tere

PeParade C

chargeill increore toten byion me

oportion as water to cement ratio decrease, concrete cover thickness increaseand when concrete has lower alkalinity reserve. The tendency of reinforcement reaching passive condi-

of chloride ions higher than a certain critical content [2,3]. In thespecic case of concrete carbonation, the pH goes down due to car-bonates precipitation as a consequence of hydroxides and otherhydrated phases reaction with carbon dioxide [4].

[1519], efciency of different electrolytes on realkalisation [20],effects on concrete properties [21,22] and side effects [2326],have been carried out during the last years.

On site applications were introduced at the end of 1980s and asignicant number of structures have been treated with thistechnique. Some documents report these experiences [15,27] andshow the ability of the technique in recovering protective pHlevels. These studies also show that, even after some years, thealkalinity remains at high levels, which would be enough to protectthe reinforcement [15].

Corresponding author at: Federal Institute of Education, Science and Technol-ogy of Paraba, R. Dep. Balduno M. de Carvalho, 155/1104, 58036-860 Joo Pessoa,Brazil. Tel.: +55 83 3246 5124; fax: +55 83 3208 3088.

E-mail addresses: gibsonmeira@yahoo.com, gibson.meira@ifpb.edu.br

Construction and Building Materials 40 (2013) 280290

Contents lists available at

Construction and B

ev(G.R. Meira).Keywords:CarbonationConcreteElectrochemical realkalisationRepassivation

tion is observed by corrosion potential measurements. For this variable, concretes that demand morecharge and time for realkalisation reach the passive condition in a shorter time. Corrosion current densityshows a decrease tendency over time and, although not reaching the level that represents passive condi-tion in most of the cases, it was not far from this reference.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Concrete is a material that usually presents high levels of alka-linity, which helps on the formation and protection of passivationlm on metal surfaces in reinforced concrete structures. Since thisinitial condition of high alkalinity and the concentration of depass-ivating ions below a certain threshold are maintained, corrosionprocess does not start [1]. However, this protective lm can be bro-ken down due to carbonation or in the presence of concentrations

As a rehabilitation method or as a preventive treatment, therealkalisation method has been used to recover concrete alkalinitynearby the reinforcing region [57]. The advantage of this methodis that when the treatment nishes the system can be unassembledand concrete structure can follow its role without larger destruc-tive interventions as in conventional parch repair treatment [8].

Studies about realkalisation treatment which focused on under-standing the phenomenon characteristics, like transport mecha-nisms involved [914], efciency on reinforcement repassivationAvailable online 6 December 2012

Accepted 23 September 2012

show the treatment efciepassed are demanded in pr" Pozzolanic addition means more total" Increasing concrete cover thickness w" Lower water to cement ratio means m" Reinforcement repassivation can be se" More charge demanded for realkalisat

a r t i c l e i n f o

Article history:Received 22 May 2012Received in revised form 6 September 20120950-0618/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.09.076demanded for the treatment.ase total charge for realkalisation.al charge demanded for the treatment.corrosion potential measurements.ans a shorter time for repassivation.

a b s t r a c t

This work studies the inuence of concrete characteristics, such as cement type and water to cementratio, and concrete cover thickness on the electrochemical realkalisation of carbonated concretes. Results

ncy for recovering concrete alkalinity. More time and total electrical chargeh i g h l i g h t sElectrochemical realkalisation of carbonacharacteristics and thickness of concrete

P.H.L.C. Ribeiro a, G.R. Meira a,b,, P.R.R. Ferreira b, N.aUrban and Environmental Engineering Postgraduate Programme, Federal University ofb Federal Institute of Education, Science and Technology of Paraba, R. Dep. Balduno M.

journal homepage: www.elsll rights reserved.d concretes Inuence of materialinforcement cover

razzo a

ba, 58051-900 Joo Pessoa, Brazilarvalho, 155/1104, 58.036-860 Joo Pessoa, Brazil

SciVerse ScienceDirect

uilding Materials

ier .com/locate /conbui ldmat

On the other hand, the effectiveness of the treatment in repass-ivating the reinforcement has been questioned [28,29] and,although some work is being carried out on this specic topic[30], there is still not a consensus and more research is necessary.Moreover, the inuence of material or structure characteristics onthe method behaviour and its nal results was studied at a lowerlevel [30,31].

