Aplicación de tratamientos electroquímicos combinados al hormigón armado: extracción electroquímica de cloruros más protección catódica

Availableonlineat

ScienceDirect

www.sciencedirect.com

www.e-ache.com HormigónyAcero2018;69(S1):43–51 www.elsevierciencia.com/hya

Application

of

combined

electrochemical

treatments

to

reinforced

concrete:

Electrochemical

chloride

extraction

plus

cathodic

protection

Aplicación

de

tratamientos

electroquímicos

combinados

al

hormigón

armado:

extracción

electroquímica

de

cloruros

más

protección

catódica

Miguel

Ángel

Climent

,

Jesús

Carmona

,

Pedro

Garcés

a_{Dr.enCienciasQuímicas,Catedrático,DepartamentodeIngenieríaCivil,UniversidaddeAlicante,Alicante,Spain} b_{Dr.IngenierodeCaminos,ProfesorAyudanteDoctor,DepartamentodeIngenieríaCivil,UniversidaddeAlicante,Alicante,Spain}

Received11December2017;accepted17May2018 Availableonline14July2018

Abstract

Ithasbeenstudiedthepossibilityofusingagraphite–cementpasteanodeforcombinedtreatmentsofelectrochemicalchlorideextractionplus

cathodicprotectiononreinforced concreteelements.These treatmentswouldbeinterestingincases ofstructuresheavilycontaminated with

chloridesandcontinuouslyexposedtoaharshchlorideenvironment.Ithasbeenshownthat,intheexperimentalconditionsofthiswork,theanode

isnotdamagedduringtheelectrochemicalchlorideextractionstep.Theapplicationofapreviouschlorideextractionstepallowsworkingwitha

lowercurrentdensityatthecathodicprotectionstep,whichreducestheriskofdamagesoftheanodicsystem.Achloridebarriereffectparameter

hasbeendefined,basedonthereductionofthechlorideuptakebyconcrete,duetocathodicprotectionaction,referredtothechlorideuptakeof

thereference(nottreated)specimens.Thevaluesofthechloridebarriereffectparameterfoundinthisworkareabout20%.

©2018Asociaci´onEspa˜noladeIngenier´ıaEstructural(ACHE).PublishedbyElsevierEspa˜na,S.L.U.Allrightsreserved.

Keywords:Steelreinforcedconcrete;Corrosion;Electrochemicalchlorideextraction;Cathodicprotection;Cathodicprevention

Resumen

Sehaestudiadolaposibilidaddeutilizarunánododepastadegrafito-cementoparatratamientoscombinadosdeextracciónelectroquímicade

clorurosyproteccióncatódicadeelementosdehormigónarmado.Dichostratamientosseríaninteresantesencasosdeestructurascontaminadas

conclorurosypermanentementeexpuestasaunambientesalinoagresivo.Sehademostradoque,enlascondicionesdeestetrabajo,elánodono

resultada˜nadodurantelaetapadeextraccióndecloruros.Laaplicacióndeunaetapapreviadeextraccióndeclorurospermiteaplicarlaprotección

catódicaconunadensidaddecorrientemenor,hechoquereduceelriesgodeda˜nosdelsistemaanódico.Sehadefinidounparámetrodeefecto

barreradecloruros,basadoenlareduccióndelapenetracióndecloruros,debidaalaproteccióncatódica,referidaalapenetracióndeclorurosde

referencia(sinproteccióncatódica).Losvaloresdedichoparámetroencontradosenestetrabajosondel20%,aproximadamente.

©2018Asociaci´onEspa˜noladeIngenier´ıaEstructural(ACHE).PublicadoporElsevierEspa˜na,S.L.U.Todoslosderechosreservados.

