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 Corrosion of reinforcing steel bars (KTH 2008 10.1)
Corrosion of reinforcing steel bars (KTH 2008 10.1)

Reinforcement steel should be protected from the corrosion, before its placement in the concrete and during its life inside the concrete.

Before final placement, steel should be exempted from obvious surface scales, alterations or accidental deformities and wounds, which accelerate corrosion.

The speed of steel corrosion is increased, inside and outside of concrete, with:
  • increase of temperature and relative humidity
  • reduction of PH
  • increase of presence of salts (eg marine environment)
  • contact of steel with soil, water etc.
  • contact of steel with different materials or environments.
Frequent changes of above factors influence further corrosion speed. Other factors that increase corrosion speed are:
  • existence of active centers on the surface of steel (as eg or wounds, bends on mandrels with small bending diameter etc.)
  • existence of surface denaturation due to pre-existing corrosion
  • contact of steel of different type and different electrochemical potential between each other
  • contact of steels of different degree of corrosion
  • cold treatment (drawing, rolling etc)
  • increased porosity of concrete.
Protection of steel from corrosion after incorporation in concrete, is generally assured by passive protection offered from alkaline environment of concrete (provided PH is bigger than 9,5) and from the concrete cover.

In special cases, better protection of steel reinforcement from the corrosion in the concrete is required. In the case of intensely corrosive environment, or in the case that the life-time of the construction is longer than usual, additional measures can be applied accordingly as follows:
  • Increase of cement content in concrete or use of special cements
  • Use of concrete of decreased porosity or water-permeability
  • Increase of concrete cover
  • Use of corrosion resistance steel such as galvanized steel, epoxy-coated steel, steel coated with stainless steel and stainless steel. Attention should be paid at the use of such types of steel because of:
    • danger of creation of galvanic cell
    • possibility of local detachment of coating.
Especially for zinc-galvanized steel, chemical reaction of zinc with the Ca(OH)2 in the pores of concrete, results to production of hydrogen bubbles close to reinforcement area, that decreases the bond between steel and concrete (especially in the case of plain steel). In order to avoid this phenomenon, use of chromic salts (either in concrete or in galvanised steel), or replacement of zinc coating with alloy of zinc with iron or aluminium, is recommended.

Use of coatings of suitable thickness
Coating of concrete surface with thin protective layer from suitable organic or inorganic substances
  • Use of corrosion inhibitors
  • Cathodic protection
These protective measures have different cost and protection duration and each one can be applied accordingly. It is likely however that unfavourable consequences may occur, that must be taken into account.

Visual testing and estimation of corrosion (KTH 2008 4.5.9)

If the surface of steel presents important alteration due to the general or located corrosion, the possibility of partial or total rejection of checked steel quantity is examined. For this reason, a special laboratory test for the determination of quantity of products of corrosion, is carried out.

Specifically for the tests of corrosion, and since a possibility of rejection is examined, the samples would be received from the most corroded regions of bars. Losses of corrosion products during cutting and transport samples should be avoided. For this purpose, six samples of minimum length 0,25m will be taken.

In each sample the mass of oxides is calculated as described below. If each value is less than 300gr/m then the checked quantity is acceptable. If a value is higher than 300gr/m, there should be further examination for existence of pitting corrosion. In each sample, measurement of the depth of pitting holes in three corroded location is carried out. If the measured values in all locations is less than the critical one fixed in KTH 2008 10.3, then steel can be used, after previously being cleaned. If the bottom of pitting holes is higher than the critical value, even in one place, then the checked quantity is rejected.

In any case, for the final acceptance of the checked quantity, the requirements for the nominal mass and the geometry of ribs should be satisfied.

Trial of determination of products of erosion (KTH 2008 10.3)

Each test-piece is weighted out with precision at least 0,01g. Afterwards it is immersed in hydrochloric acid solution (water solution 12gr HCl in 100ml of solution) and hexamethylenetetramine (solution 0,35g hexamethylenetetramine in 100ml of solution). The test-piece is weighted in intervals of time of 30min, up to constant weight.

The difference of weight is calculated:
                     Dw = initial weight - final weight
The area of surface of test-piece is calculated and the difference of weight is expressed  in g/m2.

If the calculated value is less than 300g/m 2, steel can be used. Pitting corrosion could lead to reduction of ductility or to stress corrosion, having as a result brittle fracture.

If the calculated value exceeds 300g/m 2, then existence of pitting corrosion should be investigated.

The test for pitting corrosion is carried out with the microscopy method according to the DIN 50905-Part 3/87.

The critical depth of pitting holes, h crit, is the depth above which important reduction of ductility with danger of brittle fracture happens.

Currently, until the establishment of more reliable values , for the determination of critical depth of pitting holes, h crit, is proposed the application of following semi-empirical equation:

hcrit=

 where d the nominal diameter of bar in mm and h crit the critical depth of pittinh holes in mm.

If the depth of pitting holes is determined above critical depth, steel is rejected. If is determined depth is less than critical, steel can be used under the following conditions:
  • Mechanic cleaning of products of corrosion in order to decrease quantity at less than 300g/m 2
  • Increased protection measures of reinforcement against corrosion, according to Paragr. 10.1, because even if the products of corrosion are reduced in values less than 300g/m 2, this kind of steel presents higher rate corrosion in comparison with steel thad has not been corroded.
The trials for the control of corrosion must be executed exclusively in laboratory.


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