Taking into account this gap in knowledge, this work studies theinuence of cement type, water to cement ratio and reinforcementconcrete cover on electrochemical realkalisation of carbonatedconcretes and its efciency on repassivation of reinforcements.

2. Fundamentals of electrochemical realkalisation

In the electrochemical realkalisation treatment, an externalpower supply is connected to the reinforcement (cathode) and toan auxiliary anode embedded in an external electrolyte (Fig. 1).The auxiliary anode is usually made of inert titaniummesh and so-dium carbonate solution is the most common electrolyte. An elec-trical current density between 0.8 and 2 A/m2 in respect toconcrete surface is applied to the system for a few days [5], untilthe concrete cover is realkalised.

Material realkalisation takes place by hydroxyl generationaround the steel reinforcement due to water electrolysis, as de-scribed in Eqs. (1) and (2) [10] and it is helped by electro-osmosisof electrolyte solution [9,1114]. Additional mass transport by cap-

At the anode region, hydroxyl ions are oxidised or water is elec-trolysed, as described in Eqs. (3) and (4) [5], which decreases pHlevel in this region.

2OH ! 1=2O2 H2O 2e 3

2H2O! O2 4H 4e 4The water electrolysis increases the OH concentration close to

the reinforcement surface and pH reaches a level close to 14 justafter the treatment. In time, pH rises for levels that, theoretically,are still enough for the steel reinforcement to reach the passivecondition [11]. As hydroxide ions are negatively charged, theymove towards the anode, as well as Na+ ions move towards thecathode. Considering the differences on ion transport numbers,about 20% of alkalinity generated in cathode surroundings remainsin this region [8].

Taking into account that water electrolysis is the main phenom-enon involved, the total charge passed is the variable that stronglyinuences realkalisation [19]. The number of hydroxides generatednearby the reinforcement is a consequence of the applied currentduring the treatment. It means that, the longer the treatment, morehydroxides are generated and higher alkalinity levels are reachedat surrounding reinforcement region.

Although electro-osmosis is claimed to transport carbonates to-

P.H.L.C. Ribeiro et al. / Construction and Building Materials 40 (2013) 280290 281illary absorption can take place in specic cases where the sub-strate is not saturated, but this effect is usually bordered toconcrete surface [8]. Mass transport by diffusion can take place,but it is usually neglected due to its low inuence in this process[9]. As an electrical eld is on, electro-migration of ions also takepart, making ions move according to their electrical polarity. Allthese mechanisms are represented in Fig. 1.

1=2O2 H2O 2e ! 2OH 1

2H2O 2e ! 2OH H2" 2Fig. 1. Simplied design of the electrwards the reinforcement and play an important role in realkalisa-tion process [13], some works did not observe this kind of masstransport [27]. With regards to the contribution of electro-osmoticux, the alkaline solution moves from the external anode region tothe cathode region through the concrete pores network [9,12]. Thisux depends on the electrical eld applied, but it is mainly inu-enced by the zeta-potential [11], which is continuously changingdue to the changes of species in the aqueous phase and consequentchanges of the electrical double layer in the walls of the pores [13].Thus, these conditions need to be favourable for the electro-osmo-tic ux to happen at a signicant level.

Concerning the realkalisation treatment, an important pointwould be to know when the realkalisation system can be turnedochemical realkalisation process.

off based on total charge passed or treatment time. Values between200 and 450 A h/m2 have been reported in literature [32], whichmeans a period between 8 and 18 days for a current density of1 A/m2 related to concrete surface. However, as these values alsodepend on aspects related to material and structure characteristics,they can vary in the long term, as discussed in the present work.

3. Experimental work

3.1. Casting and curing concrete specimens

Prismatic concrete specimens with 8 8 8 cm were cast using Brazilian Port-land Cements V (high early strength Portland cement ASTM Type III) and IV (Port-land pozzolan cement ASTM Type IP), which physical and chemical characteristics

procedures. All specimens were cured in a wet chamber for 7 days, to represent acondition closer to eld applications, and remained in laboratory environment until

dures. When all the concrete cover was realkalised and thus the treatment was n-

SiO2 (%) 18.96 25.75Al2O3 (%) 3.92 5.44

282 P.H.L.C. Ribeiro et al. / Construction and BuFe2O3 (%) 2.95 2.44CaO (%) 61.06 51.56MgO (%) 3.08 4.51Na2O (%) 0.15 0.23K2O (%) 1.03 1.86Insoluble residue IR (%) 0.67 16.81Loss on ignition Li (%) 1.15 4.47Free CaO 2.96 1.13completing 180 days, when it is expected that the concrete reach a more stablemicrostructure. During this period, they were covered by a plastic lm to avoidearly carbonation.

