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Session 1
Corrosion of steel in reinforced concrete
Influence of cover cracking and concrete quality
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Session 1
Session 1 Corrosion initiation
Presenter: Mark Alexander Email: [email protected]
Session I
08:00 to
10:00
(a) Corrosion of steel in concrete - background
(b) Causes (carbonation and chloride ingress)
(c) Corrosion initiation
(carbonation-induced and chloride-induced)
(d) Factors affecting corrosion initiation
(focus on influence of cracking)
(e) Prediction of time to corrosion initiation
(f) Conclusions and discussions
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Session 1
Introduction – Corrosion of steel in concrete
Corrosion of steel is today the greatest cause of durability
failure in RC structures.
Number of corrosion-affected RC structures continues to
increase.
As a result: Most structures are failing to meet their design service
life - even with maintenance and repair.
Substantial resources are spent to maintain and repair the structures.
The risks associated with corrosion-induced failures are high.
What are we not doing or not getting right ?
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Session 1
What is corrosion?
…destructive attack of a metal by chemical or electrochemical
reaction with its environment
Fundamental cause of corrosion - inherent instability of metals
in the metallic form. Metals tend to revert
to more stable forms, such as oxides,
hydroxides, etc.
Introduction – Corrosion of steel in concrete
OHMz+
Electrolyte solutionAnode Cathode
Metal A Metal Bze− O2
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Session 1
Introduction – Steel corrosion in concrete
Requirements for corrosion:
Anode: site where corrosion occurs and from which current
(electrons) flows. The metal is oxidized at the anode to form
soluble ions
Cathode: site where no corrosion occurs and to which current
flows. O2 is reduced here to produce OHˉ ions
Metallic path: connection between the anode and cathode
(completes the circuit). Steel serves this purpose in RC
Electrolyte: a medium capable of conducting electric current by
ionic current flow. In the case of concrete, the alkaline pore
solution constitutes the electrolyte
→ Steel
→ Steel
→ Steel
→ Concrete (salts and moisture)
CATHODE ANODE Electron flow, eˉ Corrosion
OHˉ flow
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Session 1
Rebar ANODE Electron flow, eˉ
Concrete surface
Corrosion
O2, H2O
Ingress of chloride ions and/or carbon dioxide
Hydroxyl (OHˉ) flow
Cover
CATHODE
Electrolyte
Anode: 𝟐𝑭𝒆𝟐 → 𝟐𝑭𝒆𝟐+ + 𝟒𝒆−
Cathode: 𝟐𝑯𝟐𝑶 + 𝑶𝟐 + 𝟒𝒆− → 𝟒𝑶𝑯−
Anode Cathode e-
OH-
Rust
Schematic illustration
Introduction – Steel corrosion in concrete
Corrosion Rate = corrosion current/ unit area of steel
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Session 1
Steel embedded in concrete is naturally protected by:
the concrete cover - limits the ingress of O2, H2O, CO2 and
Clˉ [cover depth and quality important]
the high alkalinity of the pore solution (pH > 12.5) – enables
the formation of a very thin (~1-10 nm) passive protective layer
on the steel surface which supresses
corrosion
[𝒎𝒂𝒈𝒉𝒆𝒎𝒊𝒕𝒆: ϒ − 𝑭𝒆𝟐𝑶𝟑 or 𝑭𝒆𝟑𝑶𝟒]
Introduction – Steel corrosion in concrete
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Session 1
Causes of steel corrosion in concrete:
Ingress of carbon dioxide – carbonation-induced
Ingress of chlorides – chloride-induced
Presence of dissimilar metals – galvanic corrosion
Presence of stray currents e.g. in structures close to electric train
lines
Causes of steel corrosion in concrete
Main areas of concern in R.C.
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Session 1
Transport mechanisms of corrosion-inducing species
Ingress of corrosion-inducing substances
largely the cause of corrosion in R.C. Thus, we need to
understand Transport Mechanisms
primarily influenced by the penetrability of the concrete.
Penetrability: the degree to which a material permits the transport
through it of gases, liquids, or ionic species. It encompasses all
transport mechanisms.
The porous nature of concrete facilitates
ingress of deleterious substances.
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Session 1
Transport mechanisms of corrosion-inducing species
Why is it important to understand transport mechanisms of
corrosion-inducing species in concrete?
govern the rate of ingress of the species into concrete.
explain and quantify the deleterious effects of the substances.
fundamental to developing service life prediction models of RC
structures i.e. durability prediction.
develop mitigation strategies
assessment of residual service life
assists in material selection (quality control testing)
transfer of lab experiments on accelerated corrosion tests
to the behaviour of structures under field exposure
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Session 1
Transport mechanisms of corrosion-inducing species
Permeation
Sorption
Convection
Diffusion
Migration
The above is mainly based on uncracked concrete.
In cracked concrete, more complex transport mechanisms are
involved, and are still not clearly understood!
