The effect of other ions on chloride migration in concrete.

Transcription

1 3 rd Internatonal Conference on the Durablty of Concrete Structures Paper Number XX September 2012 Queen s Unversty Belfast The effect of other ons on chlorde mgraton n concrete. ABSTRACT Peter Classe Coventry Unversty Juan Lzarazo Marraga Unversty of Columba, Bogota Measurng chlorde mgraton s essental for predctng the durablty of embedded steel n concrete. The Rapd Chlorde Permeablty Test to ASTM C1202 s ganng popularty as a method for ths but It has severe lmtatons, partcularly when pozzolanc admxtures are used. The NTBuld-492 Mgraton test overcomes some of these but stll does not take full account of the complex processes nvolved. Ths paper wll provde an explanaton of these problems whch s based on mult-speces modellng. An overvew of a major expermental programme to test and valdate the models wll be presented. The programme concluded that even n dffuson tests n whch there s no electrc feld appled or electrcal measurements made, there are stll sgnfcant effects of charge buld up from hydroxyl, sodum and potassum ons as well as the chlorde. 1. INTRODUCTION The purpose of ths paper s to dscuss the concept of the dffuson coeffcent for mgratng speces n concrete. The problems wth measurng ths parameter are llustrated by dscussng some of the measurement technques and the applcaton of the results when they are obtaned (Lzarazo- Marraga 2009). The three tests that wll be dscussed are the Rapd Chlorde Permeablty Test to ASTM C1202, the Nordtest NTBuld-492 test and a smple dffuson test wth no appled voltage. The applcatons that wll be consdered are the comparson of dfferent mxes and also the calculaton of the workng lfe of structures usng models such as lfe-365. Before consderng these stuatons the basc defntons are presented. 2. DEFINITIONS OF DIFFUSION COEFFICIENTS. 2.1 Dffuson n a homogeneous non-adsorbng system. Dffuson s drven by concentraton gradent and s defned n equaton 1 (Fck s law) (Crank 1976): F = D dc kg/m 2 /s (1) where: F s the flux n kg/m 2 /s D s the dffuson coeffcent n m 2 /s C s the concentraton n kg/m 3 1 x s the poston. 2.2 Adsorpton n a porous sold Adsorpton s consdered n ths paper because t mportant to show that the conclusons wll apply equally to an adsorbng or a non-adsorbng system. The concentraton of ons n a porous sold may be measured n two dfferent ways (Atknson and Nckerson 1984): C l kg/m 3 s the concentraton of ons per unt volume of lqud n the pores and C s kg/m 3 s the total concentraton (ncludng adsorbed ons) per unt volume of the sold. 2.3 Dffuson wth adsorpton. Because there are two dfferent ways of measurng concentraton n an adsorbng system there are also two dfferent ways of measurng dffuson: The apparent dffuson coeffcent D a s defned from: F = D a dc s kg/m 2 /s and the ntrnsc dffuson coeffcent (whch s the dffuson coeffcent for the pore soluton) s defned from: F ' = D dc l kg/m 2 /s where F ' s the flux per unt cross sectonal area of the lqud n the pores. Thus:

2 F = ε D dc l kg/m 2 /s where ε s the porosty 2.4 Electromgraton If an electrc feld s appled across the sold the negatve ons wll move towards the postve electrode. Classcal theory of electrochemstry defnes the flux due to electromgraton usng the same dffuson coeffcent as used n Fck s law n equaton (2): DzECF J = RT (2) ether ts sodum counter-on moves wth t or a dfferent negatve on such as a hydroxyl on s dsplaced ahead of t. If charge neutralty s not preserved electrc felds buld up to prevent the movement. Thus the ablty of the chlorde on to move through the regon the regon of the sample s determned by the concentraton and moblty of other ons n the regon. Referrng to the Nernst- Ensten equaton ths may be represented as a change n conductvty (or resstvty) of the regon. One effect of ths s shown n fgure 2. The dfferent resstvtes n dfferent regons of the sample make the voltage drop across t non-lnear. The dfference between the measured voltages and the lnear change s known as the membrane potental. where J s the flux n kg/m 2 /s z s the valency of the on F s the Faraday constant E s the electrc feld n volts/m R = gas constant T s the temperature n o K Rearrangng ths gves the Nernst-Ensten equaton: D = RTσ z 2 F 2 c Where σ s the electrcal conductvty 2.5 Combnng Dffuson and Electromgraton. Fgure 1. The ASTM C1202 test The general law governng the onc movements due to the chemcal and electrcal potental s obtaned by combnng equatons (1) and (2) and known as the Nernst-Planck equaton: J c zf = D + Dc x RT V x Where V = Voltage, thus δv/δx = Electrc feld (Crank 1976) 3. TESTS FOR DIFFUSION 3.1 Applyng the theory of dffuson to the ASTM C1202 Test The test method (ASTM 2005) The ASTM C1202 test s shown schematcally n fgure 1. In ths test sodum chlorde s added to one cell and sodum hydroxde to the other and a voltage s appled across the sample. In order to calculate what happens next t s necessary to consder the physcal stuaton n the sample. Charged ons can only move f charge neutralty s preserved. Thus, for example, a negatve chlorde on can only move to a regon of the sample f 2 Fgure 2. Schematc dagram of the effect of changes n resstvty Modellng the dffuson (Lzarao-Marraga and Classe 2009 a,b). The Nernst-Plank equaton can be ntegrated to gve changes of concentraton and current wth tme, but ths takes no account of the effect of other ons. In order to do ths t s necessary to use a numercal model. The model whch s dscussed

3 n ths paper uses repeated applcatons of equatons (1) and (2) throughout space and tme. However n order to apply equaton (2) t s necessary to know the electrc felds and, as shown n fgure 2. Ths changes and s extremely dffcult to calculate. The model therefore avods ths calculaton by applyng Krchoff s law whch states that the current must be the same at all ponts through the sample,.e. charge neutralty must be mantaned. In the computer code the net charge s calculated for dfferent values of electrc feld on an teratve bass untl the correct value s obtaned to acheve neutralty. Ths process s shown schematcally n fgure 3. Voltage Voltage adjustments at dfferent tmes Dstance from negatve sde mm Fgure 4. Md-pont voltages The man result that s obtaned from ths test s the total charge passng. Ths s taken as a measure of the dffuson coeffcent of the sample. Fgures 5 and 6 show modelled current for a gven sample (the area under the curve s the charge). Fgure 5 shows the result for the full model and fgure 6 shows the same result wthout the voltage control algorthm used to acheve charge neutralty. Fgure 3. The bass of the model. The model can be used to get dffuson coeffcents from expermental results from the test by usng Artfcal Neural Networks whch have been traned on repeated runs of the model wth dfferent coeffcents and capacty factors for the dfferent ons Results for the ASTM C1202 tests. Before consderng results, the model was valdated. Ths was acheved by measurng the md-pont voltage n test sample (as shown n fgure 1). Expermental results for ths are shown n fgure 4 and these were shown to agree wth the model. Total Current mamps Fgure 5. Current vs. tme wth voltage control Total Current mamps 3.000E E E E E E E Tme hours 4.000E E E E E E E E E Tme hours Fgure 6 Current vs tme wthout effect of other ons 3

4 It may be seen that the requrement for charge neutralty (.e. the effect of the other ons) s very sgnfcant. Tm e = E E E E E E E E potassum sodum chlorde hydroxyl Fgure 7. Showng the current carred by the dfferent ons n Amps vs. the dstance from the cathode sde n mm 14 hours after the start of a test. Fgure 9. Modelled results for the NTBuld-492 test usng the Nernst-Plank equaton Fgure 7 shows how the current durng the test s carred by a number of dfferent ons. The charge neutralty requrement makes the total current the same at each locaton. 3.2 The Nordtest NT Buld-492 Test (Nordtest 1999) Ths test s shown n Fgure 8 s smlar to the ASTM C1202 test n that a voltage s used to drve chlordes nto the sample. However the results are obtaned by cuttng up the sample and measurng the chlorde content at dfferent depths at the end of the test. The results are then analysed usng an ntegrated form of the Nernst-Plank equaton. No account s taken of the on-on nteractons. Fgure 10. Modelled results for the NTBuls-492 test wth on-on nteractons. Fgures 9 and 10 show the effect of on-on nteractons on ths test. They may be seen to be very sgnfcant 3.3 A Smple Dffuson Test Fgure 8 Arrangement of the mgraton test (NTBuld-492) Fgure 11. Smple dffuson test 4 Fgure 11 shows a smple dffuson test. Despte the fact that there s no appled voltage there are stll electrc felds present as soon as ons start to