3.2. Accelerated carbonation

After this initial period of 180 days, the specimens were subjected to acceler-ated carbonation in a saturated carbonation chamber under 75 5% of relativehumidity and 28.5 1 C of temperature. The usual accelerated carbonation condi-tions reported in literature with CO2 concentrations below 10% and relative humid-ity between 50% and 65% [33] could not be adopted in this case. During thiscarbonation period, sacricial non-reinforced concrete specimens (cylinders) wereused to monitor the carbonation front advance. This was done by means of a phe-nolphthalein test, which is based on a pH indicator (1% phenolphthalein solution)that becomes dark pink in environments with pH higher than nine and colourlessfor lower values of pH. The carbonation front was veried on 1 cm slices cut per-pendicularly to cylinder axis.

Simultaneously to carbonation front advance monitoring, electrochemical mea-surements were taken to identify reinforcement depassivation. This was indicatedby corrosion potentials more electronegative than 350 mV (CSE copper sulphateelectrode) [34] and corrosion current density higher than 0.1 lA/cm2 [35] as com-mented in Section 3.3.

Table 1Chemical composition and physical properties of cements.

Composition/property Cement I Cement II

SO3 (%) 3.19 2.69are presented in Table 1 as cements I and II, respectively. In this case, the commer-cial Portland pozzolan cement used incorporated calcined clay as pozzolan additionin its composition. Water to cement ratios were set at 0.55 and 0.65, comprisingconcrete mixtures and properties presented in Table 2. Coarse aggregate and sandhad 9.5 mm and 4.8 mm maximum diameter, respectively.

Two reinforcement bars with 6.3 mm diameter and 10 cm length were embed-ded in each specimen and were placed respecting 1 or 2.5 cm of concrete cover(Fig. 2), comprising the specimens characteristics and codes presented in Table 3.The option of embedding two bars in each specimen was to consider an arrange-ment that can take place in eld applications. Two reinforced concrete specimenswere cast for each experimental condition. Reinforcements were previously cleanedby mechanically brushing their surfaces until removing any previous surface oxida-tion products. Afterwards, these bars had their extremities covered with an insulat-ing tape, delimiting an intermediate constant exposing area of 6 cm2 (Fig. 2).Duplicate reinforced specimens were cast to monitor realkalisation of concrete cov-er as described in Section 3.4.

Furthermore, additional non-reinforced cylindrical specimens (10 20 cm)were cast to monitor carbonation front advance during accelerated carbonationBlaine (cm2/g) 4020 4670Specic density (g/cm3) 3.14 2.973.5. Electrochemical measurements

Electrochemical measurements were carried out during the carbonation andelectrochemical realkalisation periods. This was done on single measurements ofopen circuit corrosion potential (Ecorr) and instantaneous corrosion current density(icorr) measurements. A potentiostat, model VMP3 from PAR (Priceton Applied Re-ished, the specimens were broken and samples close to the reinforcement werecollected using a diamond-based tool. These samples represent an 8 mm thicknesslayer nearby the reinforcement surface and were collected for reference (REF), car-bonated (CARB) and realkalised (REA) specimens. Additionally, similar sampleswere also extracted at three different depth bands (08; 816 and 1625 mm withrespect to concrete surface) for specimens with 25 mm concrete cover thickness.After being extracted, these samples were crushed to small pieces (about 2 mm)and were immersed in deionised water for 24 h respecting the 1/1 ratio and storedin an atmosphere with nitrogen to avoid further carbonation. After this period, itwas assumed that the medium was close to the equilibrium and the resultant solu-tion was used to analyse the hydroxide concentration and, consequently, the pH ofthis equilibrium solution. This extraction procedure is similar to that described byCastellote et al. [36]. No further mathematical data treatment was adopted to tpore-pressing condition as proposed in the same work [36] due to its limited vali-dation. It is clear that this equilibrium solution does not accurately represent theconcrete porous network solution, but in some way it represents a potential alka-linity of the samples and thus it was used only for comparative analysis, in a similarway to other previous works [37,38].3.3. Realkalisation procedures

For realkalisation treatment, an electrical eld was generated between the rein-forcement (cathode) and an external anode attached to the concrete surface using apower supply, which was continuously controlled to produce an average cathodiccurrent density of 2 A/m2 in respect to the exposed concrete surface. In thisarrangement, the anodes were made of activated titanium mesh placed on the sur-face of the specimen and the electrolyte was a sodium carbonate (NaCO3) 1 M solu-tion, in a similar arrangement as the one presented in Fig. 1.

