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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
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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-
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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.
ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012
LNEC
• ICDS1
2
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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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
ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012
LNEC
• ICDS1
2
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
7
≈ 100 nm= 10-5 cm
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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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
ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012
LNEC
• ICDS1
2
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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The non-steady state period is longer (twice) under AC migration.
The steady state period higher slope under AC.
RESULTS
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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The high frequency impedance spectra evolve with testing time.
ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012
LNEC
• ICDS1
2
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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.
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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
ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012
LNEC
• ICDS1
2
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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))
ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012
LNEC
• ICDS1
2
DURABLE STRUCTURESLNEC Lisbon 31 May - 1June 2012
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
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
ICDS12 International Conference DURABLE STRUCTURES: from construction to rehabilitation LNEC • Lisbon • Portugal • 31 May - 1 June 2012
LNEC
• ICDS1
2