Palabrasclave: Hormigónarmado;Corrosión;Extracciónelectroquímicadecloruros;Proteccióncatódica;Prevencióncatódica

1. Introduction

Severalelectrochemicaltechniques havebeensuccessfully usedtotheprotectionandremediationofsteelcorrosionin

rein-∗_{Correspondingauthor.}

E-mailaddress:ma.climent@ua.es(M.Á.Climent).

forced concretestructures. Allthese techniquesare based on theloweringoftheelectricpotentialofsteel[1–7].Thiseffect canbe obtained bothbyconnectiontoaless noble metal,as incathodicprotection(CP)bysacrificialanodes,orby connec-tiontothenegativepoleofanelectricdirectcurrentsource,as inCPbyimpressedcurrent[8–12].Thetechniquescanalsobe classifiedinpermanentandtemporary techniques.Permanent

https://doi.org/10.1016/j.hya.2018.05.003

techniquesareCP,whoseaimistostopthesteelreinforcement corrosion,anditsvariantcalledcathodicprevention(CPre), fol-lowingPedeferrietal.[9,11].TheobjectiveofCPreistoprevent theinitiationofsteelcorrosioninnewstructures.Ontheother hand,temporary techniques are thoseintended tochange the prevailing corrosion conditions of the structures by lowering its chloride content, such in electrochemicalchloride extrac-tion(ECE)[13–18],orbyincreasingthepHofconcrete,such inelectrochemicalrealkalization(ER)[19,20].Inthisworkwe concentrateonapplicationsofthetechniquesaimedtoprevent theonsetofcorrosion(CPre),improvetheprevailingconditions (ECE)orstopthecorrosionofsteelreinforcement(CP),dueto chloridecontaminationofconcrete,excludingsacrificialanode applications.

Alltheelectrochemicaltechniqueshaveincommonthe reac-tionsatthesurfaceofsteel,thecathode,i.e.oxygenreductionto producehydroxideions(thebasisofER),andeventual hydro-gen gas evolution if negative enough potentials are reached. Hydrogen production is considered as a potentially harmful process, dueto the possibility of hydrogen embrittlement of highstrength steels [5,6].Besides to thesereactions the sys-tembenefitsalsofromeffectsduetomigrationofionsinduced bytheelectricfield:hydroxide(OH−)andchloride(Cl−)ions movefromcathodetotheexternalanode. Thislattereffectis thebasisofECE.Thepermanenttechniques,i.e.CPreandCP canbeconsideredasmoresophisticatedinthesensethatthey needtoguaranteethemaintenance(CPre),orthenew forma-tion andmaintenance(CP), of the passivatinglayer onsteel, bymaintaining the potentialof steel belowthe pitting poten-tial imposed by the presence of a certain Cl− concentration in the concrete’s inner pore solution [9–12]. These electro-chemical conditions induce a reduction of the ratio of the concentrations of chloride andhydroxideions, [Cl−]/[OH−], near the steel. A non-negligible contribution to the mainte-nanceofthepassivatinglayercomesfromtheso-calledchloride barriereffectactinginCPreandCPapplicationsonsteel rein-forcedstructurespermanentlyexposedtoanaggressivechloride environment[9,11].

The operation conditions of the abovementioned electro-chemicaltechniquesdifferineachcase.CPandCpre,applied toreinforced concrete,needusingapermanentlyfixedanode, whichisusuallyanembeddedmetallicwiremesh,typicallya titaniummeshactivatedwithmixedmetaloxides,ortheyuse conductive coatings or overlays, such as conductive cemen-titious overlays [21–24]. CPtypically applies current density values between 5 and 20mA/m2, while CPre needs only 1–3mA/m2. Nevertheless, in the case of CP the actual cur-rentdensityneededtoeffectivelyprotectthesteelishigherthe higheristheCl−contentofconcrete.ThesuccessofCPorCPre dependsgreatlyonanoptimumdesignoftheanodicsystem,able toprovideanadequatecurrentdistributiononthesteel reinforce-ment[9].ECE,asatemporarytechniqueaimedtoreducetheCl− contentofconcrete,usesexternalanodesandelectrolytes[25], althoughit hasbeendemonstratedthataconductive cementi-tious anodic overlay can also be used as the anode for ECE

[26,27,28,29].ThetypicalcurrentdensityforECEapplications onreinforcedconcreteelementsisintherange1–5A/m2,while

thetotalelectricchargepassedisusuallybetween1×106and 5×106C/m2.Fromanelectrochemicalpointofviewthe cur-rentdensityshouldbedefinedasreferredtoelectrodesurface, i.e. the surface ofthe steel cathode. However,in engineering fieldapplicationsitissometimesdifficulttoknowthesteel rein-forcementarea.So,manytimesthecurrentdensityisreferredto exposedconcretesurface,which,inthecaseofanodicoverlay systemsisequaltotheanodearea.