The option for this level of current density and sodium carbonate based electro-lyte was a consequence of experiences presented in some previous papers[5,9,15,19], which show the efciency of this arrangement without signicant risksto change concrete microstructure.

Based on this set up and on electrical current and potential difference measure-ments directly obtained on this circuit to monitor the target current density, it waspossible to obtain the ohmic resistance of the system. This helped on visualising thehigh initial resistance of the system and its gradual decrease due to the ionic uxincrease.

3.4. Alkalinity monitoring

The system arrangement for the treatment was turned on until the concretereinforcement cover was completely realkalised. This was monitored in two dupli-cate specimens cast with this objective. These specimens were periodically (up tove times during the treatment period) sliced close to the reinforcements(Fig. 3a) using a diamond-based tool under a dry cutting process. This was done ini-tially after approximately 24 h and 90 h of treatment for more and less porous con-cretes, respectively, and it was followed at shorter intervals of time depending onthe alkalinity advance in the previous measurement. After being sliced, the speci-mens had their surfaces cleaned with high pressure air jets and then, a 1% thy-molphthalein solution was sprayed on them as a pH indicator (Fig. 3b). Thiscolorimetric indicator becomes blue for pH above and around 10.5 and colourlessfor lower values of pH, showing the region where concrete alkalinity was recovered.

Taking into account the blue area indicated by the phenolphthalein solution andthe previous carbonated area, considering the area in front of the exposed reinforce-ment length as shown in Fig. 3b, it was possible to monitor the realkalisation ad-vance in the concrete cover region by means of the ratio between the realkalisedarea and the previous carbonated area. This ratio was expressed as a realkalisedfraction of the total carbonated area in the cover region. Digital images capturedfrom sliced surfaces obtained for each alkalinity analysis were used for this purpose,as represented in Fig 3b.

Complementary pH analyses were also carried out using the following proce-After identifying reinforcement depassivation, specimens were removed fromthe carbonation chamber, were saturated by immersion in deionised water untilmass stabilisation, which took a maximum period of 7 days. Then, the specimenswere subjected to electrochemical realkalisation treatment. The saturation proce-dure was adopted to avoid the wicking effect during the treatment.

ilding Materials 40 (2013) 280290search), was used to perform these measurements. Linear polarisation techniquewas used for corrosion measurements. A potential difference of 10 mV with re-spect to the free corrosion potential and a scanning rate of 0.17 mV/s were applied.

Table 2Concrete mixtures and properties.

Concrete C1 C2 C3 C4Cement type I I II II

Cement (kg/m3) 362 349 360 347Sand (kg/m3) maximum diameter 4.8 mm 858 828 853 823Coarse aggregate (kg/m3) maximum diameter 9.5 mm 934 902 928 896Plasticiser (kg/m3) 1.77 3.51 w/c ratio 0.55 0.65 0.55 0.65

Slump (cm) 6 16 5 9Compressive strength (MPa) 90 days 40.2 36.3 28.9 22.3Absorption (%) 3.14 4.62 4.52 5.67Water porosity (%) 7.11 10.21 10.04 12.32Density (g/cm3) 2.26 2.21 2.22 2.17

Fig. 2. Schematic design o

Table 3Specimen codes.

Concrete Reinforcement concrete cover (mm) Specimen code

C1 10 C1/10C2 10 C2/10

25 C2/25C3 10 C3/10C4 10 C4/10

25 C4/25

Fig. 3. Evaluation of the realkalised area specim

P.H.L.C. Ribeiro et al. / Construction and Building Materials 40 (2013) 280290 283The ohmic drop was compensated and the icorr values were obtained by Stern andGeary equation [39]. The measurements set up used a copper sulphate electrode(CSE) as reference electrode and a stainless steel plate as a counter electrode (Fig. 4).