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Session 1
(Unacceptable) Performance limit
Initiation period Propagation period
Leve
l of d
amag
e
- Ingress of contaminants: CO2, Cl- - No corrosion signs
· Corrosion with minor cracking
· Macro-cracking and concrete cover-cracking · Spalling · Loss of steel cross-section
Stages in the life of a corrosion-affected RC structure
Propagation Acceleration
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Session 1
Corrosion
initiation
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Session 1
What is corrosion initiation?
…the process by which aggressive substances (CO2 and Clˉ)
destroy the passive protective film on the steel surface, thus
rendering it liable to corrode
The aggressive substances destroy the protective film in
different ways → will be covered later.
Other requirements:
oxygen
Moisture
Introduction – corrosion initiation
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Session 1
Carbonation-induced corrosion
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Session 1
Carbonation-induced corrosion
Common in:
car parks
industrial areas
many building facades
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Session 1
Caused by ingress of carbon dioxide
𝑯𝟐𝑶 + 𝑪𝑶𝟐 → 𝑯𝟐𝑪𝑶𝟑
𝑯𝟐𝑪𝑶𝟑 + 𝑪𝒂 𝑶𝑯 𝟐 → 𝑪𝒂𝑪𝑶𝟑 + 𝟐𝑯𝟐𝑶
The process leads to reduction in the pH of the pore solution
(from 12.5 to ~9.0).
Characterized by generalized / microcell / homogeneous corrosion
Carbonation-induced corrosion
Carbonation-induced corrosion
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Session 1
Carbonation-induced corrosion Favourable conditions for increased carbonation rates include:
temperatures ~ 20 °C
relative humidity ~ 50-70%
increased CO2 concentrations (exceeded 400 ppm this year!)
w/b ratios at or above 0.6
use of fly ash or slag
Carbonation does not occur if the concrete is water-saturated
or in very dry conditions because:
moisture is required to form carbonic acid which attacks the
Ca(OH)2 and
Diffusion of CO2 through water is very slow
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Session 1
Carbonation-induced corrosion Favourable conditions (RH) for corrosion are different from
those for progress of carbonation: relative humidity > ~ 80%
Low to moderate resistivity
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Carbonation Corrosion
Susc
epti
bili
ty t
oth
e ac
tio
n
Relative humidity (%)
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Session 1
Carbonation-induced corrosion Carbonation progresses into the concrete as a ‘front’.
When the pH at the steel level drops to ~9, corrosion initiates.
pH 8.3
pH 12.5Carbonationfront
Concretesurface
Carbonated concrete
Non-carbonated concrete
Clear colour
Pink/purple colour
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Session 1
Carbonation-induced corrosion
Carbonation is very slow and generally follows the square root
time law:
𝒙 = 𝒌𝑪𝑶𝟐𝒕
x = carbonation depth after time t
𝒌𝑪𝑶𝟐= carbonation coefficient/factor for a given concrete
The model has been modified over the years to account for factors such as relative humidity and reduction in concrete porosity during carbonation process.
𝐞. 𝐠. 𝐂𝐄𝐁 𝐓𝐆 𝐕, 𝟏𝟗𝟗𝟔 𝐌𝐨𝐝𝐞𝐥
𝒙 =𝟐𝒌𝟏𝒌𝟐𝑫𝒆𝒇𝒇𝑪𝒔
𝒂𝒕
𝒕𝒐
𝒕
𝒏
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Session 1
Chloride-induced corrosion
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Session 1
Initiated by ingress of chlorides
Relatively fast compared to carbonation – much more
pernicious!
Corrosion initiates when the chloride concentration
reaches a critical value called chloride threshold.
Chloride threshold is not a single value for all concretes.
Characterized by macrocell / pitting / localized corrosion
Chloride-induced corrosion
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Session 1
Chloride-induced corrosion Sources of chlorides:
Sea-water (marine environment) - tidal, splash,
spray, submerged
De-icing salts
Thin passive iron oxide film
Solid corrosion product
2eˉ Anode
Cathode Cathode
O2 + 2H2O + 4e− → 4OH−
O2 H2O Cl−
Cover concrete
Steel reinforcement
Concrete surface
O2 + 2H2O + 4e− → 4OH−
↔ ↔
(Not covered here)
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Session 1
Chloride-induced corrosion
0.0
0.5
1.0
1.5
2.0
0 10 20 30 40 50 60
Chl
orid
e co
ncen
trat
ion
(w
eigh
t % c
emen
t)
Depth x (mm)
Chloride ingress is modelled using Crank’s solution to Fick’s
2nd law of diffusion:
𝑪𝒙,𝒕 = 𝑪𝒔 𝟏 − 𝒆𝒓𝒇𝒙
𝟐 𝑫𝒂𝒕
Depth to steel in concrete
Compare with chloride threshold e.g. 0.4%
Clˉ content Cx,t at steel level
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Session 1
Chloride-induced corrosion – marine exposure classes
EN 206-1 classification: based on the severity of chloride-
induced corrosion i.e. in terms of the corrosion-induced
damage over a given period of time.
