Corrosion of Steel Repair & Rehabilitation of Structures Steel Corrosion

1. Repair& Rehabilitation of Structures
RamanuJ Jaldhari
Asst. Prof.
KITE, Jaipur

2. Corrosion
Corrosion is defined as the destruction or
deterioration of materials due to chemical or
electrochemical reaction with the environment & also
the loss of steel due to formation of rust.
The corrosion of steel is the depassivation of steel
with reduction in concrete alkalinity through
carbonation.

3. Corrosion

4. Corrosion

5. Corrosion
•Corrosion deteriorates concrete because the product of
corrosion-FERRIC OXIDE brown in colour occupies a greater
volume than steel & exerts substantial bursting stresses on the
surrounding concrete.
•The outward manifestations of rusting include staining,
cracking, & spalling of concrete.
•The progress of process of corrosion is generally in geometric
progression with respect to time.
•Consequently, the cross-section of steel is reduced. With time,
structural distress may occur either due to loss of the bond
between steel & concrete, due to cracking and spalling of
concrete, or as a result of reduced steel cross-sectional area.

6. Volume Change Process

7. Corrosion

8. Electrochemical Corrosion
The presence of electrical potential is the prerequisite for
the occurrence of electrochemical corrosion.The
electrochemical potential may be created by any of the
following:
 Differential aeration (difference in the concentration of
oxygen on the steel surface)
 Differential ion concentration (metal ions, dissolved
salts, & pH of concrete in the vicinity of the steel may
cause this)
 Differential surface properties (small blemishes on the
surface of the reinforcement formed during rolling
generally termed as mill scales or breaks in coatings,
impurities in concrete, etc may be responsible for this).

9. Factors affecting Corrosion
pH Value
 The pH value of the moist concrete is normally about 12.0,
which is sufficient to passivate the reinforcement against
corrosion.
 When it reduces to below 8.0, the carbonation of concrete
takes place and in turn, corrosion initiates.

10. Factors affecting Corrosion
CARBONATION OF CONCRETE
Carbonation of concrete is a process by which carbon dioxide from the air
penetrates into concrete and reacts with calcium hydroxide to form calcium
carbonates.
•The conversion of Ca(OH)2 into CaCO3 by the action of CO2 results in a
small shrinkage. CO2 by itself is not reactive.
•In the presence of moisture, CO2 changes into dilute carbonic acid which
attacks the concrete and also reduces alkalinity of concrete.
•The concentration of CO2 in rural air may be about 0.03 per cent by volume.
In large cities the content may go up to 0.3 per cent or exceptionally it may go
up to even 1.0 per cent.

11. Influencing Factors
Rate of Carbonation: The rate of carbonation depends on
the following factors.
◦ The level of pore water i.e., relative humidity.
◦ Grade of concrete
◦ Permeability of concrete
◦ Whether the concrete is protected or not
◦ Depth of cover
◦ Time

12. Factors affecting Corrosion
REACTION WITH CHLORIDE
Chloride attack is one of the most important aspects for
consideration when we deal with the durability of concrete.
Chloride attack is particularly important because it primarily
causes corrosion of reinforcement. The presence of CaCl2
even in small percentages can lead to rapid corrosion of
reinforcement, as it reduces the electrical resistivity of
concrete and helps to promote galvanic cell action.

13. Factors affecting Corrosion
COVER TO REINFORCEMENT
Lack of adequate cover contributes much to corrosion in an
aggressive environment. A well compacted & continuous,
even if thin, cover of good quality concrete on reinforcement
is sufficient to protect it from corrosion.

14. Factors affecting Corrosion

15. Damages Caused by Corrosion
FORMATION OF WHITE PATCHES
Carbon-dioxide reacts with calcium hydroxide in the cement paste to form
calcium carbonate. The free movement of water carries the unstable calcium
carbonate towards the surface & forms white patches. This indicates the
occurrence of carbonation.
BROWN PATCHES ALONG REINFORCEMENT
When reinforcement starts corroding, a layer of ferric oxide is formed. This brown
product resulting from corrosion may permeate from the steel surface along with
moisture to the concrete’s outer surface without cracking the concrete. This leads
to brown discolouration on the surface of concrete.
OCCURRENCE OF CRACKS
Increase in volume of corrosion products exerts considerable bursting pressure on
the surrounding concrete, resulting in hairline cracking.
SNAPPING OF BARS
The continued reduction in the size of bars results in snapping of the bars. This
occurs at ties & stirrups. At this stage, the size of main bars is also reduced.

