It includes mechanism of corrosion of steel reinforcement in concrete. It includes concepts like passivation, chloride ingress and chloride binding. It deals with the durablity aspects of a concrete structure.

1. CORROSION OF STEEL
REINFORCEMENT IN CONCRETE

2. CONTENTS
1. GENERAL.
2. STEEL CORROSION IN REINFORCED CONCRETE.
 corrosion of metals.
 Galvanic corrosion.
 Corrosion mechanism in steel.
 Passivation.
 Steel corrosion in reinforced concrete.
 Detrimental influence of corrosion on concrete performance
3. CHLORIDE INGRESS INTO CONCRETE.
 Chloride in the environment.
 Ingress mechanism.
 Chloride binding.
 Role of chloride in corrosion.

3. Cont…
 Protection from chloride induced corrosion.
4. CARBONATION.
 Carbonation reaction
 Factors affecting rate of carbonation.
 Changes in the physical properties.
 Avoiding carbonation.

4. GENERAL
 Steel reinforcement in concrete
 Corrosion involves loss of material from metal surface as a
result of chemical reaction.
 This leads to loss of cross-sectional area and finally loss of
load bearing capacity.
 Provision of cover block extends the life of reinforcement.
 In addition to that, the chemical environment in the pores of
concrete discourages the corrosion of steel reinforcement.
uses drawbacks
The ductility and high tensile
strength of steel reinforcement
when used in concrete , the
structure can be used in flexure
and in direct tension.
Plain steel when used as
structural material is susceptible
to corrosion.

5. MECHANISMN OF
CORROSION
CORROSION OF METALS
 Corrosion of metals involves an oxidation reaction,
 The proportion of oxygen and metal in this reaction depends
on oxidation state of the metal .
 This form of corrosion is only of minor concern for steel in
civil engineering application at ambient temperature.
 Since this reaction is very slow.
 Corrosion becomes a problem for steel in conventional
structures where water is present.
 In such cases , galvanic corrosion may occur which causes
more damage..

6. GALVANIC CORROSION
 Galvanic or wet corrosion describes an electrochemical form
of corrosion in which the close proximity of two different
metals in contact with themselves and water containing an
electrolyte leads to corrosion of one of the metals.
 This corrosion is dependent on the strength with which each
metals atom are bound to each other.
 This is indicated in terms of metal’s standard electrode
potential.
 Standard electrode potential – potential difference between a
metal electrode and a hydrogen electrode across an
electrolyte solution junction under standard conditions.
 A more positive standard electrode potential denotes a
material is more prone to corrosion and is thus more active
(anodic).

7. CORROSION MECHANISM IN STEEL
 Iron in steel – more anodic .
 It undergoes oxidation which takes the form of ionization at
its surface:
 The metal ion dissolves.
 The iron can undergo further oxidation in the presence of
water:
 At the other metal surface , under neutral pH conditions, a
reduction reaction occurs:

8.  Iron hydroxides are then formed :
 The hydroxides may subsequently undergo various
dehydration reactions to give a mixture of hydroxides and
which collectively make up rust
that is a familiar feature of surface of plain steel that has been
exposed to the elements.

9.  Requirement for galvanic corrosion to occur are
1. Both water and oxygen is essential
2. The water must be capable of conducting electricity, ie.,
presence of electrolyte is essential.
 The list of metals in the order of their standard electrode
potentials in a given electrolyte solution- galvanic series.
(for identifying more anodic metal in a metal pair).
 Comparing galvanic series explains why joining plain steel
sections with stainless steel bolts is not a good idea and
galvanizing steel with zinc.
 But galvanic corrosion in steel occurs even without
presence of two metals.
 Steel alloy forms crystals of different phases (ferrite and
cementite phase), these phases posses different electrode
potential, a vast number of microscopic electrochemical
cells are set up.
 Ferrite – anode and cementite – cathode.

10. PITTING
 Corrosion due to differences in oxygen concentration.
 Differences in oxygen concentration drive a common
corrosion process in steel reinforcement – pitting.
 Part of steel exposed to lower concentration of oxygen –
more anodic but water is accessible.
 This leads to formation of pits .
 The variation in oxygen concentration at the bottom of the pit
compared with elsewhere on the steel surface causes the pit to
grows.
 Similarly for varying concentration of electrolyte corrosion
occurs.
 The rate of corrosion depends of surface area of anode and
cathode.
 Small anodic surface compared to larger cathodic surface area
leads to higher rate of corrosion.