Tidal
Splash
Spray
Submerged
Or combination thereof
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Session 1
Chloride-induced corrosion – marine exposure classes EN 206-1 Marine exposure Classes (“XS” classes):
EN206-1 Class Description
XS1 Exposed to airborne salt but not in direct contact with seawater
XS2a Permanently submerged
XS2b* XS2a + exposed to abrasionXS3a Tidal, splash and spray zones
XS3b* XS3a + exposed to abrasion
* Sub-clauses added for South African coastal conditions
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Session 1
Factors affecting corrosion initiation
Concrete cover thickness
Concrete cover quality – i.e. resistance to penetration of
corrosion-inducing species (Clˉ, CO2, O2 and H2O)
Binder type, NOT CONTENT (PC vs. blended cement concretes)
w/b ratio
Curing
Presence of moisture (especially for carbonation-induced corr.)
Chloride threshold value (for chloride-induced corrosion)
Chloride binding capacity of the binder (chloride-induced)
Cover cracking
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Session 1
Factors affecting corrosion initiation
Affects resistance to penetration of deleterious species (Clˉ,
CO2, O2 and H2O)
time for carbonation front to reach steel, or chlorides to
reach a threshold at the steel level
Influenced (controlled) by:
w/b ratio (affects alkalinity, more important than binder content)
curing
binder type (PC vs. blended cement concretes)
“binder content” – of lesser importance; min. binder
content – less critical
Influence of concrete quality
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Session 1
Factors affecting corrosion initiation
PC (CEM I) vs. blended cement concretes
Influence of concrete quality cont’d.
(Mackechnie, 1997)
Increasing concrete penetrability
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Session 1
Factors affecting corrosion initiation
Moisture in concrete is required
for the transport of corrosion-
inducing species
Moisture content affects:
ingress of oxygen
progress of the carbonation front
resistivity of the concrete
(important mainly for corr.
propagation)
Influence of moisture content and relative humidity
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Session 1
Factors affecting corrosion initiation
…varies from concrete to concrete
Blended cement concretes have lower threshold values than PC
concrete but this is offset by their significantly lower penetrability.
Influence of chloride threshold value
Binder Type (w/b ratio, 0.58) Concentration (% by mass of binder)100% PC 0.5375/25 PC/GGBS 0.4150/50 PC/GGBS 0.0870/30 PC/FA 0.3650/43/7 PC/GGBS/SF 0.08
(Scott, 2004)
… a value of 0.4% is usually taken as a rough estimate!
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Session 1
Factors affecting corrosion initiation
Significantly increases the penetrability of the concrete
Travel path for corrosion-inducing species is shortened
Influence of cover cracking
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Session 1
Factors affecting corrosion initiation
Transport mechanism(s) of the corrosion-inducing species are
altered:
Transport properties of cracked concrete cannot be correlated with
those measured on the uncracked concrete → more complex
transport mechanisms are involved.
Ingress of aggressive agents into cracked concrete is still not clearly
understood → cannot be accurately estimated from uncracked
concrete. Further studies are still required.
Influence of cover cracking
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Session 1
Factors affecting corrosion initiation
If a certain combination of crack characteristics, concrete quality,
resistivity and corrosion-inducing agents exist, the initiation
phase can be effectively ‘eliminated’ – not desirable!
Influence of cover cracking
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Session 1
Prediction of corrosion initiation
This has already been covered but of importance are:
1. Understand the type of corrosion - is it carbonation-induced or
chloride-induced?
2. What is the transport mechanism of the corrosion-inducing species?
3. What models are applicable to test concrete for the given transport
mechanism?
4. What valid reference test methods are available to obtain the required
model input parameters?
Durability index (DI) tests
a) Oxygen permeability index test
b) Chloride conductivity index test
c) Water sorptivity index test
Each of the tests is linked to a transport mechanism relevant to a particular deterioration process.
…will soon be incorporated in SANS…
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Session 1
Prediction of corrosion initiation
Chloride conductivity index test
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Session 1
Prediction of corrosion initiation
Oxygen permeability index test
Concrete sample in rubber collar, in rigid sleeve
Clamping cover plate
Permeating gas outlet
Gas outlet valve
Pressure gauge
Gas inlet valve
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Session 1
Prediction of corrosion initiation
Water sorptivity index test
Capillary rise
Test surface of concrete specimen
Epoxy coating or packaging tape
Concrete disc specimen (68 ± 2 mm diameter, 30 ± 2 mm thick)
Wet paper towels in (Ca(OH)2 solution
Concrete discs (after pre-conditioning)
Wet paper towels in (Ca(OH)2 solution
Scale
Stopwatch
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Session 1
Summary - corrosion initiation
Main causes of steel corrosion in concrete: CO2 and Chlorides
Chloride-induced corrosion is more aggressive than
carbonation-induced corrosion
Service life of a corrosion-affected RC structure comprises of
“initiation” and “propagation” phases
It is more desirable to extend the initiation phase
Other factors kept constant, cover depth, quality and condition
(cracked / uncracked) can be used to control corrosion initiation
At the end of the initiation phase, the structure is still in “perfect”
structural condition but in a “compromised” durability state.
Points to remember
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Session 1
Questions
. . .discussion
Thank you