Regarding electrochemical measurements to indicate reinforcement depassiva-tion, this work accepted that icorr values higher than 0.1 lA/cm2 indicate metaldepassivation, as well as Ecorr values more electronegative than 350 mV (CSE)[34,35]. This electrochemical monitoring was continued during 1 year after therealkalisation treatment was over. Throughout this period, the specimens were pro-tected with a plastic lm and stored in a laboratory environment to avoid furthercarbonation.

f concrete specimens.

en cutting (a) and comparison of areas (b).

mica

284 P.H.L.C. Ribeiro et al. / Construction and Building Materials 40 (2013) 2802904.1. Ohmic resistance of the system

The ohmic resistance of the system (reinforcement (cathode)concreteelectrolyteanode) was obtained from electrical currentand voltage measurements and it is represented in Fig. 5. It canbe seen that ohmic resistance starts at high levels but tends to de-crease during the treatment. As carbonated concretes usually havehigher resistivity [11] and ionic ux is starting, these high valuesare expected. As time goes by, the ionic ux increases as well asthe electrical current, which contributes to lower resistance values.

The ohmic resistance increases for lower w/c ratio concretesand for concretes made with cement I (C1, C2). This is related tothe lower porosity levels observed (Table 2), which makes ionictransport in cover layers of concrete during the treatment moredifcult. In a similar way, specimens with thicker concrete coveralso present higher ohmic resistance of the system, due to its thick-er barrier to ionic transport. This makes resistance data forarrangements with specimens with 25 mm concrete cover thick-ness higher than those with thinner covers for measurements afterstabilisation (see data after 70 h in Fig. 5).

4.2. Total electrical charge passed for completing realkalisation4. Results and discussion

Fig. 4. ElectrocheThe total charge capable to realkalise carbonated concrete coverwas monitored by multiplying current density and time. Results

Fig. 5. Ohmic resistance of the system during the treatment.are presented in Fig. 6, which shows the relationship between totalcharge passed (in respect to concrete surface) and realkalised frac-tion of the total carbonated area in the cover region, as described inSection 3.4.

Results show that concretes made with Portland pozzolan ce-ment (C3, C4) required more charge to complete concrete realkali-sation. It could be expected that, as these concretes presentedhigher porosity (Table 2) and lower ohmic resistance, less chargewould be necessary. However, it did not happen and concreteswith pozzolanic addition were harder to be realkalised.

To explain this behaviour, two aspects may be taken into ac-count. First, as concretes made with pozzolan cement have lessalkalinity reserve due to the portladite consumption in pozzolanicreactions, they tend to reach lower levels of alkalinity after carbon-ation (see Section 4.4). Thus, comparing concretes without mineraladditions (C1, C2) with those that use Portland pozzolan cement(C3, C4), the last ones need more total electrical charge passed tocomplete the realkalisation of a same thickness of concrete cover.Furthermore, as more total charge passed is demanded as a conse-quence of the rst aspect, more hydrogen cations are generated inthe anodic region, which contributes to neutralise some of thehydroxides produced at the cathode region.

The inuence of w/c can be observed in Fig. 6. Comparing con-cretes C1 and C2, it can be seen that concrete C1 need330 A h=m2concrete to complete realkalisation, in proportion as con-

l measurements.crete C2 need 240 A h=m2concrete, which means an additional chargeof 90 A h=m2concrete when w/c changed from 0.65 (C2) to 0.5 (C1). As

Fig. 6. Progress of the realkalised fraction as a function of the total charge passed.

d Bucommented by Bertolini et al. [27], concretes with lower w/c needa higher total charge passed to complete the treatment and thus,under the same current density, the treatment takes longer to real-kalise a given concrete cover thickness. In the case of concretes C3and C4, there is a similar behaviour with a difference of about50 A h/m2.

Concretes with higher reinforcement cover thickness need morecharge passed to complete realkalisation. Although this seems tobe an obvious aspect, it was not measured in previous papers. Itwas necessary to change total charge passed from 240 to400 A h=m2concrete when increasing reinforcement concrete coverfrom 1 to 2.5 cm for concrete C2, which means an increase of morethan 66% in total charge. For concrete C4, it took place in a similarway. The total charge increased from 310 to 500 A h=m2concrete,which means an increase of more than 61%. This is related to theamount of OH ions that need to be generated to realkalise thewhole concrete cover thickness.

