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Electrochemical extraction of chlorides from reinforced concrete using a conductive cement paste as the anode
Corrosion Science 52 (2010) 1576–1581
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Electrochemical extraction of chlorides from reinforced concrete using a conductive cement paste as the anode A. Pérez a, M.A. Climent b,*, P. Garcés b a b
Structural Design Engineering (SDE), C/. Santa Hortensia, 15, 28002 Madrid, Spain Departament d’Enginyeria de la Construcció, Obres Públiques i Infrastructura Urbana, Universitat d’Alacant, Ap. Correus 99, 03080 Alacant/Alicante, Spain
a r t i c l e
i n f o
Article history: Received 5 December 2008 Accepted 16 January 2010 Available online 22 January 2010 Keywords: A. Concrete A. Steel A. Conductive cement paste C. Cathodic protection C. Electrochemical extraction of chloride
a b s t r a c t The results of this research show the viability of using a conductive cement paste anode for the electrochemical extraction of chlorides from reinforced concrete with an efficiency similar to that obtained with a classic Ti–RuO2 anode. The obtained efficiencies are within the typical range of values of overall efficiencies found for such treatments. The thickness of the conductive cement paste anode has a great influence on the capacity of the anode to retain an important part of the extracted chlorides after finishing the electrochemical treatments. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The electrochemical extraction of chlorides (EEC) is a nondestructive methodology to prevent corrosion of steel rebar, a principle problem in structural concrete. There is an extensive bibliography on this technique beginning with its initial utilization in the 1970s [1–3]. The method basically consists in applying an electric field between the steel rebar (the negative pole or cathode) and an externally deposited electrode at the concrete surface (the positive pole or anode). Since chlorides (Cl) are negatively charged ions, the imposed force field causes them to migrate from the rebar to the exterior electrode through the concrete pores [4–6]. Nevertheless, some doubts have been expressed concerning features such as end-point determination, side effects and durability of structures submitted to EEC trials [7]. The main crux of research on cement materials used in public works and building has traditionally concentrated on mechanical properties in line with its principal structural function. However, a new tendency has recently emerged along this line of research; the integration of materials with multiple functions [8]. Included among these multifunctional materials are cementitious conductors. They obtain their electrical properties through the addition of conductors like carbonaceous materials [9], for example, graphite powder or carbon fibers. Some potential applications of electrically conducting cementitious materials are: electromagnetic interference shielding (to protect electronics from external radia* Corresponding author. Tel.: +34 965903707; fax: +34 965903678. E-mail address: [email protected] (M.A. Climent). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.01.016
tion interference or avoid electromagnetic forms of spying or terrorism), self heating for deicing of concrete surfaces, and the possible function of strain sensitivity of cement matrix composites under the action of internal structural forces (without embedded sensors). These last applications can include structural vibration control, traffic monitoring, room occupancy monitoring, etc. [8]. Cementitious conductors have been used as anodes for cathodic protection in research [9–12]. A commercial conductive, polymer-modified, cementitious mortar has been successfully used as the anode for applying cathodic protection to more than 40,000 m2 of reinforced concrete [13]. The principal objective of this research is to study the viability of the application of a conductive cement paste (CCP) as an anode for the EEC technique because of the potential advantages it has when compared to the usual anodic systems [14]. Some of these advantages include the possibility of the application as a fine layer, the adaptation to various types of surfaces, and the possibility of reutilization for repeated EEC treatments, which may be necessary for certain structures and conditions [15]. A single EEC trial may be unable to eliminate enough Cl ions, in cases of heavy or progressive chloride contamination of concrete, to reduce permanently the steel corrosion rate below its threshold value of about 0.1–0.2 lA/cm2 [16]. Another interesting possibility is the combination of an initial EEC treatment, followed eventually by cathodic protection applied using the same CCP anode. The second of these actions may be deemed necessary or convenient when analysing the corrosion state of the structure after the EEC trial. Furthermore, the current density needed for the effective cathodic protection of steel, may be significantly lowered as a consequence of the reduction of Cl content of concrete
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near the steel [17], produced by the initial EEC treatment. A disadvantage of using CCP anodes is the impossibility of using the half cell potential mapping technique for assessing the corrosion state of reinforcement after the EEC treatments, since the conductive character of the overlay homogenizes the potential values. A number of secondary objectives arose during our research: the analysis of the effect of the applied charge density, the thickness of the anodic conductive cement paste and the quality of the concrete.