InthecaseofCPacriticalpointofconcernisthepossibilityof malfunctionoftheanodicsystem,duetotheacidityproducedby theelectrochemicalreactionsattheanode,whichcanimpairthe contactbetweentheembeddedconductingmaterialoftheanode andthesurroundingcementpaste.Thisriskincreaseswhenthe appliedcurrentdensityishigherthan20mA/m2_[21].

The Departmentof CivilEngineering of the University of Alicantehasbeenconductingresearchontheuseofconductive cementpastes(CCP),basedonmixesofcementwithdifferent carbonaceousmaterials,asanodiccoatingsforapplicationsof electrochemicaltechniquesonreinforcedconcrete[24,26–31]. Oneofthemostappealingpossibilitiesofferedbythese perma-nentanodicoverlaysisthepossibilityofcombinedsuccessive treatments ofECEplusCP,withoutchangingtheanode.This combination of treatments may be convenient in the case of structuresheavilycontaminatedwithCl−ions,whosechloride environmental load is expected tobe maintained highin the future, forinstancethecaseofreinforcedconcreteexposedto de-icingchloridesalts,oraharshmarineenvironment.Inthese casesitmaybedeemednecessarytofirstreducetheCl−content ofconcretebyapplyingECE,andlatermaintainthesteel pro-tectedbyapermanentCPtreatment,withouttheneedtoapplya toohighCPcurrentdensity,thusreducingtheriskofmalfunction oftheanodeduetotheacidityproducedatitssurface.

2. Methodology

Table1

Nomenclatureofspecimensforapplicationoftheelectrochemicaltechniques.

Studiedtechniques Initial%Cl−inconcrete Referencesample Treatedsample

CP 2 R(noelectrochemicaltreatment) A(CPtreated)

CPre 0 P(noelectrochemicaltreatment) B(CPretreated)

ECE+CP 2 ER(treatedonlywithECE) EA(ECE+CPtreated)

ECE+CPre 2 EB(ECE+CPretreated)

Figure1.Sketchofreinforcementofsamplesandconnectionofthecathodicsystem(steelreinforcement). Adaptedfrom[24].

ontotheconcreteoranodicoverlaysurface,inordertosimulate thecontinuedchloridecontaminationduetoexposuretoavery aggressiveenvironmentasmentionedabove.

2.1. Reinforcedconcretespecimens

Thespecimenswereprism-shapedreinforcedconcrete ele-ments, with a dimension of 18×18×8cm3, which were reinforced by agrid 16×16cm2 _{composedof six steel bars} (5mmdiameter)weldedsymmetricallyformingsquaresof5cm side,and placed 2cm under the anode, Fig.1.The concrete dosagewasasshowninTable2.Twomixeswereprepared:one withoutaddedchloride for preparing the specimens intended for pure cathodic prevention (CPre) treatments, and another mixcontaining2%Cl−relativetocementmassforthe speci-mensusedincathodicprotection(CP)applicationsorcombined treatments (ECE+CP) and (ECE+CPre), see Table 1. Once the formworkwasremoved, thespecimens were moist-cured at95–98%relativehumidity(RH)for28days.The

character-Table2

Concretedosageforpreparationofthetestspecimens.

Material Dosage

PortlandcementCEMI42.5R 250kg/m3

w/cratio 0.65

Limestoneaggregatemax.size 12mm

1890kg/m3

Superplasticizer 2.50kg/m3

NaCl Nilor3.3%(2%Cl−

relativetocementmass)

istics of the hardenedconcretewere as follows: compressive strength37.8N/mm2[32],porosity11.1%[33],andbulkdensity 2.38T/m3[34].

Fig. 1 also shows the system used for connecting the reinforcement(cathodesystem)tothenegativepoleofthe elec-tric powersource, through plastic isolated copperconnectors screwedtotherebar.

2.2. Commonexperimentaldetailsoftheelectrochemical tests

Figure2. Sketchofspecimenassembly. Adaptedfrom[24].

theratiobetweenthesurfaceofconcretecoveredbytheanodic overlayandthetotalsurfaceofthesteelbarswas1.7,Fig.2.All current densityandelectric chargedensityvalues reportedin thisworkarerelativetosurfaceofconcrete,(equaltotheanode surface),exceptotherwisestated.