4.3. Visual monitoring of realkalisation front advance

Fig. 7 shows the advance of the realkalised fraction in concretecover region during the treatment based on digital images, as de-scribed in Section 3.4. C4/25 specimen was taken as reference tothis purpose. By Fig. 7, it can be clearly seen that the realkalisationprocess started near to the reinforcement and progressively ad-vanced towards the concrete surface. This indicates that waterelectrolysis in the cathode region was the main phenomenon in-volved in concrete cover realkalisation in this experiment. It is alsonoticed that the treatment increases concrete alkalinity in thewhole concrete cover, including the layers close to the concretesurface. This behaviour was observed for all studied concretesand conrmed the efciency of electrochemical realkalisationtreatment.

Fig. 8 compares the realkalised fraction of concrete specimensmade with the different cements used in this work and shows that,for a similar total charge passed (about 350 A h=m2concrete), the spec-imen made with cement I present higher realkalised fraction,which corroborates the point that Pozzolan cement-based speci-mens reached lower levels alkalinity during carbonation and needmore charge to restore a similar level of alkalinity.

4.4. Relative alkalinity changes due to the treatment

Additional alkalinity measurements were carried out aiming tohelp on the identication of concrete alkalinity changes nearby thereinforcement and in the outer layers. Reference (REF), carbonated(CARB) and realkalised (REA) concrete specimens were consideredfor this purpose.

Fig. 9 compares the alkalinity of concrete samples extractednearby the reinforcement for REF, CARB and REA specimens. Inthe last case, samples were extracted 30 days after the treatmentwith the objective of representing a more stable condition, regard-ing the polarisation applied during the electrochemical realkalisa-tion and the chemical equilibrium nearby the reinforcement. As pHmeasurements taken from the water equilibrium samples (Sec-tion 3.2) may not accurately represent the pH in concrete solutionporous network, but helps on relative analysis, the relative alkalin-ity was adopted in this approach. Two samples were taken for eachanalysed condition and the average of each pair of samples was ex-pressed as a relative alkalinity value in terms of the highest aver-age pH value observed among all the analysed conditions, whichwas 13.4.

For all studied concretes, specimens subjected to electrochemi-

P.H.L.C. Ribeiro et al. / Construction ancal realkalisation presented higher alkalinity than carbonatedspecimens and quite close to the alkalinity of reference specimens.This shows the efciency of the treatment in recovering theconcrete alkalinity, as observed in previous works [15,19,27]. Onthe other hand, it can also be noticed the inuence of the cementtype. Both concretes C3 and C4 presented lower values for REF,CARB and REA specimens. This can be explained by its lower alka-line reserve (see Fig. 9, reference samples), which makes themreach lower levels of alkalinity during carbonation and afterrealkalisation.

There are also differences of alkalinity between specimensmadewith the same concrete mixture and different reinforcement con-crete covers (C2/10, C2/25 and C4/10, C4/25). These differencescan be related to the skin effect for REF and CARB specimens, whichleads to differences on concrete microstructure in outer layers andalso makes the carbonation of this concrete region easier. Further-more, for REA specimens, the higher total charge demanded to real-kalise a thicker reinforcement concrete cover generates more OH

ions at the reinforcement surrounding region (see Section 4.2).Fig. 10 shows average relative alkalinities at three different

depth bands for concrete specimens C2/25 and C4/25, in a similarway as Fig. 9. These depth bands are related to concrete surfacedistance (08 mm, 816 mm and 1625 mm). The efciency ofthe treatment for recovering concrete alkalinity can be noticedeven at layers close to concrete surface. However, the alkalinitynearby the reinforcement, after the treatment, is a bit higher thanthat observed at the outer layers for C2/25 specimen. This behav-iour is in accordance with a process that starts at reinforcement le-vel and progressively advances towards the specimen surface (seeFig. 7).

4.5. Electrochemical parameters and tendency of reaching passivecondition

Figs. 11 and 12 show the comparative behaviour of corrosionpotential (Fig. 11) and corrosion current density measurements(Fig. 12) during the experiment, considering REF, CARB, REA spec-imens and the classication ranges for these variables regardingthe corrosion process. This classication is based on ASTM C-876[34] and in previous published papers [35].

