4 CAndrade [Modo de Compatibilidade]durati.lnec.pt/pdf/ICDS12_Presentations/K_4_CAndrade.pdf · CISDEM, CSIC-UPM C/ Serrano Galvache nº4, 28033, Madrid, Spain MOTIVATION • The

LIMIT STATE CALCULATION OF REINFORCEMENT CORROSION

C. Andrade, F. Tavares and D. Izquierdo.

Center for the Durability and Safety of Structures and Materials, CISDEM, CSIC-UPM C/ Serrano Galvache nº4, 28033, Madrid, Spain

MOTIVATION• The requirement of 100 years of service

life is being more commonly specified in the design of large infrastructures.

• However, this specification is not always accompanied with the mention to the type of models and their constitutive parameters and their related tests.

• In particular is not mentioned which is the chloride threshold and the probability of corrosion.

ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012

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LIST OF CONTENTS

• Initiation Period

– Modelling• Impact of surface concentration

• Aging factor of Dap

– Testing methods

• Corrosion Onset

– Chloride threshold: statistical distribution

– Probability of depassivation

• Limit State of corrosion

– Relation between difusivity and Corrosion rate

– Structural limit states: SLS and ULS

SERVICE LIFE STEPS

CORROSION THRESHOLD

tlife= ti + tp


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TRANSPORT MODELS BASED IN FICK’ s SECOND LAW

Non stationary conditions

jc jc + djcCtotdx

Initial conditionsBoundary conditions

Cs and D are constant

MODEL FOR CHLORIDE INGRESS

[Cl-]

x

LABORATORY

TEST


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TEST STATION IN A BEACHIN REAL STRUCTURES D and Cs ARE NOT CONSTANT

HIGH

TIDE

LOW

TIDE

THE MODELS FAIL BECAUSE THE BOUNDARY CONDITINS ARE NOT

RESPECTED• Cs is not constant

• D is not constant

OTHER LIMITATIONS

•The climate is not constant


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Skin effectextrapolation for Cs value

AGING FACTOR OF DapDecrease of the diffusion coefficient with time

q ≈ 0,8n


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RILEM BENCHMARKING

any MODEL can or not predict correctly

All models may fail

Natural diffusion

NaCl

NaCl

Cell:D1: Ds

D1-P/M: DsD4:Dns

Ponding:D3: Dns

Immersion:D2

Resistivity

R1, R2, M-R

migration

C1

Migrationexperiments are Instantaneouswhile the natural diffusion tests give the integration during the test time period.

The comparison of natural and accelerated tests may not agree and therefore the use of a value o another may mean a different prediction.

At European level the proposed method are based in natural exposure.

OTHER SOURCE OF UNCERTAINTY :TEST TYPES


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SPECIFICATIONS OF SERVICE LIFEparameters to be defined

• The external concentration of Cl

• The diffusion coefficient

• The aging factor and for how long to apply it

• The chloride threshold– The probability of corrosion

• The tolerances or variations

CHLORIDE THRESHOLDSTEPS DURING DEPASSIVATION

TIME

Cl-

CO2


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DEPENDENCE OF THE CHLORIDE THRESHOLD FROM THE CORROSION POTENTIAL

0

0,2

0,4

0,6

0,8

1

0 0,5 1 1,5 2 2,5

Co

rro

sio

n p

rob

abili

ty

Chloride thershold (%) cement weight

D. Izquierdo et al

Which is the chloride threshold ?

STATISTICAL CHARACTERIZATION OF THE CHLORIDE THRESHOLD

LAB(= 0.700.20)

SITE (= 0.770.24)


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CALCULATION OF PROBABILITIES OF DEPASSIVATIONfor a chloride concentration to reach a cover depth in a

certain time

Initiation Period Propagation period

Degree of corrosion

Time

0

0,2

0,4

0,6

0,8

1

0 0,5 1 1,5 2 2,5

Co

rro

sio

n p

rob

abili

ty

Chloride thershold (%) cement weight

D. Izquierdo et al

SEQUENCE OF THE PROCESS OF DEPASSIVATION

TIME

Cl-

Time to corrosion for c= 50mm, Cs= 2% and Dapp= 10E-8

t1 for Clth= 0.2% t2 for Clth= 0.2% t3 for Clth= 0.2% t4 for Clth= 0.2%

14.65 years 24.13 years 36.90 years 55.96 years


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Calculation of the accumulated corrosion in each time

A DCl-

Condition stateSTEPS DURING DEPASSIVATION

DUE TO CARBONATIONTIME

CO2

6789

1011121314

0 5 10 15 20 25

pH

cover depth

10 years 20 years 50 years


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WHICH IS THE ACCEPTABLE DAMAGE LEVEL?

The answer should come from the analysis of the consequences of the corrosion.

How to calculate it?through the loss in cross section from the

accumulated corrosion

HOW TO ESTIMATE THE FUTURE CORROSION RATE???

FROM THE CONCRETE RESISTIVITY

ρ

D Icorr


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Example of quivalence between diffusivity-corrosion rate through the

resistivity

Dapp

(cm2/s)

0.1E-8 1E-8 10E-8

Icorr

(m/year)

1 10 100

VERIFICATION OF SLS AND ULS FROM THE LOSS IN CROSS SECTION

initiation propagation

t5

t4

t3

t2t1

Levelofdamage

time

Despasivation

Corrosion

Cracking

SectionLoss

StructuralSafetyloss

initiation propagation

t5

t4

t3

t2t1

limit states

Levelofdamage

time

Despasivation

Corrosion

Cracking

SectionLoss

StructuralSafetyloss


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CONCLUSIONS1. Models for calculation of the service life need better

definition if incorporated in specifications2. It seems to exit a unique statistical distribution of

the chloride threshold in aerial structures with a mean in 0.70±0.20 % cement weight,

3. It enables the calculation of the probability of corrosion (by Montercarlo i.e) at the most external surface of the bar,

4. The accumulated corrosion Pcorr can be calculated from the resistivivity/diffusivity.

5. From the loss in cross section it is possible to calculate the deterioration level (crack appearance and deflections for instance).

6. The acceptable degree of corrosion should be adopted by verifying the SLS and the ULS at the structural level.

Thanks for your attention

[email protected] de Seguridad y Durabilidad de Estructuras y

Materiales

CISDEM (UPM-CSIC)

Madrid-Spain


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Documents

4 CAndrade [Modo de Compatibilidade]durati.lnec.pt/pdf/ICDS12_Presentations/K_4_CAndrade.pdf · CISDEM, CSIC-UPM C/ Serrano Galvache nº4, 28033, Madrid, Spain MOTIVATION • The