12. PASSIVATION
 Presence of concrete cover acts as a barrier to the movement
of oxygen and substances capable of promoting corrosion
towards the reinforcement and thus prolonging the life of
steel.
 The alkaline environment in concrete also provides protection
to steel .
 Under high pH , a highly impermeable layer of 1micrometre
thickness is formed at the steel surface. This protection is
known as passivation.
 The stability of this impermeable layer is dependent on the
pH of the pore solution. If it decreases below 11.5 then the
passive layer decomposes.
 Additionally these passive layer can also be destroyed by
presence of certain dissolved like chloride ions.

13. STEEL CORROSION IN REINFORCED
CONCRETE
 Both water (relative humidity in concrete pores) and oxygen (extent
oxygen can access steel surface) is essential for galvanic corrosion.
Other factors are temperature and electrical resistivity.
RELATIVE HUMIDITY
 An increase in internal relative humidity within the concrete pores
leads to increase in the rate of corrosion expressed in terms of
corrosion current density (Icorr) .
 Corrosion current density is the current in the steel reinforcement
during corrosion per unit of surface area.
EXTENT OF OXYGEN ACCESSIBLITY
 The extent of oxygen able to reach the reinforcement is dependent
on how easily oxygen can enter the concrete and how rapidly it can
subsequently diffuse towards the steel.
 In submerged concrete , oxygen cannot enter easily – low rate of
corrosion.
 However , in cases like continous exposure to air and alternate
drying and wetting cycles – higher rate of corrosion since oxygen
can enter.

14. RATE CORROSION DEPENDING ON RELATIVE
HUMIDITY AND W/C RATIO FOR VARIOUS
EXPOSURE CONDITIONS:

15. TEMPERATURE VS RATE OF CORROSION
 The rate of corrosion increases with the increase in the
temperature
ELECTRICAL RESISTIVITY
 The corrosion require transport of
through solution.
 For this steel must permit the movement of ions and the level
of moisture must be sufficiently high.
 The mobility of ions is expressed in terms of electrical
resistivity.

17. DETRIMENTAL INFLUENCE OF
CORROSION ON PERFORMANCE OF
STRUCTURE CONCRETE
 Reinforcement itself undergoes a loss in cross sectional area
which compromises its ability to carry tensile stresses.
 Formation of rust at the steel surface eventually leads to formation
of cracks in the concrete cover.
 Corrosion leads to loss of load bearing capacity beyond certain
level of loss of mass .
 The reason is initial corrosion enhances the bond between steel
and concrete.
 Initially there is a small increase in flexural strength followed by
decline.
 This increase is attributed to the frictional stress between
reinforcement and concrete as a result of rust formation.
 The decline is due to the loss of bond strength resulting from
removal of ribs form reinforcement.

18.  The corrosion product is less dense compared to the steel
metal , so there is a expansion in volume up to four times.
 This expansion leads to development of cracks originating
from the steel reinforcement and extending to the concrete
surface.
 Wider reinforcing bars closer concrete surface will produce
cracks earlier than narrower bars at greater depths.
 The corrosion leads to loss in load bearing capacity and the
resulting increased structural deflection.

19.  Development of crack has the effect of easing of oxygen and
substances that promote corrosion . Pores can be bypasses in
a more direct route.
 Long term deterioration in the load bearing capacity of the
structural members follows the following type of behavior.

20.  Initially the slow rate of loss in load carrying capacity. Till t1
t1 – time period between construction and the initiation of
reinforcement corrosion.
 Beyond this point, corrosion continues until the performance of
the element falls below the serviceability limit after a period t2.
 In case of reinforcement of corrosion , the point at which
serviceability limit is reached is defined in terms of development
of surface cracks for various exposure conditions.
 Service life of structure = t1 + t2.
 Development of cracks marks the accelerated rate of corrosion
that initiates the beginning of time period t3 (residual life stage).
 Beyond t3 , load carrying capacity falls beyond ultimate limit
state.
 Corrosion manifest cracking , spalling with the appearance of
rust staining .
 Cracks may run parallel to the direction of main reinforcement
indicating the formation of expansive corrosion product.