5 dffuse. The modellng of the test s dfferent from the electrcal tests because the boundary condton s one of zero current rather than a fxed voltage. Indeed a voltage can be measured across the sample durng a test t works as a type of smple battery. Fgure 12 shows modellng and expermental results from a test Cl Concentraton [mol/m3] Dstance [mm] (1) Current control model - zero current (propertes calculated) (2) Model wth non-zero current, no voltage correcton (propertes calculated) (3) Model wth no bndng, no voltage correcton and just dffuson of Cl (Dnt-cl calculated) (4) Equaton 7 (Dnt-cl calculated) (5) Equaton 7 (Dnt-Fck) Fgure 12. Results of smple dffuson test. (1) s the full model wth zero current (2) s the model wth current permtted (.e. smulatng what would happen f the ends of the sample were electrcally connected) (3) and (4) show the computer model wth the onon nteractons swtched off and the result from ntegratng Fck s law. These gves the same results so the lnes concde valdatng the computer model. (5) Is based on expermental data From fgure 12 t may be seen that obtanng values for dffuson coeffcents from ths type of experment by smply ntegratng Fcks s law wll gve msleadng results. 4. APPLICATIONS FOR THE DIFFUSION COEFFICIENT 4.1 Comparng dfferent mxes (Lzarazo- Marraga and Classe 2010) It s common practce to use the ASTM C1202 test to compare dfferent mxes when publshng research on potental new cementtous components for concrete. 5 Fgure 13. The membrane potental (.e. the dfference between the voltage and the lnear voltage drop) for md-pont voltages n an ASTM C1202 test for OPC (CEM1), 30% PFA (Pulversed fuel ash) and 50% GGBS (Ground Granulated Blastfurnace Slag) From Fgure 13 t may be seen that changng the cementtous components n a mx has a sgnfcant effect on onc voltages. Ths s because they sgnfcantly change the onc concentratons (e.g. pozzolans deplete hydroxyl ons). Fgure 14. Showng current vs. tme data for the GGBS and PFA mxes shown n fgure 13. From fgure 14 t may be seen that the total charge passng s very dfferent for the dfferent mxes. Ths dfference s caused by on-on nteractons and does not ndcate a change n the dffuson coeffcent. 4.2 Modellng Concrete Durablty Software packages such as Lfe-365 (NRMCA 2011) us an ntegrated verson of Fck s law and take no account of on-on nteractons. These packages are used to predct the lfe of structures and nfluence desgn decsons such as the depth of cover to the renforcement of the water/cement raton of the mx. The package s set up so recommended values of the dffuson coeffcent are gven for dfferent

