including mill scale • In exceptional cases, if sand blasting not possible, power tool cleaning shall be done

  39. Equipment SILICA SAND COMPRESSOR CHILLED IRON GRIT (MAGNIFIED) HOPPER

  40. Sand vs Grit Blasting • For small components, grit blasting shall be done • Grit blasting has to be done in enclosure for collecting the shots for reusing. • For large components and on-site, sand blasting shall be done • Issues • Causes silicone particles in air, bad for health • Shall be done with adequate precautions • Not cause too much problem to railway users.

  41. Blasted Surface: Shall be to Sa2½ as per IS:5909

  42. Scraping vs Sand blasting • Sand blasting is very costly • Use sand blasting for expensive paint systems • Use scraping and cleaning for Ordinary painting • Also for expensive paints in inaccessible locations

  43. Blasting Parameters • Spray sand/grit with pressure to remove rust, paint and other foreign particles • Nozzle Pressure: Not less than 2.109 Kg/sq cm • Nozzle Dia: Not more than 12 mm • Nozzle Position: At Right Angles to surface, Approx 22.5 cm away.

  44. Blasting

  45. Metallising • Metallising done using special gun • Has acetylene/ LPG, Oxygen and Air feeds • Aluminium wire fed separately • Heat from fired LPG/ Acetylene melts the aluminium wire • Molten metal sprayed by air pressure onto prepared steel surface.

  46. Metallising Gun

  47. Metallising being Done

  48. Metallising • Parameters for Spraying Aluminium • Air Pressure: Not less than 4.218 Kg/sq cm • Wire dia 3 mm or 5 mm • Aluminium wire 99.5% pure as per BS:1475, material 1-B or as per IS:739 • In minimum two layers • At least one layer within 4 hrs of blasting, and finishing within 8 hrs of blasting

  49. Metallising • Check coating thickness using any magnetic thickness gauge. • Thickness minimum 115 microns • Thickness average 150 microns

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