21. CHLORIDE INGRESS INTO
CONCRETE
 Chloride ion is one of the greatest threats to steel
reinforcement .
 May enter concrete from the external environment via various
mass transport process.
 These are introduced as a contaminant of the constituent
materials or as calcium chloride used as an accelerating
admixture.
 This chloride is no longer permissible in reinforced concrete
and prestressed concrete as a result of its corrosive nature.

22. CHLORIDES IN THE
ENVIRONMENT
 Reason for chloride ingress is the large number of
opportunities for chlorides to come into contact with
reinforced concrete.
 Two sources of soluble chloride – sea water and de-icing salts
on highways.
Seawater :
 sodium , magnesium and calcium chloride.
 concentration of chloride vary with the salinity.
 say 35g/l is present at a concentration of 19000 mg/l.
De-icing salts:
 Mostly sodium chloride. Other salts are magnesium and
calcium chloride.
 Exposure to hydrochloric acid also causes corrosion.

23. INGRESS MECHANISMS
 Chloride ingress can occur in concrete as a result of
i. Concentration gradient (diffusion).
ii. A pressure gradient causing the flow of chloride bearing
solutions through pores.
iii. Capillary action.

24. DIFFUSION
 In the absence of cracks , chloride diffusion through
concrete is very much dependent on the nature of
porosity.
 Low diffusion coefficient is achieved when volume
fraction of porosity is low.
 For low diffusion constrictivity is low and tortuosity is
high.
 Constrictivity – measure of the extent to which changes
in the width of pores along their length , hinder the
diffusion of chemical species.
 Tortuosity- measure of the extent to which a chemical
species must deviate from a direct route while diffusing.

25. FACTORS AFFECTING
DIFFUSION
 Porosity of concrete.
 Crack width and spacing.
 Concentration difference.
 Type of salt (calcium or sodium salts )- eletrical double layer
configuration.

26. CHLORIDE DIFFUSION Vs WATER
CEMENT RATIO
 The degree of cement hydration increases , the total volume
of porosity falls , reducing diffusion coefficient.
 Chloride ingress progresses, the volume of porosity declines
in the outer layer of the concrete as a formation of friedal’s
salt within the pores.
 For maximum pore size , constrictivity increases leading to
higher diffusion coefficients.
 Increasing proportion of macro pores in cement matrix leads
to increase in the chloride diffusion coefficient.

28. CHLORIDE DIFFUSION
COEFFICIENT Vs CRACKS
 Diffusion through cracks is same as the diffusion through
pores although the width of cracks are several magnitudes
greater than pore width.
 Cracks present unimpeded path for chlorides though concrete
cover.
 So cracked concrete higher diffusion coefficient compared to
undamaged material.
 Measurement of concentration profile under flexural loading
shows higher concentration in tension zone.
 Crack spacing factor ,
Where l – average distance between cracks along a straight line
on the concrete surface.
w- average crack width.

29.  The relationship in the graph can be described as
Where D – chloride diffusion cofficient in .
D0 – chloride diffusion coefficient in concrete without
cracks in .
D1 – diffusion coefficient in free solution in .
D/D0 – equivalent diffusivity- the proportion by which
the diffusion coefficient of cracked concrete
exceeds that of the uncracked concrete.

31.  In a very narrow cracks self healing or autogenous healing
occurs due to re-precipitation of crystals of calcium carbonate
and calcium hydroxide.
 The growth of precipitate is controlled by diffusion of
calcium ions.
 Increase in temperature leads to increase in self healing
because at increased temperature rate of transport is higher.

32. CONCENTRATION DIFFERENCE
 Effect of chloride
concentration on the rate of
chloride ingress is complex.
 The rate of diffusion
decreases as the external
chloride concentration
increases.
 The decrease results from
greater interaction between
ions at higher concentration
which hinders movement.
 Although rate of diffusion is
slow the total quantity of
chloride that has entered is
still much higher for the
higher external concentration.