16. Damages Caused by Corrosion
FORMATION OF MULTIPLE CRACKS
As corrosion progresses, multiple layers of rust are formed on the
reinforcement, which in turn exert considerable pressure on the surrounding
concrete resulting in the widening of hairline cracks. The bond between
concrete & reinforcement is considerably reduced. If this is happening, there
will be a hollow sound when the concrete is tapped at the surface with a
light hammer.
SPALLING OF COVER CONCRETE
Due to loss in bond between steel & concrete and the formation of multiple
layers of cracking, the cover concrete starts peeling off.
BUCKLING OF BARS & BULGING OF CONCRETE
The spalling of cover concrete & snapping of ties causes the main bars to
buckle. This results in the bulging of concrete in that region. This is
followed by collapse of the structure.
Corrosion can also cause structural failure due to reduction in cross-section
of the main bar, hence leads to reduced load-carrying capacity.

17. Corrosion Preventive Measures
USING CORROSION RSISTANT REINFORCEMENT
 EPOXY COATING OF REINFORCEMENT
Fusion bonded epoxy coating, also known as fusion-bond epoxy powder
coating and commonly referred to as FBE coating, is an epoxy based powder
coating that is widely used to protect steel pipe used in pipeline construction,
concrete reinforcing bars and on a wide variety of piping connections, valves
etc. from corrosion. FBE coatings are thermo set polymer coatings. They
come under the category of 'protective coatings' in paints and coating
nomenclature.
Advantages of Epoxy coating Protection
Epoxy coated rebar is designed to protect the rebar against rust and corrosion.
Applying an epoxy coating to steel rebar prevents oxygen and chlorides from
reaching the steel surface reducing corrosion.
Environmentally friendly materials - Unlike many paints, the fusion-bonded
epoxy coatings used for steel reinforcement do not contain appreciable
solvents or other environmentally hazardous substances.

18. Corrosion Preventive Measures
Disadvantages of Epoxy coating Special Handling ECR requires delicate
handling to prevent damage to the epoxy coating. It is essential that damage to the
coating is minimized.
UV damage - ECR coatings break down under UV ray exposure. Touch up after
placement may be required due to scratching and chipping during transport and
placement
Coating inconsistencies - ECR specifications allow for a percentage of the bar to
have holidays and pinholes, compromising its protection mechanisms before it
reaches the job site.
Abrasion resistance - The coating lacks abrasion resistance, and thus is easily
damaged in transport to the job site and installation.
Poor bond to concrete - Loss of bond strength as a result of the epoxy coating.
Under film corrosion - ECR coatings are permeable and once corrosion begins, it
spreads throughout the bar underneath the epoxy film.
Installation conditions - ECR coatings may crack when handled in temperatures
less than 50 F.
Increased corrosion process - Pin holes or discontinuity of the epoxy layer on a
bar will enhance the corrosion process faster and more aggressive than in the case
of a bare uncoated bar.

19. Corrosion Preventive Measures
GLASS FIBRE REINFORCED POLYMER REBAR (GFRP)
Composed of resin-impregnated glass fibers and containing no steel, GFRP
rebar are immune to chloride and chemical attack. In addition, the bars are
nonconductive and have high strength-to-weight ratios. They have a tensile
strength as much as twice that of conventional steel reinforcement yet are only
one-fourth the weight. Because GFRP rebar are nonconductive, they don’t
affect magnetic fields and radio frequencies, making the bars ideal
reinforcement for concrete in the vicinity of magnetic resonance imaging
(MRI) equipment, radio and compass calibration equipment, or high-voltage
transformers, cables, and substations.

20. Corrosion Preventive Measures
HOT DIPPED GALVANISED REINFORCEMENT (HDG)
Galvanized reinforcing steel is effectively and economically used in concrete
where unprotected reinforcement will not have adequate durability. The
susceptibility of concrete structures to the intrusion of chlorides is the primary
incentive for using galvanized steel reinforcement. Galvanized reinforcing
steel is especially useful when the reinforcement will be exposed to the
weather before construction begins. Galvanizing provides visible assurance
that the steel has not rusted.