Theresistivityofthe graphite–cementpastewasmeasured through the four-probemethod.To thisend, pastespecimens werecastin4×4×16cm3moulds,andmoist-curedat95–98% RH during 14 days. The experimental details of the mea-surementscanbefoundelsewhere[35].Theaverageobtained resistivitywas1.5m.

Themeasurementsofsteelcorrosionpotential(Ecorr)andall thesingleelectrodepotentials,wereperformedusingAg–AgCl referenceelectrodes.Theseelectrodeswerehousedinrespective holesdrilled fromthe exposedsurface of the concrete speci-men(that bearing thegraphite–cement anode)tothe vicinity oftherebar,Fig.2.Forthispurpose,theholesweresheathed withaplastictube,andfilledwithaKOH0.2Maqueous solu-tion,tryingtoapproachthephysical-chemicalconditionsofthe concrete’sinnerporesolution.

In thisway ninespecimens were prepared,seven of them with salt in the mixing water and two without it. Two of thesaltedspecimens wereusedonlyfordetermining the effi-ciencyoftheECEprocess.Concretecoreswereextractedfrom them,andtheir chloride contentprofiles weredetermined, in one casebefore and in the other caseafter ECE. Thesetwo specimens were discarded after coring.The seven remaining specimenswereintendedforthetestscorrespondingtoCP,CPre, and the combined treatments (ECE+CP) and (ECE+CPre), seeTable1.

TheECEefficiencieswerecalculatedasthepercentagesof reduction of the initial chloride content. Obtainingthe chlo-ride content profiles before and after the ECE trials allowed calculatingthelocalandoverallefficiencies.Attheendofthe 24weeksof exposuretoasevereCl−load,i.e.thefirstphase of CP andCPretreatments,see Section2.4,chloride content profileswere obtained from all the reinforced concrete spec-imens. The Cl− profileswere measuredfollowingessentially RILEM recommended procedures [36]. Cylindrical concrete cores,95mmdiameterand20mmheight(uptotherebardepth),

wereextracted.Fromthesecores,concretedustsampleswere obtainedbygrindingthin(2mmthick)successiveparallellayers totheexposedsurface[36].Inthisway10concretedust sam-ples weregainedfromeachcore, thusallowingthe obtention ofsufficientlydetailedchloridecontentprofiles.The determina-tionofthesamples’acidsolublechloridecontentswascarried outbypotentiometrictitration[37,38].Allthechloridecontent valuesareexpressedinthisworkas%Cl−relativetocement mass.

2.3. ECEtreatments

Fourofthespecimensmadewithsalinemixingwaterwere subjected toECE.Forthispurpose,thespecimenswere elec-tricallyconnectedinseriesbypairstoadirectcurrentsource. The relevant parameters of the ECEtreatments are shownin

Table3.Alowchargedensitywasapplied,only1.5MC/m2 rel-ativetoconcretesurface(2.6MC/m2relativetosteelsurface). Thecurrentsourcefeedingvoltage(Efeed)wascontrolledata levelbelow40Vduringtheprocesses,forsafetyreasons.Itwas necessary inthis respect tointerrupt the current passage two times (twopausesof 24h eachone)alongthe treatment.The wholeprocesseswereperformedinsideafumehoodto elimi-natethechlorine,Cl2(g),producedbyelectrochemicaloxidation of the Cl− ionsextracted from the concrete. When the ECE processeswerefinished,thepHvaluesoftheelectrolyteswere measuredinordertochecktheacidifyingeffectcausedbythe electrochemicalanodicreactions.

2.4. ApplicationofCP,CPreandthecombinedtreatments ECE+CPandECE+CPre

Thissectiondescribesthedetailsoftheexperimentscarried outtodemonstratetheapplicabilityoftheGCPanodesfor com-binedprotectivetreatmentsofECE+CPandECE+CPre,please refertoTable1.

Table3

SummaryofECEdata.

