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DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012

MECHANISMS INVOLVED IN CHLORIDE TRANSPORT THROUGH CONCRETE SIMULATIONS OF THEIR EFFECTS ON

CHLORIDE PROFILES

Durable Structures: From Construction to RehabilitationICDS12 Lisbon: 31 May - 1 June 2012

O.Amiri & A. At-MokhtarUniversity of La Rochelle CNRS LaSIE FRE 3474, France

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OUTLINE

1/ Introduction : corrosion of steel in reinforced structures

2/ Limitations of the simplified models: Second Ficks law

3/ Multiphysics approach - an alternative?

4/ Difficulties of identification of the inputs of the model 5/ Simulation of chloride profile: influence of main parameters

6/ Conclusions

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

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The steel in concrete is protected by 3 ways

the concrete cover

the basicity of concrete (pH 13) the existence of a passive film on steel

What is the problem ??

Chlorides

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Corrosion of steel in the bridge pier P26 (R bridge, built on 1988 : La Rochelle (France)

Example of degradation

Economic Issues: OCDE countries

4% of PIB spent in the Framework of structures repair

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Solution in the case of infinite medium(where D considered constant)

Although many studies have shown the limitations of this law, it is often used to predict chloride ingress: Simplicity

Limitations of second Ficks Law

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Limitations of second Ficks Law

Interactions between the ions of the pore solution neglected

Pore solution considered as ideal solution

Convection (drying and wetting cycles) not considered

Electrical Double Layer (EDL) effect neglected

Physical limitations

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The diffusion is not the sole phenomenon involved in the penetration of chlorides:

Typical chloride profile obtained for field samples and chloride profile predicted by Ficks diffusion model (Marchand et al, 2009)

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Transfer in liquid phaseTransfer in gas phase Coupling With the consideration of physical

phenomena mentioned previously

Approach : Modeling according 3 steps :

Chloride transport in unsaturated concrete:Multiphysics Approach

Main Hypothesis:

Medium tri-phasic (solid, liquid, gas), the solid phase is considered compact (without deformations

Phase change only between the liquid and the gas phases

Transfer isotherm

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1 - Liquid phase (water + ions):

Water (l) :

Ions (i = Cl-, Na+, K+, OH-) :

Nernst Planck Equation

electrostaticInteractions

Diffusion Ionic Activity Convection

Outputs of the model

Inputs of the model

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Water Vapor (v):

Air (a) :

Clapeyron Equation for a saline solution

3 - Interface liquid - gas

2 Gas phase (air + vapor) :

22nlo-2olores ),(f (Millington, 1960)

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(electroneutrality condition)

Identification of the model inputsSummary of the transport equations :

krl ; krg ; frs ;pc = f(l) ; i ; al ; kl ; DeCl

CCl, b = f(CCl,f) ; DeCl, sat ; kg ; o

l ; pv ; pa ; CClf ; CNaf ; CKf ; COHf

krl ; krg ; frs ;pc = f(l) ; i ; al ; kl ; DeCl

CCl, b = f(CCl,f) ; DeCl, sat ; kg ; o

l ; pv ; pa ; CClf ; CNaf ; CKf ; COHf

The outputs

Experimental parameters

Bibliographical parameters

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Influence of the diffusion coefficient

The least variation of the diffusion coefficient generates important differences in Chloride concentration: The method of determination of D is important

In saturation, the main driving of chloride transport is the diffusion

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Determination of diffusion coefficient by migration test

- Chloride Dosage

- Electrical Current measurement


Chloride Flux measurement in downstream compartment

Chloride Flux measurement in upstream compartment

Chloride Flux measurement by electrical current

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(Andrade, 93; Amiri et al, 97)

(Truc, 2000)

(Friedmann et al, 2004)


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Method Dosage in downstream Dosage in upstream Electrical

Advantages taking into account the chlorides binding with cement matrix

- rapid- anodic reactions avoided

- high accuracy (no dosage required)-multi-species approach

Disadvantages - need of electrodes with good quality (expensive method)- mono-species approach- Renewal of solutions- Inaccuracy of chloride dosage due to the risk of release of Cl2 and formation of ClO

- need of several dilutions (inaccuracies)-mono-species approach-the chlorides binding with cement matrix neglected

-the migration test is carried out in 2 steps (time consuming)-renewal of solutions

Let us consider the Advantages and Disadvantages of the different Methods

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Comparison between simplified and physical models

Cement paste (w/c = 0.5) immersed in saline solution during 100 Days (Sergi et al, 2002)

Good agreement between experimental and multiphysics model in the case of humidification 16


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Good agreement between experiments and model in the case of drying-wetting cycles

comparison between the model and experimental results obtained on mortar submitted to 2 days humidification and 1 month drying (Francy, 98)

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Materials

Density (g/cm3) water/cement Porosity

Coefficientdiffusion

GasPermeability

W/C porosity Dsat (m2/s) K (m2)

C22 2.1 0.63 0.14 5.2*10-12 2*10-17

C30 2.2 0.5 0.1 1*10-12 9*10-18

Simulation of chloride profile in two concretes: C22; C30 :manufactured during MEDACHS project

Simulation of chloride profile :The tested materials

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Simulation of chloride profile:Influence of wetting and drying

Chloride transport is accelerated when drying period is longuer Comparatively to saturated case, the chloride ingress is accelerated of 20 years 19


Similar behavior than C22:The critical concentration is reached 5 years rather 20


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Simulation of chloride profile :Influence electrostatic interactions

The chloride transport is accelerated slightly: interaction potentialThe effect could increase if concrete contains slag, fly ash...? 21


Simulation of chloride profile :Influence of chloride binding

Whatever the binding considered, the kinetic chloride ingress is decreased

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Great influence of water permeability: driver of convection

Concrete C22 submitted to a wetting during 3 months

Simulation of chloride profile :Effect of water permeability

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Simulation of chloride profile :Influence of water adsorption isotherm

The chloride ingress depends on water isotherm:GAB and BET: whatever the saturation type Bradley: high saturation 24


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Simulation of chloride profile :Influence of the saturation of the material

The saturation of the material controls the chloride penetration: when the material is not saturated, convection occurs 25


Simulation of chloride profile :Influence of the concrete cover

The least variation of the concrete cover generates important differences in [Cl-]26


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1. Multiphysics approach is more realistic but requires several inputs

2. The chloride profile is influenced by the:

Chemical binding of cement matrix with chlorides Initial saturation degree of the concrete and water isotherm and

water permeability Thickness of the concrete cover Longer drying, shorter wetting Electrostatic interactions

Conclusions

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1. Actually modeling requires more experimental validation, for example in the cases of tide, splash and spray zones: Only very few studies were carried out until now

2. To reduce the empiricism of some inputs, the heterogeneity of the material should be taken into account

Required improvements

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Thank you

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