33. CHLORIDE SALT TYPE
 The chloride ingress rate with calcium chloride produces
higher diffusion coefficients than sodium chloride.
 The reason is presence of electrical double layer at the pore
surface.
 Electrical double layer is formed as a result of electrostatic
charge developing at the surface of the hydration product and
the surface becomes ionised.
 The resulting negative surface attracts cations and create a
fluid layer rich in cations.
 Fluid away from the surface becomes rich in anions.
 Lower concentration of calcium ions are sufficient to ionise
the surface due to higher charges.
 By the principle of electroneutrality – the concentration of
anions in the pore fluid must balanced with the cation in the
fluid such that the net charge is zero.

34. This effect increases the rate of chloride ion diffusion, as these
ions follow cations diffusing in the double layer.

35. FLOW
 The rate of flow of chloride
bearing solutions into the
concrete under a given
pressure difference is
dependent on the permeability
of the material which is
strongly influenced by the
pore structure of the concrete.
 In short the same
microstructural characteristics
that influence diffusion also
influences rate of flow.
 Self healing of cracks occurs
where pressure gradient is
high.

36.  Environmental conditions that
will influence the rate of
ingress of chlorides are the
pressure difference between
the interior and the exterior of
the concrete, external
concentration of chlorides and
temperature.
 Viscosity of water decreases
with the increase in the
temperature reducing the
resistance to flow.
 Marine environment , due to
presence of magnesium
chloride a layer of brucite is
formed (magnesium
hydroxide) and calcium
carbonate leading to reduction
in permeability.

37. CAPILLARY ACTION
 When unsaturated pores at the surface of concrete come into
contact with water, the process of capillary action will draw
the liquid into the interior.
 Clearly water contains dissolved chlorides, this process will
also act as a further ingress mechanism.
 The rate of uptake of water by concrete as a result of capillary
action is dependent on the gradients of volume fraction .
 The hydraulic diffusivity (D in m/s2 ) is a measure of ability
of concrete to transmit water via capillary action .
Where θ – volume fraction saturation. (ratio of liquid volume to
bulk volume).
t - time in s.

38.  Capillary action plays its significant role where alternate drying and
wetting occurs.
 such situation include those in the tidal, splash and atmospheric zones
of coastal and offshore structures and in highway environment.
 In repetition of drying and wetting cycle, the chloride is deposited in
pores during drying followed by fresh supply of chlorides during
wetting. This leads to accumulation of chloride beneath the surface.
 Slightly modified concentration profile compared to diffusion.
 The rate of drying is itself influenced by the quality of the concrete.
 Concrete with low porosity will dry at a slower rate.
 In most cases combination of ingress mechanisms will be operating
simultaneously.

40. CHLORIDE BINDING
 As chloride ions move into concrete, chemical process act to
remove some of these ions from the solution, thus rendering them
unavailable for contributing towards the corrosion process. This
process is chloride binding.
 It involves two mechanisms:
1. Formation of friedal’s salt( ).
2. Immobilisation of ions that come into contact with the calcium
silicate hydrate gel (CSH). – by chemisorption within the spaces
between the disordered layers that make up the crystal structure
of the gel increasing the strength of binding.
 Chloride induced corrosion occurs even in absence of oxygen –
green rust.

41. ROLE OF CHLORIDE IN
CORROSION
 cl- ions reaching the surface of steel breakdown the passive
layer at the surface and allow corrosion to progress.
 Depassivation involves the formation of chloride complexes
with iron from the passive layer.
 The iron chloride complex is soluble in the pore solution .
 Thus material is removed from the passive layer at some
localised points on the steel surface.
 For this minimum concentration of chloride known as
threshold concentration is required.

42.  To determine the concentration of free chlorides – extraction
type technique.
 Powered sample brought in contact with volume of water and
concentration of the solution is determined.
 This technique overestimates the concentration.
 Because the cement hydration products containing chlorides
will be in equilibrium with the pore solution.
 But powered specimen may be brought in contact with
volume of water greater than that present as pore solution.
 Another important parameter that influence corrosion is
hydroxyl ion.
 Cl- and OH- ions play opposing role – one destroys passive
layer and other forms passive layer.
 However presence of chloride ion promotes pitting in steel
reinforcement.

43. PROTECTION FROM CHLORIDE-
INDUCED CORROSION
 Mix propotions and depth of cover.
 Corrosion inhibitors and other admixtures .
 Alternative reinforcement materials.
 Reinforcement coatings.
 Fibres.
 Surface coatings.