ECE.SpecimenswithGCPasanode. Electrolyte:dammeddistilledwater

Initial%Cl− 2%relativetocementmass

Currentdensity 2A/m2ofconcreteexposedsurface(3.4A/m2ofsteelsurface)

InitialΔEfeed 16–24V FinalΔEfeed 23–22V

Electricchargedensity 1.5MC/m2_{ofconcreteexposedsurface(2.6MC/m}2_{ofsteelsurface)}

handtheapplicationofCPrewith2mA/m2ofcurrentdensity (relativetoconcretesurface)toasamplepreviouslyECEtreated (EBinTable1)andtoanothermanufacturedwithoutsaltandnot ECEtreated(BinTable1).Thevaluesofthecurrentdensities relativetothesteelbarssurfaceare25.5mA/m2and3.4mA/m2 fortheCPandCpretreatments,respectively.Thedirectcurrent sourcewasprogrammedtomaintainaconstantcurrentdensity alongthe wholeprocesses. Eachapplicationhasits reference samplewithouttreatmentofCPorCPretocomparetheresults. Thereferencesampleforthespecimenssubjectedtocombined treatments(EAforECE+CP,andEBforECE+CPre)isER.R isthereferenceforA,andPisthereferenceforB.

TheapplicationofCPandCPreconsistedoftwophases: Phase1.First24weeks.TheaforementionedtreatmentsCP andCPrewerecontinuouslyappliedduringthefirst13weeks. Then,thecurrentwasswitchedofffor4weeksandafterthat, treatmentswereresumedtotheend.Chloridecontaminationwas continuouslyapplied,evenduringtheswitchoffperiods.

While applying the CP and CPre treatments some elec-trochemical parameters were measured. During the current passingperiodsthefeedingvoltageofeachspecimen,Efeed, wasobtainedas thepotential differencebetweencathode and anode; andthe individual anodicand cathodic potentials,Ea andEc,respectively,weremeasuredagainstthereference elec-trode Ag–AgCl. Finally, in order to check the efficiency of CPandCPreasmaintainersof protectionconditions ofsteel, the “100mV decay” criterion was used, as is specified in ISO12696:2012[39].Thiscriterionhasbeenalsoextensively employedforthispurposebyseveralresearchers[21,40].The methodconsistsinobtainingthe4hpotentialdecay(Edecay), thatisthedifferencebetweenEc4h(thevalueofEc4hafterthe currentswitchoff),andtheinstant-offcathodicpotentialEcio, whichinthiscasewasmeasured1safterthecurrentswitchoff. Theminimumvalueofthe4hdepolarizationmustbe100mV foranadequatecorrosionprotectionofsteel[39].Thevaluesof Ecioweremonitoredwithanautomaticdataloggerabletoobtain andrecord500measurementsin6s,afterthecurrentswitchoff. Once the 24 weeks processes were fulfilled, cores were extractedfromallspecimensofTable1,andtheirrespectiveCl− contentprofileswereobtained.Thiswasdonewiththepurpose ofevaluatingtheneteffectoftheelectrochemicaltreatmentson theCl−ionuptakebythereinforcedconcretespecimensduring thecontinuedexposuretoaveryaggressiveenvironment.

Phase2.Attheendof Phase1 itwasobservedthatallthe specimens had lost the steel protection condition, evidenced by Edecay values lower than 100mV. Then, it was decided tostartthissecond phasewiththeobjectiveof recoveringthe

protectionconditionsofsteelbyadjustingthecurrentdensityof theCPtreatments.Theprocedurewastoincreaseprogressively the current density during 4 weeks, starting with a value of 20mA/m2,untilobtainingtheprotectionconditions.

3. Resultsanddiscussion

3.1. ApplicationofECE

Four of the reinforced concrete specimens weresubjected toan ECEtreatment beforestarting thefirst phaseof theCP andCPretreatments,see Section2.3.Oncefinished the ECE processwiththesettledparameters,Cl− contentprofileswere obtained,correspondingtothestatesbeforeandaftertheECE treatment. ThelocalECE efficiencies,understood as percent-ageofCl−contentremoved,areplottedinFig.3.Theaverage of removedCl− was51%of theinitialcontent,i.e. the resid-ualCl− contentof concreteafterECEwasapproximately1% referredtocementmass.Thisindicatesagoodperformanceof the ECEprocessapplied onaconventionalordinaryPortland cement concrete with the GCP anodic overlay system, for a relativelylowchargedensityof1.5×106C/m2relativeto con-cretesurface.Thisresultcanbecomparedtothe41%efficiency obtainedforaverysimilarreinforcedconcreteelement,withthe sameinitialamountofCl−,subjecttoanECEtreatment,using aTi–RuO2 meshanode,andpassingatotalchargedensityof 1MC/m2relativetoconcretesurface[26].