Before starting realkalisation procedures, depassivated rein-forcements presented corrosion potentials ranging between 535and 640 mV (CSE) due to carbonation and saturation of concretespecimens. At the end of the treatment, corrosion potential dropsto strongly negative values due to the imposed polarisation. How-ever, these potentials gradually become less electronegative reach-ing values close to 250 mV (CSE) 1 month after nishing thetreatment and close to 200 mV (CSE) after 2 months. After6 months, almost all the bars presented potentials less electroneg-ative than 200 mV (CSE).

By corrosion potential measurements, it is possible to see a ten-dency of reinforcement reaching the passive condition. This ten-dency is clearly seen 6 months after the end of the treatment.Before this period, there are still some values in the zone of uncer-tain corrosion (Fig. 11).

Considering the corrosion current density, the reinforcementspresented values between 0.6 and 14 lA/cm2 after concrete car-bonation and saturation. These values increased during the treat-ment due to the imposed polarisation, but started to decreasejust after turning off the system. It presented some uctuation dur-ing the depolarisation period and only 1 year after the treatmentthe corrosion current indicated a tendency of stabilisation of itsvalues. Measurements after 6 months were not taken due to apotentiostat fault at that time.

Fig. 12 shows that, although only some bars presented valuesbelow 0.1 lA/cm2, which indicates the passive condition, most of

ilding Materials 40 (2013) 280290 285the bars reached the low corrosion current level after 12 months(6-month measurements were not taken for this variable as previ-ously explained). This implies in current values signicantly lower,

d Bu286 P.H.L.C. Ribeiro et al. / Construction anwhich means a metal dissolution rate signicantly lower and, con-sequently, a longer residual service life. On the other hand, a ten-dency of reaching values even lower throughout time is notrejected.

Similar behaviour about corrosion current measurements afterrealkalisation treatment was observed in literature [30]. Concern-ing this aspect, Redaelli and Bertolini [30] found that although cor-rosion current density obtained by potentiostatic tests signicantlydecreased over time they do not match the condition of passivesteel in alkaline media. On the other hand, no signicant amounts

Fig. 7. Visual monitoring of realkalisation front advance during the treatment (case of tilding Materials 40 (2013) 280290of corrosion products were found on steel surface after breakingthese same specimens, which indicates that this kind of measure-ment could not be suitable for monitoring the effectiveness of steelrepassivation. However, this aspect is not still a consensus [40].The lack of an effective electro-osmotic ux has been reported asthe reason for not reaching the effectiveness of reinforcementrepassivation in some cases [40]. Considering the present work,the electro-osmotic ux was not clearly observed, but the levelsof corrosion current density even not being enough to reach thepassive condition in most cases, were quite close to this reference.

he C4/25 specimen) 88.5 h (a); 123.5 h (b); 142 h (c); 183.5 h (d) and 207.5 h (e).

d BuC2/25

P.H.L.C. Ribeiro et al. / Construction anThis way, further research is necessary to clarify the interpretationand applicability of corrosion current measurements after electro-chemical realkalisation.

Fig. 13 shows the tendency of reinforcements reaching the pas-sive condition by drawing tendency lines for corrosion potential.

(a)

Cathode

Fig. 8. Differences on realkalised fraction for specimens made with

Fig. 9. Relative alkalinity nearby the reinforcement for reference (REF), carbonated(CARB) and realkalised (REA) concrete specimens.

(a)

(b)

Fig. 10. Relative alkalinity proles in concrete cover for reference (REF), carbonated(CARB) and realkalised (REA) concrete specimens C2/25 (a); C4/25 (b).(b)

C4/25

Cathode

cements I and II and subjected to a similar total charge passed.

ilding Materials 40 (2013) 280290 287Average values and standard deviations representative of eachset of reinforcements used for each experimental condition wereconsidered and only the measurements taken after around 10 daysafter turning off the electrical arrangement were used for ttingthe tendency lines. This was done to avoid the strong polarisationinuence just after the treatment. The linear function was the onethat best tted the presented data. By tendency lines it is possibleto obtain the time required for these values to become less electro-negative than 200 mV (CSE), which is the reference assumed forthe passive condition.

These values are presented in Table 4. A relationship betweentotal charge passed and time for reinforcement repassivation couldbe observed. For same concretes, a higher charge passed meansless time for reinforcement to reach the passive condition. Thehigher total charge demanded (Section 4.2) and the higher relativealkalinity for specimens with higher concrete thickness (Sec-tion 4.3) helps to explain this behaviour. As a consequence, it canbe pointed out that the total charge passed directly inuences con-crete alkalinity and this indirectly inuences the time required forreaching the passive condition.

