Download pdf - A4_2_XRNovoa

DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012

CHLORIDE TRANSPORT THROUGH CEMENTITIOUS MEMBRANES USING

PULSED CURRENT

B. Díaz a, X.R. Nóvoa a, B. Puga a, V. Vivier b

a Grupo ENCOMAT, Universidade de Vigo, ETSEI, Campus Univ., Spain,b LISE – UPR 15 du CNRS,-4 place Jussieu, 75252 Paris Cedex 05, France


MOTIVATION

Most of the corrosion related problems inmarine environments are due to chlorides.

The diffusion coefficient of chlorides is akey parameter in the estimation ofservice life.

Electrochemical chloride extractionand realkalinisation are passivity-restoration techniques under development

Cathodic protection and cathodicprevention are widely used techniquesin corrosion control in structures exposedto the marine environment.

2

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

LNEC

• ICDS1

2


BACKGROUND

3

Chlorides in concrete can be:Chemically bonded (Friedel’s salt)

Adsorbed (at the C-S-H gel layers)Free (important in rebar’s corrosion)

Two main hydrated phases are present in hydrated cement paste:calcium hydroxysilicate, (C-S-H) 3CaO·SiO2·nH2Oalumina-ferric oxide monosulphate, (AFm) 3CaO·(Al,Fe)2O3·CaSO4·nH2O

Friedel’s salt results from the transformation of the AFm phase:

2 Cl- + 3CaO·(Al,Fe)2O3·CaSO4 .10H2O 3CaO·(Al,Fe)2O3·CaCl2 .10H2O + SO42-


BACKGROUND

4

Cementitious materials are of porous nature. Hardened OPC paste has about 15% porosity.

Most of the porosity is concentrated in the nanometer size range.

The size of gel pores and nano-pores approaches that of the double layer (Debye length), 2 nm. (S. Chatterji, M. Kawamura, Cem. Conc. Res. 22(1992)774)

Double layer properties will influence the electrokinetic phenomena induced by migration experiments.


LNEC

• ICDS1

2


5

Porous cementitious materialscan be modelled as a compactcementitious matrix in parallelwith the porous structure.

The physical image: anelectrical equivalent circuit withresistive and capacitiveparallel paths.

M. Cabeza, M. Keddam, X. R. Nóvoa, I. Sánchez, H. Takenouti, Electrochim. Acta 51 (2006) 1831–1841

R1

BulkBulk

Occludedpores

Percolatingpores

C1

R2C2

The percolating porosity isclosely linked to gel pores, thusin the few nm range: ionicmigration will be affected bypore wall charge (double layereffects).

BACKGROUND


6

The high frequency domain (MHz) corresponds to the contribution of the bulk. The low frequency (kHz) to the pore structure.

NaOH pH 12

NaOH pH 12

1M NaCl

NaO

H p

H 1

2, 1

M N

aCl

V

R1, percolating porosity

BACKGROUND B. Díaz, X.R. Nóvoa, M.C. Pérez, Cem. Conc. Composites 28 (2006) 237–245


LNEC

• ICDS1

2


7

≈ 100 nm= 10-5 cm


8

Cement paste: CEM I 52.5R, w/c = 0.5

[OH-]= 0.5M (NaOH+KOH)

ANOLYTE

[OH-]= 0.5M (NaOH+KOH)

NaCl 1M

CATHOLYTE

AC/DC

–

Cl-

-0.6 mA.cm-2

-0.3 mA.cm-2

0

t

DC

AC: 0.1, 1, 10 kHz

50% duty cycle

2.5 cm

Z: 40 MHz 10Hz

Periodic impedance measurements: 40 MHz to 10 Hz.

Periodic speed of sound measurements. MIP at the end of the tests.

EXPERIMENTAL


LNEC

• ICDS1

2


9

The non-steady state period is longer (twice) under AC migration.

The steady state period higher slope under AC.

RESULTS


10

The high frequency impedance spectra evolve with testing time.


LNEC

• ICDS1

2


11

The resistivity of the sample varies along the experiment. Non-SS and SS are clearly differentiated. Synchronised with Cl- flux

The frequency of the AC signal affects the magnitude of the resistivity.


12

The geometry of the cell was chosen so that:

Accurate R1 values could be obtainedAlmost no current flow at the edges reference material for MIP.

B. Díaz, L. Freire, X.R. Nóvoa, B. Puga, V. VivierCem. Conc. Res. 40 (2010) 1465–1470


LNEC

• ICDS1

2


13

AC 10 kHzDC

Conduction enhanced through gel pores

Conduction through capillary pores (dissolution of portlandite)

Pore size / nm

Rel

ativ

e c

ontr

ibut

ion

to p

oros

ity /

%

Anode Reference

Cathode Bulk


14

Sound waves (0.5 MHz) emitted/collected in direction normal to the current flux. v0~ 1 km.s-1.

The speed of sound increases more under DC condition (higher water-saturation degree higher porosity (degradation))


LNEC

• ICDS1

2


15

The pulsed current can enhance the transportthrough the gel pores.

The frequency of pulses shall match the of pores.

The frequency of pulses modifies the percolatingresistivity, which indicates that different levels of thegel pores network are reached.

Impedance Spectroscopy allows to follow thestructural changes associated to the transport/bonding of chlorides

CONCLUSIONS


CHLORIDE TRANSPORT THROUGH CEMENTITIOUS MEMBRANES USING

PULSED CURRENT

B. Díaz a, X.R. Nóvoa a, B. Puga a, V. Vivier b

a Grupo ENCOMAT, Universidade de Vigo, ETSEI, Campus Univ., Spain,b LISE – UPR 15 du CNRS,-4 place Jussieu, 75252 Paris Cedex 05, France


LNEC

• ICDS1

2