ThepHvaluesdeterminedfortheelectrolytes,aftertheECE experiments,wereintherange5–5.5.Thismeansthattheacidity

0 20 40 60 80

0 2 4

0 4 8 12 16

Ef

fi

c

ie

nc

y(

%

)

Depth (mm) Efficiency

Initial content of chlorides

Final content of chlorides

Cl

- co

n

ten

t (

%

r

e

la

ti

ve t

o

cemen

t

ma

ss)

Figure3.ChlorideconcentrationprofilesbeforeECE(initial)andafterECE (final),andlocalefficienciesoftheextractionprocess.ECEdetails:current density:2A/m2_{,chargedensity:1.5MC/m}2_{,bothrelatedtoexposedconcrete} surface.

-100 100 300 500 700 900

0 5 10 15 20 25

Δ

Edec

a

y

(m

V)

Time (weeks of treatment) B

A

EB

100 mV EA

Figure4.EvolutionofΔEdecayduringtheCPorCPretreatments.A:CP;B: CPre;EA:ECE+CP;EB:ECE+CPre.AllofthemsubjectedtoCl− contam-inationduringthe24weeks.Theelectrochemicaltreatmentswereinterrupted betweenweek13andweek17.

Adaptedfrom[24].

oftheelectrolyteshadincreasedslightly,(theinitialelectrolyte wasdistilledwater).Thisacidifyingeffectmaybeduetoseveral ofthe electrochemicalanodicreactions,namelytheoxidation ofhydroxideionsorwatertoO2(g) andthe oxidationof car-bontoCO2(g).Itispossiblethatthealkaline characterofthe graphite–cementpasteanodemayhavereducedtheacidity pro-ducedbythoseelectrochemicalprocesses.Itisworthnotingthat nosharpincreaseofthefeedingvoltagewasobservedduringthe ECEtreatments.Thissuggeststhatnoimportantdamageofthe GCPoverlayhasbeenproducedduringECE[21],eventhough thesetrialsareperformedwithahighcurrentdensityof2A/m2. WemustrecallthattheECEtrialsareperformedunder continu-ousliquidpondingoftheanode,anditsdurationisshort,about eightdays.

3.2. FirstphaseofCPandCPretreatments

Inthissectionwedescribetheresultsoftestscarriedoutto investigatetheperformanceoftheGCPanodesduring protec-tiveelectrochemicaltreatmentstoreinforcedconcreteelements affected by steel corrosion due to chloride contamination, such are CP, CPreand combined treatments of ECE+CP or ECE+CPre,seeTable1.

ToverifytheeffectivenessofCPandCPretreatmentsfor pro-tectingsteelfromcorrosion,the“100mVdecay”criterion[39]

wasused,asstatedinSection2.4.Fig.4 showstheevolution oftheEdecayvaluesforthespecimensinTable1,duringthe 24weeksexperiments.TheEdecay valuesoftheAspecimen, treatedonly withCP, practically never reached the threshold valueof100mV.Itseemsthata15mA/m2currentdensitywas notsufficienttoprovideprotectiontosteelinsuchharsh condi-tions:initialCl−contentof2%plusthecontinuedsaltingregime (65mlNaCl0.5Mweeklysprayedontotheanodicoverlay sur-face).RegardingtheEAspecimen(ECE+CP),theprotection conditionsof steelwerekeptduring11weeksbecause ofthe currentcirculation,despiteexternalCl−loading.Cathodic pro-tectionof15mA/m2currentdensity,relativetoconcretesurface, wasabletokeepprotectiveconditionsforthesteelreinforcement iftheinitialCl−contentofthespecimenwasabout1%(EA).

0 2 4 6

0 5 10 15 20

C

lori

de c

o

n

ten

t

(%

r

el

a

tiv

e to

c

em

en

t

mass)

Depth (mm)

%Cl-_ER

%Cl-EA

%Cl-EB

Figure5.ProfilesofCl−contentinspecimenstreatedpreviouslywithECEand afterwithCP(EA),withECEandafterCPre(EB),andinthereferencesample onlytreatedwithECE(ER),allofthemsubjectedtoCl−contaminationprocess, afterECEandfirstphaseofCPorCPre(24weeks).