44. MIX PROPORTIONS AND
DEPTH OF COVER
 Strategies that limits the penetration of
chloride ions and the way to achieve
it.S
NO.
STRATEGY WAY TO ACHIEVE IT
1 Reducing the volume of capillary porosity Simple way is to reduce water
cement ratio.
2 Reducing the pore diameter •Achieved by combination of
cement fraction particle sizes that
produce refined porosity.
•This can done by using pozzolonic
materials like GGBS , SF with the
portland cement.
3 Increasing the tortuosity, surface area and
constrictivity of the porosity

45.  The use of other cement components also increases the chloride
binding capacity of the concrete.
 This can occur in 2 ways:
1. When cement component (GGBS, FA) contains higher level
of Al2o3 then higher quantity of friedel’s salt is formed.
2. Cementitious material with higher Sio2 content will normally
lead to formation of more CSH gel during hydration . This
may increase the proportion of immobilised chlorides formed.
 There are minimum cover specifications for various exposure
for adequate protection for specified working lives of the
intended structures.
 Higher cover depths are used for aggressive environment.

46. CORROSION INHIBITORS AND
OTHER ADMIXTURES
 Corrosion inhibiting admixtures are agents that increase the
chloride threshold level required to cause depassivation and
also slow down the rate of corrosion once depassivation
occurred.
 Eg: calcium nitrate ((ca(No2)2) , sodium nitrate , tin (II)
sulphate, sodium fluorophosphate, malonic acid etc…
 Inhibitors – anodic or cathodic inhibitors.
 Anodic inhibitors- oxidize Fe(oH)2 to Fe(oH)3 which is more
stable to react with cl- ions. Eg: calcium and sodium nitrate.
 Cathodic inhibitors- passive layer by cathodic mechanism eg:
malonic acid, disodium glycerophosphate.
 Corrosion inhibitors applied to the surface of concrete to
provide protection after setting and hardening have occurred.

47. ALTERNATIVE REINFORCEMENT MATERIALS
 One form of corrosion resistant steel is stainless steel.
 Three common classifications – martensitic, ferritic and austenitic.
 Martensitic – alloy of iron and chromium, which is quenched.
less resistant to corrosion than other types. So not suitable for
construction purpose.
 Ferrite – alloy of iron and chromium but not quenched. Again this
is less resistant to corrosion.
 Austenitic – chromium- nickel- Aluminium alloy . More resistant
to corrosion. However welding can weaken this steel by reversing
work hardening.but oxide layer is formed while welding.
 Other materials: Weathering steel – low alloy steel that display
enhanced resistance to corrosion and forms protective layer.

48.  Fibre reinforced polymer –high strength fibres impregnated
with polymer resin has high corrosion resistance but not
ductile.
 FRP not used if fire is design consideration because
additional cover may be required in that case.
 For prestressing tendons – glass fibre composites are not
suitable because it undergoes stress corossion.
 Aramid and carbon fibres are suitable for pretsressing
although maximum prestress level is limited to ablout 50%
for design life of 100 year.

49. REINFORCEMENT
COATINGS
 Takes two forms.
 First is , a layer of impermeable material that acts as a physical
barrier between steel and outer environment – using epoxy resin.
 If damage to these coating during bending then it still allows
corrosion.
 So used in precast concrete where coating provided to prefabricated
steel.
 Second form is sacrificial coatings.
 Galvanised coating is produced by coating the surface steel with a
thin layer of zinc.
 Zinc is more anodic so steel is uncorroded and zinc gets corroded .
 The corrosion product of zinc is non expansive.
 Under low chloride concentration zinc is passivated and forms
protective layer.

50. FIBRES
 Presence of steel, polymer of glass fibre in concrete
provide greater resistance to chloride ingress.
 Steel fibres with additional stiffness control cracking
and reduce the width of crack.

