Surface blasting and metallizing of steel girders is done as per procedures mentioned in Appendix-VII of IRS B-1. Subsequent operations of applying primer and paints are to be done in accordance with Clause 39.2.1 of IRS B-1. This is applicable for all locations and for all types of new steel girder bridges including all components as per ACS no. 8 dated 27.08.2014 of IRS B-1.
As per annexure VII of IRS B1, For stage 1 that is for surface blasting, compressed air blasting or centrifugal blasting with a suitable abrasive material in accordance with Clause 3 of IS 6586 should be used. The achieved surface should be comparable in roughness with a reference surface of Appendix-A of IS 5905. IS 5905 is code for sprayed aluminium and zinc coatings on iron and steel.
Stage 2 is for aluminium metallizing. The chemical composition of aluminium to be sprayed shall be 99.5% aluminium conforming to IS 2590. IS 2590 is code for ‘primary aluminium ingots for remelting for general engineering purposes’. The nominal thickness of coating shall be 150 microns. The minimum local thickness shall be not less than 110 microns.
Stage 3 is the stage at which one coat of wash primer or etch primer conforming to IS 5666 shall be applied. After four to six hours of the application of this etch primer, one coat of zinc chrome primer conforming IS 104 shall be applied as covered in Stage 4. Zinc chrome to be used in the manufacturing of primer shall be conforming to type two of IS 51.
After hard drying of one coat of zinc chrome primer, Stage 5, painting that is one coat of aluminium paint conforming to IS 2339 shall be applied. This coat is to be applied in fabrication workshop.
In the last stage that is in Stage 6, the final coat of aluminium paint conforming to IS 2399 shall be applied after completion of erection at site. This coat should be applied after touching up the damages to earlier coats occurred during transit and during erection operations.
It is most important to note that the portion of girders which are to be tightened with HSFG bolts should be treated differently. In order to achieve appropriate slip factor, at bolt hole locations, only surface blasting and metallizing should be done. No painting coats after Stage – II needs to be applied at interface between the plies. This is a common mistake done by fabricators. Inspecting officials should invariably checked and see that this provision is adhered to. Otherwise, it will be difficult to achieve proper friction grip between steel plies at a joint.
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