Adaptedfrom[24].

In the caseofthe specimen withinitialCl− contentof about 2%(A),ahighercurrentdensitywouldbeneededforreaching theprotectionconditions[9,11].Theseobservationscorroborate themainhypothesisofthepresentresearch,i.e.incasesof rein-forcedconcretestructureswithahighCl−contaminationsubject toveryharshchlorideenvironments,itwouldbeadvantageous toapplysuccessivelyaninitialECEtreatmenttoreducetheCl− content,andthenmaintainprotectiveconditionstosteelthrough acontinuousCPtreatmentwithouttheneedofusingatoohigh CP currentdensity,whichcouldeventuallyimpairthe perfor-manceoftheanodicsystem[21].Thesecombinedtreatments, ECE+CP,wouldbemoreconvenientlyimplementedwiththe CCPcoatings,sincethesameanodecanservebothfortheECE andfortheCPtreatments.

RegardingtheCPretreatedspecimens(cathodiccurrent den-sity2mA/m2relativetoconcretesurface),specimenBshowed

ΔEdecayvalueshigherthan100mVuptothecurrentswitchoff at13thweek, confirmingthecapacityofcontinuouslyapplied CPretreatmentstokeepsteelprotectionconditionsfor an ini-tially Cl− freereinforced concrete, despiteextensiveexternal Cl− load[9,11,41,42]. Onthe other handsuch alow current densityisunabletoprotectsteelifconcreteispreviously contam-inatedatalevelofabout1%relativetocementmass,andsteel hasstartedtocorrode,asshownforspecimenEB(ECE+CPre) inFig.4.

AttheendofPhase1allthereinforcedconcretespecimens hadreached avery highdegreeof Cl−contamination, ascan beappreciatedinTable4.Nevertheless,somecomparisonscan be doneonthebehaviour ofthedifferentcases. Forinstance,

Table4

Finalaveragedchloridecontents(expressedin%Cl−relativetocementmass)attheendofthe24weeksofexposuretoasevereCl−load(firstphaseofCPand CPretreatments).

Specimen InitialCl−content (%ref.cem.mass)

Electrochemicaltreatmentprevious tothe24weeksfirstphase

Electrochemicaltreatmentduring the24weeksfirstphase

Finalaverageda_Cl−

content(%ref.cem.mass)

P 0 – – 4.93

R 2 – – 6.08

ER 2 ECE – 4.26

A 2 – CP 5.39

B 0 – CPre 3.94

EA 2 ECE CP 3.41

EB 2 ECE CPre 3.42

a _ThefinalCl−_{contentwascalculatedasthemeanvalueofthosefoundintheCl}−_{contentprofiledeterminedthroughtheconcretecoverzone(20mmwidth).}

(Cl−BEP),whichisdefinedhereasthepercentageofreduction oftheCl−uptakebyconcrete,duetoCPorCPreactions,referred totheCl−uptakeofthereference,(nottreated)specimens,see Eq.(1).

Cl−BEP=(Cl

−_{UptakebyReference)−(Cl}−_{UptakebyTreatedSpecimen)}

(Cl−UptakebyReference) ∗100 (1)

Table 5 shows the net values of the Cl− barrier effect parameterfoundforthespecimenstestedinthiswork,putting in evidence that this parameter reaches values of about 20%intheexperimentalconditionsofthiswork.

3.3. SecondphaseofCPtreatments

Giventhatafterthe24weeksoftreatmentsofPhase1, includ-ing rest periods between the 13th and 17th weeks, the steel reinforcementsinallconcretespecimens hadcompletely lost theirprotectionconditions,andhavingbeendemonstratedthat aCPof15mA/m2wasunabletorestoretheprotective condi-tions(Fig.4),thePhase2wasstarted.TheexternalCl−loadwas discontinuedsinceallthespecimenshadreachedveryhighCl− contents,seeTable4.Intheseconditions,CPwasappliedwith highercurrentdensities.Thequestionwasifitwouldbepossible torecovertheprotectionconditionsofsteelbyincreasingthe cur-rentdensitytotheappropriatevalue.Inthebeginningofthislast phase,thecurrentdensitywassetat20mA/m2.After4weeks inoperation the thresholdvalue of 100mV ΔEdecay was not reached,i.e.theprotectionconditionswerenotobtained,Fig.6. Neithersuccesswasobtainedinasecondattemptat25mA/m2 (datanotshowninFig.6).Finally,athirdstepof40mA/m2was set.Inthiscase,after4weeks,theruleof100mVofΔEdecay wasachievedforEA,AandBspecimens.