51. SURFACE COATINGS
 Surface coatings for concrete where protection against
chloride ingress is major priority .
 Most common forms – hydroprobic impregnants in cyclic
wetting and drying cases.
 Silane compounds ender the surface and near surface of pores
hydrophobic and restricts chloride ingress.
 In addition to that they allow the watervapour from concrete
interior to escape.
 Eg : isobutyl(trimethoxy) silane used in various parts of
bridges like retaining walls, deck beams, piers etc…

52. CARBONATION
 Carbonation is a reaction between cement hydration products
in concrete and atmospheric carbondioxide which leaves steel
vulnerable to corrosion.
 Rising level of co2 due to human activities like combustion of
fossil fuels is a major corncern.
 Atmospheric cabondioxide – 390 ppm by volume .
 But localised carbondioxide near industrial and agricultural
carbon sources - upto 700 ppm by volume.

53. CARBONATION REACTION
 Chemical reaction occurs in two stages:
1. Dissolution of carbondioxide in water.(carbonic acid is
formed)
2. Reaction of product of dissolution with hydration products
within the cement phase of concrete.(calcium carbonate is
formed).
 CSH gel is destroyed leaving silica gel beyond certain
level of carbonation.
 The process of carbonation leads to reduction in pH . When
pH reduces below 11.5 the passive layer around steel is
destroyed.
 However , the drop in pH develops s a front that progresses
into the concrete cover with time.

54.  Straightforward means of monitoring carbonation is spraying
freshly fractured concrete surface with solution of
thymolphthalein or phenolphthalein.
 In these pH indicators colour change occurs around pH where
depassivation occurs.
 Carbonation decreases pH and increase in chloride ion
concentration in pore fluid is also observed.
pH indicator pH range for
colour change
Colour change
Thymolphthalein 9.3 -10.5 Colourless to blue
phenolphthalein 8.2-10 Colourless to
pink

55. FACTORS INFLUENCING RATE
OF CARBONATION
 Three environmental factors are concentration, relative
humidity and temperature.
 Mass transport of co2 into concrete occurs by diffusion.
 When the co2 molecule in concrete pore comes in contact
with a water molecule then it forms carbonic acid and
consequently calcium carbonate.
 The rate of reaction is given by
 So the rate at which carbonation front moves in concrete
depends on rate of diffusion of co3
2- molecule and the rate of
reaction.

56.  Concentration of co2 in air influences carbonation in two ways.
 First , greater diffusion rate if higher concentration gradient
between exterior and interior of concrete.
 Second, Higher concentration will lead to higher rate of reaction.
 Relative humidity of air in contact with cement will determine the
amount of moisture in the concrete pores.
 Carbonation reaction cannot occur without the presence of water.
 Water filled voids limits air filled volume of pores as a result
diffusion and carbonation is limited.
 The rate of reaction is dependent on temperature.
 This is given by arrhenius equation , k = Ae-Ea/RT .
 Temperature increases then the rate of reaction also increases.
 However solublity decreases with increase in temperature. Thus
carbonation rate again decreases beyond 60c.
 The other factor is nature of porosity , tortuosity and
constrictivity also influence the rate of carbonation.

57. CHANGES IN PHYSICAL
PROPERTIES
 Carbonation reduces the total porosity due to precipitation o
caco3 crystals in the pores.
 Thus resistance to further co2 ingress is increases
 The carbonation reaction yields water which limit mobility of co2
in pores until pore moist is equilibrated with the external relative
humidity.
 The formation of caco3 crystals lead to slight increase in strength.
 Cracking (in form of crazing) occurs as a result of stresses caused
by differences in volume of carbonated and uncarbonated parts of
concrete.
 However, Diffusion coefficient of co2 through carbonated layer of
a cracked concrete is higher.
 Reduced w/c ratio not omly limits carbonation rate but also the
cracking of concrete.

58. AVOIDING CARBONATION
 Carbonation is of lesser threat compared to chloride.
 Ficks second law, theoretical depth of carbonation
 The diffusion coefficient can be reduced and the level of hydration
products available for carbonation can be achieved by reducing w/c
and increasing the cement content .
 The influence of concrete cover depth in carbonation can obtained
by rearranging ficks equation and obtaining the time taken to reach
the depth can be seen.
 Greater cover depths.
 Other protective measures – same as that for chloride ( corrosion
inhibitors, fibres , surface coatings etc…)
D – diffusion coefficient.
C1-external concentration.
C0 – amount of co2 required to
complete carbonation of entire
volume of concrete considered.

59. THANK YOU