corrosion of steel in reinforced concrete - comsiru · session 1. introduction – corrosion of...

Session 1

Corrosion of steel in reinforced concrete

Influence of cover cracking and concrete quality

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

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 ?

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

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

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


Corrosion Rate = corrosion current/ unit area of steel

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]


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.

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.

Session 1


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

Session 1


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!

http://www.comsiru.uct.ac.za/downloads

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

Session 1

Corrosion

initiation

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

Session 1

Carbonation-induced corrosion

Session 1


Common in:

car parks

industrial areas

many building facades

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



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

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 (%)

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

Session 1


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

Session 1

Chloride-induced corrosion

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


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)

Session 1


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

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

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

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

Session 1


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

Session 1


PC (CEM I) vs. blended cement concretes

Influence of concrete quality cont’d.

(Mackechnie, 1997)

Increasing concrete penetrability

Session 1


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

Session 1


…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!

Session 1


Significantly increases the penetrability of the concrete

Travel path for corrosion-inducing species is shortened

Influence of cover cracking

Session 1


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.


Session 1


If a certain combination of crack characteristics, concrete quality,

resistivity and corrosion-inducing agents exist, the initiation

phase can be effectively ‘eliminated’ – not desirable!


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…

Session 1


Chloride conductivity index test

Session 1


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

Session 1


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

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

Session 1

Questions

. . .discussion

Thank you

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