6 mxes however t would be possble for users to nput ther own values f expermental data was avalable. 5. DISCUSSION From the ntal defntons of the dffuson coeffcent t may be seen that there are two values, the apparent and the ntrnsc. To see the dfference, a smple dffuson test s consdered n whch chlorde mgrates nto a sample and the concentraton s then obtaned at varous depths. If the concentraton s measured by samplng the pore soluton the ntrnsc coeffcent wll be measured but f the total concentraton s obtaned by acd dssoluton of the sold ths wll gve the apparent coeffcent. The dfference between them s caused by chlorde bndng. The most mportant mechansm for ths s the formaton of chloroalumnates wth the alumnate phases n cement. There are many arguments n favour of each coeffcent for consderng durablty of structures. It can be argued that bound chlordes wll not cause corroson but t s also known that they can be released f the ph drops. However n ths smple experment nether measure wll take account of on-on nteractons. The ASTM test s not normally used to get a dffuson coeffcent. The result s smply gven as charge passng. However the Nordtest uses the Nernst-Plank equaton to get a coeffcent and smple dffuson tests just use Fck s law. The effect of on-on nteractons and the membrane potental s not normally used for any of them. If the ASTM test s used for comparng mxes t can be seen from fgures 5 and 6 that the result wll be ncorrect s pozzolanc mxes are compared wth non-pozzolanc. In some specfcatons a maxmum charge passng of 1000 Coulombs s specfed and the mx desgn s left to the contractor Charge 2000 Coulomb Slca Fume CEM Chlorde % of cementtous Fgure 15. Comparson of Charge passng wth data for a smple dffuson test for mxes wth 6 w/c=0.3 and 20% slca fume replacement (Classe 1988) Fgure 15 shows results wth a range of mxes wth smlar propertes ndcated by a smple dffuson test. The pozzolanc mxes generally have Coulomb values below 1000 and the CEM1 mxes above. Ths data ndcates that the ASTM C1202 test s not sutable for ths purpose. The dfferences between fgures 9 and 10 ndcate that the Nordtest would also be unsutable. However t should be noted that these two tests can be used very effectvely for qualty control purposes. If regular samples are taken from concrete used on ste, a change n the result from the tests wll clearly ndcate a problem. For modellng concrete durablty t has been noted that nether the smple dffuson test nor the models take account of on-on nteractons. It may also be seen that the smple dffuson test has a good resemblance to the geometry of an exposed concrete surface on ste. It may therefore be ndcated that results from the test may be used n the model subject to concerns about changng condtons whch may affect adsorpton or onc concentratons. It s ndcated that the use coeffcents from electrcal tests n these models should only be attempted f numercal smulatons are used to correct for the effect of the appled feld on the on-on nteractons. 6. CONCLUSIONS All measurements of dffuson n concrete, wth or wthout appled voltages, are sgnfcantly affected by on-on nteractons. For the comparson of mxes wth dfferent cementtous materals n them a smple dffuson test should be used unless computer modellng s used to correct the results of electrcal tests. The electrcal tests are sutable for qualty control purposes. For modellng the lfe of structures a dffuson coeffcent whch ncludes some adjustment for on-on nteractons (such as that obtaned by applyng Fck s law to a smple dffuson test) should be used. 7. REFERENCES ASTM (2005) ASTM-C1202, Standard Test Method for Electrcal Indcaton of Concrete s Ablty to Resst Chlorde Ion Penetraton Atknson, A. and Nckerson, A. K. (1984) 'The Dffuson of Ions through Water-Saturated

7 Cement ' Journal of Materals Scence 19, (9) Classe P (1988) PhD thess, Unversty of Leeds. Avalable at: asse%20thess.pdf Crank, J. (1976) The Mathematcs of Dffuson second. Oxford: Unversty Press Lzarazo-Marraga, J. and P. Classe (2009a), Effect of the non-lnear membrane potental on the mgraton of onc speces n concrete. Electrochmca Acta, (10): p Lzarazo-Marraga, J. and P. Classe (2009b), Determnaton of the concrete chlorde dffuson coeffcent based on an electrochemcal test and an optmzaton model. Materals Chemstry and Physcs, (2-3): p Lzarazo-Marraga, J. and P. Classe, Determnaton (2010) of the transport propertes of a blended concrete from ts electrcal propertes measured durng a mgraton test. Magazne of Concrete Research, (3): p Lzarazo-Marraga J. (2009) PhD thess. Coventry Unversty avalable at: 0Lzarazo%20Marraga.pdf Nordtest (1999) NTBUILD-492, NORDTEST METHOD: Concrete, Mortar And Cement-Based Repar Materals: Chlorde Mgraton Coeffcent From Non-Steady-State Mgraton Experments. NRMCA (2011) 5.htm 7