21. Corrosion Preventive Measures
 CONSTRUCTIONALASPECTS
Adequate compaction of concrete, effective curing,
production of impervious concrete, effective grouting of
tendons & periodical maintenance must be implemented and
these constitute sound corrosion engineering principles
DESIGN FACTORS
W/C ratio, high strength concrete, higher minimum cement
content, thicker concrete cover, proper detailing of
reinforcement & moderate stress levels.
CATHODIC PROTECTION
The corrosion process that takes place in concrete is
electrochemical in nature, very similar to a battery. Corrosion
will result in the flow of electrons between anodic and cathodic
sites on the rebar. For corrosion to occur four basic elements are
required.

22. Corrosion Mapping
The combination of steel and concrete is a viable construction
material of proven durability. In the normally alkaline concrete
environment, a thin oxide layer is formed on the surface of the
reinforced steel. This oxide film isolates the steel from the
environment and prevents corrosion for as long as the oxide
layer remains intact. Protected within the concrete, the oxide
layer is seldom disturbed and the structural integrity of the
concrete-steel combination is unaffected by corrosion. The
oxide is known as (Gamma-Fe2O3).
Following instrument used for determine the occurrence % of
corrosion.
1. HALF-CELL POTENTIOMETER
2. RESISTIVITY METER

23. HALF-CELL POTENTIOMETER
•The half-cell is usually a copper/copper sulphate or silver/silver
chloride cell.
•The concrete functions as an electrolyte and the risk of corrosion
of the reinforcement in the immediate region of the test location
may be related empirically to the measured potential difference.
•The instrument measures the potential and the electrical resistance
between the reinforcement and the surface to evaluate the corrosion
activity as well as the actual condition of the cover layer during
testing.
•The electrical activity of the steel reinforcement and the concrete
leads them to be considered as one half of weak battery cell with
the steel acting as one electrode and the concrete as the electrolyte.
•The name half-cell surveying derives from the fact that the one
half of the battery cell is considered to be the steel reinforcing bar
and the surrounding concrete.

24. HALF-CELL POTENTIOMETER

25. HALF-CELL POTENTIOMETER

26. HALF-CELL POTENTIOMETER
The risk of corrosion is evaluated by means of the
potential gradient obtained, the higher the gradient, the
higher risk of corrosion. The test results can be interpreted
based on the following table.
Half-cell potential (mv) relative to
Cu-Cu sulphate Ref. Electrode
% chance of corrosion
activity
Less than -200
Between -200 to -350
Above -350
10%
50% (uncertain)
90%

27. RESISTIVITY METER
RESISTIVITY TEST OF CONCRETE
Electrical resistivity technique gives an insight into the concrete
microstructure. For a homogeneous material the value of resistivity will be
constant throughout and therefore its measurement carries with it the
following advantages:
In theoretical and experimental work, there appears to be a correlation
between concrete resistivity and chloride ingress. In general, the chloride
diffusion coefficient is inversely proportional to the concrete resistivity.
Within a particular structure, more permeable zones will have a
comparatively low porosity and higher chloride penetration.
Hence this test can be used to assess the durability and corrosion resistance
of concrete.
Electrical resistivity is strongly influenced by the electrolytes in the pores,
it can be used to detect the conducive elements as in the case of fibre
reinforced concrete.

28. RESISTIVITY METER
To measure the resistivity, metallic probes are placed over the concrete
surface.
A known current is passed on the outer probes and resulting potential
drop between inner probes is measured.
The resistance is computed by dividing potential drop by the current.
A conductive gel is used between probe and concrete surface to make
effective contact.

29. RESISTIVITY METER
Limitations
It is difficult to measure resistivity in very close reinforcement
Carbonation may affect the resistivity
It cannot be used where ambient change in temperature is there.
Experience operator is required to handle this equipment.
Summary
With the help of resistivity meter, probability of corrosion can be assessed.
This is a very simple technique and can be adopted easily in the field
without any disruption to traffic.

30. Any Query???