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
Phosphate jelly is applied to the bars, with fine
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
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
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.