Coatings for Steel
The object of coating to steel bar is to provide a durable barrier to aggressive materials, such as chlorides. The coatings should be robust to withstand fabrication of ribcage, and pouring of concrete and compaction by vibrating needle.
Simple cement slurry coating is a cheap method for temporary protection, against rusting of reinforcement in storage.
Central electro chemical Research institute (CECRI), Karaikudi, have suggested a method for prevention of corrosion in steel reinforcement in concrete. The steps involved in this process are :
The reinforcement are cleaned with a derusting solution. This is followed without delay by leaning the rods, with wet waste cloth and cleaning powder. The rods are then rinsed in running water and air dried.
· Phosphate jelly is applied to the bars, with fine brush.
o The jelly is left for 45-60 minutes, and then removed by wet cloth an inhibitor solution is then brushed over the phosphated surface.
ü Cement Coating
Slurry is made by mixing the inhibitor solution, with Portland cement and applied on the bar. A sealing solution is brushed after the rods are air cured. The sealing solution has an insite curing effect. The second coat of slurry is then applied and the bars are air dried.
Two coats of sealing solution are applied to the bars, in order to seal the micro-pors of the cement coated an to make it impermeable to corrosive sails.
The is patent method evolved by CECRI, and licence is given to certain agencies. Somehow or other, this method has not become very popular. Some experienced consultants and engineers doubt the effectiveness of this method.
It is one of the effective methods of coating rebars. The fusion bonded epoxy coating is a specialized job, carried out in a factory, and not at site of work. Plants are designed to coat the straight bars in a continuous process. Initially, the bar is shot blested to remove all mill scale and to give the kind of surface finish required.
This ensures an adequate bond between epoxy and steel. The bar is then heated to a carefully controlled temperature, before passing thorugh a spray booth. Eletrostatesically charged epoxy powder particulars are deposited evenly on the surface of the bar. It looks greenish in color, the coating thickness vary from 130 to 00 microns.
Although epoxy coated bars have an excellent protection to corrosion in aggressive environment, there are a few limitations.
After the treatment, cutting and bending may injure the steel, which needs certain sit treatment. The site treatment is likely to be inefficient. The presence of any defect in the treated body can induce severe localized corrosion, which defeated very purpose. The bars cannot be welded. The epoxy is not resistant to rays of sun.
The bars should not be exposed to sun for long duration before use. The coating may get damage during vibration of concrete. The treatment is very costly, as that of steel. This method of protection to the steel is being given to all the flyovers, and other structures at Mumbai.
Galvanizing of reinforcement consists of dipping the steel bars in molten zinc. This results in a coating of zinc, bonded to the surface of steel. The zinc surface with calcium hydroxide in the concrete, to form a passive layer and prevents corrosion.