Comparing concretes made with cements I (C1, C2) and II (C3,C4), it can be seen that although concretes made with cement I de-manded lower total charge passed, they presented shorter time forreaching the passive condition. This can be explainedby the fact thatthese concretes presented higher relative alkalinity after the treat-ment and thus presented better conditions to repassivation. In otherwords, concretes made with Portland pozzolan cement (II) demandmore time for reaching the passive condition of reinforcements.

Fig. 11. Comparative corrosion potential for reference (REF), carbonated (CARB)and realkalised (REA) concrete specimens.

Fig. 12. Comparative corrosion current density for reference (REF), carbonated (CARB) and realkalised (REA) concrete specimens.

(a) (d)

(b) (e)

(c) (f)Fig. 13. Assessment of depolarisation time by means of corrosion potential.

288 P.H.L.C. Ribeiro et al. / Construction and Building Materials 40 (2013) 280290

d Bu5. Conclusions

Water electrolysis seems to be the main reason for the electro-chemical realkalisation of carbonated concretes in this study,which is deduced from the outline of the realkalised area advanceduring the treatment.

The inuence ofwater to cement ratio on this treatment happensin an inverseway to the ohmic resistance of the system, the time re-quired by the treatment and the total charge passed. It means thatas w/c increases, the ohmic resistance decreases, as well as the timeto complete the treatment and the total electrical charge passed.

The inuence of cement type happens on concrete alkalinity, to-tal charge passed required for completing the treatment and con-sequently on the time demanded for it, as well as on the timedemanded for reinforcement to reach a passive condition. Theuse of pozzolanic addition in cement contributed for longer periodsof realkalisation as a consequence of its lower alkalinity reservedue to portlandite consumption in pozzolanic reactions, as wellas for higher total charge passed to complete the treatment. Thislower alkalinity has consequences for reinforcement repassivationtendencies, demanding a longer time to reach this condition ortendency.

The concrete cover thickness inuences the time demanded forcompleting the realkalisation process. A thicker concrete cover de-mands more time and, consequently, more charge to produce en-ough alkalinity for the whole concrete cover region. On the otherhand, this contributes for a shorter time demanded for reachingelectrochemical stabilisation, which can be explained by the higherlevels of alkalinity reached nearby the reinforcement.

Taking into account the efciency of the method about rein-forcement reaching the passive condition, it is possible to say thatthe tendency of reaching this condition is clearer for corrosion po-tential measurements. For corrosion current density measure-ments, although it is not clear, they tend to decrease over time,reaching values not far to this reference and longer service lifeperiods for the structure. However, more monitoring time and fur-ther research are necessary to analyse if it is possible to conrm asimilar tendency with this parameter and if corrosion current ratecould be suitable for this purpose.

Table 4Relationship between total charge passed, relative alkalinity and estimated depolar-isation time.

Specimencodes

Total electrical chargepassed (A h=m2concrete)

Relativealkalinity afterERA

Estimateddepolarisation time(days)

C1/10 328 0.952 98C2/10 238 0.924 66C2/25 401 0.966 59C3/10 360 0.924 115C4/10 310 0.904 73C4/25 499 0.918 65

P.H.L.C. Ribeiro et al. / Construction anAcknowledgments

The authors thank the Brazilian Research Agency (CNPq) forsupporting this research and the engineering electrochemical lab-oratory of the Federal University of Campina Grande for helping onalkalinity analysis.

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Electrochemical realkalisation of carbonated concretes Influence of material characteristics and thickness of concrete reinforcement cover1 Introduction2 Fundamentals of electrochemical realkalisation3 Experimental work3.1 Casting and curing concrete specimens3.2 Accelerated carbonation3.3 Realkalisation procedures3.4 Alkalinity monitoring3.5 Electrochemical measurements

4 Results and discussion4.1 Ohmic resistance of the system4.2 Total electrical charge passed for completing realkalisation4.3 Visual monitoring of realkalisation front advance4.4 Relative alkalinity changes due to the treatment4.5 Electrochemical parameters and tendency of reaching passive condition

5 ConclusionsAcknowledgmentsReferences