Table5

Valuesofthechloridebarriereffectparameterforthetestedspecimens.

Specimen Treatmentof specimen

Reference Treatmentof reference

Cl−BEP (%)

A CP R None 17.0

B CPre P None 21.0

EA ECE+CP ER ECE 26.0

EB ECE+CPre ER ECE 25.7

-100 -50 0 50 100 150 200 250

0 2 4 6 8

Δ

Ede

c

a

y

(mV)

Time (weeks of CP phase 2)

20 mA/m² 40 mA/m²

B

EA

EB 100mV

A

Figure6.EvolutionofΔEdecayduringPhase2ofCP.Firststepof4weekswith 20mA/m2ofcurrentdensity,andthirdstepwith40mA/m2.SpecimensA,B, EAandEB.

Adaptedfrom[24].

Moreover,protectionconditionswereverifiedwiththe mea-surementof depolarization potential difference values7 days after switch off [39]. In fact, more than150mV of ΔEdecay wasreached after7days (209mVforEA,211mVforAand 153mVforB).ThisefficiencyofCPissimilartothatobtained byotherresearchers[40,43].However,thiswasnotthecasefor EB.After4weekswithacathodiccurrentdensityof40mA/m2 the protection conditions were not recovered for this latter specimen.

4. Conclusions

Theresultsofthisworkpointoutthatitispossibletousea graphite–cementpaste, overlaidonthesurfaceofareinforced concreteelement,astheanodeforsuccessivetreatmentsof elec-trochemicalchlorideextraction,toreducethechloridecontent, andthencathodicprotectiontomaintainprotectiveconditions forthesteelreinforcement.Thefollowingaspectscanbe empha-sized,namely:

• The chloridebarriereffectparameter(Cl− BEP)is defined asthepercentageofreductionoftheCl−uptakebyconcrete, duetoCPorCPreactions,referredtotheCl−uptakeofthe reference(nottreated)specimens.Thisparameterallows eval-uatingthemagnitudeofthechloridebarriereffectduetothe application of CP or CPre toreinforced concreteelements continuouslyexposedtoaharshchlorideenvironment.The foundvaluesof thisparameter,bothforsimpleCP orCPre trialsandforthecombinedtreatmentsofECE+CP,areabout 20%,intheexperimentalconditionsofthiswork.

• Acathodiccurrentdensityof2mA/m2_{,typicalofCPre} appli-cations,isunabletoprotectsteelafteranECEtreatment.

• Itispossibletorecoverprotectiveconditionsagainstcorrosion of steel reinforcementinconcretebyapplying acombined treatmentofECEfollowedbyacontinuousCPtreatment.The currentdensityvalueoftheCPstepmustbesetattheproper value,accordingtotheresidualchloridecontentinconcrete. TheneededCPcurrentdensityvaluewillbelowerthanifthe ECE wasnot applied,thusreducing the riskof damage of theanode. Inthissense theGCPoverlaysonconcretemay showanadvantageousversatility,overothertypesofanodic systems,sincethesameanodeisusedonthetwostepsofthe combinedtreatment.

Acknowledgements

ThisresearchwasfundedbytheSpanishAgenciaEstatalde Investigación(AEI),andFondoEuropeodeDesarrolloRegional (FEDER) through project BIA2016-80982-R. We acknowl-edge also funding from the Spanish Ministerio de Ciencia e Innovación (MAT2009-10866), and Generalitat Valenciana (PROMETEO/2013/035).

Theauthorswouldliketodedicatethisworktohonourour masterandfriendProf.CarmenAndradePérdrix,whosuggested ourresearchgroupundertakingresearchontheapplicationof electrochemicaltechniquestoreinforcedconcretestructures.

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