Problems associated with the measurement of chloride diffusion in - PowerPoint PPT Presentation

For citation information please see http://www.claisse.info/Publish.htm Problems associated with the measurement of chloride diffusion in concrete Peter Claisse and Juan Lizarazo Marriaga, Coventry University, Priory Street, Coventry CV1 5FB, UK Presentation contents 1. Electromigration tests 2. “Traditional” diffusion tests

ASTM C1202 – Names for the Test • Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration (in the ASTM). • The Rapid Chloride Permeability Test (after Whiting – who invented the test) • The Coulomb Test (it measures Coulombs)

ASTM C1202: Rapid Chloride Penetration Test (RCPT) Coating Reservoir 0.3N NaOH Concrete sample Reservoir 3% NaCl Mesh electrodes Solid acrylic cell 60 V Charge Passed Chloride Ion (coulombs) Penetrability >4,000 High 2,000 - 4,000 Moderate 1,000 – 2,000 Low 100 – 1,000 Very low <100 Negligible

The Problem • At the start of the test there is no chloride in the sample so the current depends on other charge carriers (primarily OH-) • Adding pozzolans to concrete depletes the OH- • Thus pozzolanic mixes can give misleading results

The new test Reference electrode SCE Capillary pipe / salt bridge Reservoir - NaOH Concrete sample KCl solution Mesh electrodes Reservoir - NaCl Solid acrylic cell Coating Potential difference cathode and sample mid point D.C. power supply

Using the mid-point voltage to identify cement replacements

Electro-diffusion model for chlorides in concrete ∂ ∂ c z F E = + i i J D D c • Nernst-Planck equation: ∂ ∂ i i i i x RT x Diffusion Migration ∑ = • Charge electroneutrality (Kirchoff’s law): 0 F z i J i i External External solution solution Concrete 2 n-1 n 0 1 n+1 ∆ t ∆ X n

Solving the hard way – assuming E is constant 2 2 α α β − − ( ) α β 2 1 2 2 β 16 + − I = FADc a[ e erfc( )] o β β π 2 4 where a = zFE RT α = ax β = 2a Dt

Section through sample during test Chloride zone Sodium zone Low resistance (high D) High resistance (low D) ? Electrostatic field E is ? gradient Voltage

Membrane Potential External voltage OH - Na + 2OH - Na + K + OH - Na + Ca + OH - Cl- Membrane potential Voltage External voltage Distance

Modelling a thin slice of the sample for a short time step Apply Kirchoff’s law : current in = current out Diffusion in and out – fixed by concentration gradient Electromigration into element - Electromigration out of element – set by field E which was we can set this for charge neutrality calculated for the last element by adjusting the field E Final adjustments are needed to get the correct total voltage across the sample.

Key innovation in the computer code INPUTS Set linear voltage drop for all space steps Calculate diffusion flux for each ion in all space steps Calculate electro-migration flux for each ion in all space steps Correct the Increase voltage in all Yes time space steps to Is there total charge surplus prevent charge in any space step? build up MEMBRANE No Reach time limit? POTENTIAL

Current in amps at different times in hours vs position in mm from the negative side Time = 0 Time = 7 3.00E-02 1.40E-02 2.50E-02 1.20E-02 2.00E-02 potassium 1.00E-02 potassium sodium 8.00E-03 1.50E-02 sodium chloride chloride 6.00E-03 1.00E-02 hydroxyl hydroxyl 4.00E-03 5.00E-03 2.00E-03 0.00E+00 0.00E+00 2.5 7.5 12.5 17.5 22.5 27.5 32.5 37.5 42.5 47.5 2.5 7.5 12.5 17.5 22.5 27.5 32.5 37.5 42.5 47.5 Time = 14 1.40E-02 1.20E-02 1.00E-02 potassium 8.00E-03 sodium 6.00E-03 chloride hydroxyl 4.00E-03 2.00E-03 0.00E+00 2.5 7.5 12.5 17.5 22.5 27.5 32.5 37.5 42.5 47.5

3.000E+01 Total Current mAmps 2.500E+01 Model output for 2.000E+01 1.500E+01 current and voltage 1.000E+01 5.000E+00 0.000E+00 0 5 10 15 20 Time hours Voltage adjustments at different times 45 40 Current vs time with no voltage 35 30 correction (average) Voltage 25 0.000 4.000E+02 20 6.802 Total Current mAmps 3.500E+02 17.013 15 3.000E+02 2.500E+02 10 2.000E+02 5 1.500E+02 1.000E+02 0 5.000E+01 0.0 10.0 20.0 30.0 40.0 50.0 60.0 -5 0.000E+00 Distance from negative side mm 0 5 10 15 20 Time hours

Optimization Model Data base Electro- Transport properties diffusion model: • Intrinsic diffusion coefficient (Cl - ) Experiments Voltage • Intrinsic diffusion coefficient (OH - ) control • Intrinsic diffusion coefficient (Na + ) - Current • Intrinsic diffusion coefficient (K + ) - Membrane potential • Porosity ( ε ) • Chloride binding capacity factor ( α ) Artificial • OH - conc. of the pore solution Neural Network Network training

Experimental programme % OPC PFA GGBS Mix w/b % % % OPC 0.49 100 0 0 30%PFA 0.49 70 30 0 50%GGBS 0.49 50 0 50 Inputs of the neural network

Chloride related properties from voltage control model You can’t get this lot with the new 5 minute test!

“Traditional” diffusion test For modelling: • The boundary condition is not zero voltage because the ends of the sample are not short-circuited. A voltage can be • measured. • The voltage in the model is set to give zero current .

Traditional diffusion test (no applied voltage) 1400 1200 Cl Concentration [mol/m3] 1000 800 600 400 200 0 0 40 80 120 160 Distance [mm] (1) Current control model - zero current (properties calculated) (2) Model with non-zero current, no voltage correction (properties calculated) (3) Model with no binding, no voltage correction and just diffusion of Cl (Dint-cl calculated) (4) Equation 7 (Dint-cl calculated) (5) Equation 7 (Dint-Fick) Equation (7) is the integral of Fick’s law. Dint = Intrinsic diffusion coefficient (3) and (4) coincide – showing that the computer model gives the same results as integrating Fick’s law if the ion-ion interactions are switched off. (5) Is based on experimental data

Future work • Controlled power tests to avoid overheating. • Voltage steps to avoid the need for a salt bridge.

Conclusions • The electrical model can be used with an artificial neural network (ANN) to give good values for transport properties. • Even when no voltage is applied, an electrical model is needed to simulate a diffusion test because of ion-ion interactions.

Thank you www.claisse.info References: J Lizarazo Marriaga and P Claisse Effect of non-linear membrane potential on the migration of ionic species in concrete Elecrochemica Acta Volume 54, Issue 10, 1 April 2009, Pages 2761-2769 2008. Juan Lizarazo-Marriaga, Peter Claisse Determination of the concrete chloride diffusion coefficient based on an electrochemical test and an optimization model Materials Chemistry and Physics. VOL 117; NUMBER 2-3 (2009) pp. 536-543 (15 October 2009) J Lizarazo and P Claisse Determination of the transport properties of a blended concrete from its electrical properties measured during a migration test Submitted to Magazine of Concrete Research. September 08. Coventry University and The University of Wisconsin Milwaukee Centre for By-products Utilization Second International Conference on Sustainable Construction Materials and Technologies June 28 - June 30, 2010, Università Politecnica delle Marche, Ancona, Italy. http://www4.uwm.edu/cbu/ancona.html