2. Experimental 2.1. Reinforced concrete specimens and conductive cement paste anodes Two concrete mixes were produced, whose dosages and characteristics are shown in Table 1. Concrete mixes A and B were purposefully designed with different cement contents and different water/cement (w/c) ratios in order to study the effect of different concrete quality on the EEC tests performed with the conductive paste anode. An ordinary Portland cement with limestone, CEM II/B-L 32,5 N following Ref. [18], was used for both mixes. The concretes were admixed with NaCl at a dosage of 2% Cl referred to cement mass. The mix used for the conductive cement paste to be used as the anode had a mass composition of 1/3 cement (same cement used for the concretes), 1/3 graphite powder and 1/3 distilled water. The hardened paste showed a porosity of 50.4%, as determined by the hydrostatic balance method [19]; and an electrolytic resistivity of 6 (Xm), as measured on a 40 40 160 mm paste specimen equilibrated with a 100% relative humidity (RH) atmosphere. The concrete blocks fabricated for the EEC tests, had dimensions of 180 180 120 mm (see Fig. 1). They were cast in appropriate wooden moulds. A steel mesh was embedded within each block at medium height, i.e. at 60 mm from the bottom. The square shaped mesh was obtained by welding 12 plain steel bars (six in each perpendicular direction), of 2 mm diameter, and 160 mm length, so that the square shaped openings of the mesh had approximate dimension of 30 mm. The electric contact to the mesh was provided by copper wires welded to the outermost bars of the mesh. These weldings were protected with coatings of epoxy resin. After pouring and compacting the fresh concrete into the moulds, a bottomless bucket made of PVC was partially sunk in the upper surface of each block, to form a reservoir in order to introduce later the CCP anode and the electrolyte for the EEC tests. The concrete blocks were cured for 28 days in a humidity chamber at 20 °C and 95% RH. After curing, the CCP anode was applied superficially like a plaster, with four embedded graphite primary anodes (6 mm diameter, 100 mm length), which provided the current leads for connecting the CCP anode to the current source during the EEC
Table 1 Dosages and characteristics of the concrete mixes tested. Materials or properties
Dosages expressed in kg/m3 Cementa Distilled water Limestone sand Limestone gravel (5–20 mm) Plasticizerb NaCl (2% Cl ref. cement mass) Water/cement ratio Mean compressive strength @ 28 days (MPa) a b
Concrete A
B
195.0 195.0 1083.2 1050.0 – 6.43 1.0 7.6
341.6 170.8 950.0 921.6 1.37 11.26 0.5 43.0
Ordinary Portland cement with limestone, CEM II/B-L 32,5 N following Ref. [14]. Plastiment HP-1, Sika.
Fig. 1. Upper (A) and side (B) views of the concrete blocks and experimental setup for the EEC tests performed with the conductive cement paste anode.
experiments, see Fig. 1. The ratio between the surface of concrete covered by the CCP overlay and the surface of the primary anodes was 5.1. The thickness of the CCP layer was 20 mm or alternatively 7 mm. The lateral and bottom faces of the blocks were covered with a sealing mortar (Thoroseal Grey, Construction Chemicals). The cement paste and the sealing mortar were allowed to set for an additional 7 days in the humidity chamber. Some concrete blocks were left without the CCP anode in order to perform reference EEC tests with a standard anode: a Ti–RuO2 wire mesh previously used with satisfactory results [14]. 2.2. Electrochemical extraction treatments The starting electrolyte used for the EEC treatments was distilled water, which was placed on the CCP anode. In the case of the reference tests the Ti–RuO2 mesh anode was immersed in the electrolyte placed on the concrete surface. The electrolyte level was kept constant during the current passing periods. All EEC trials were performed galvanostatically, at a constant current density of 2 A/m2. This area relates to the exposed concrete surface, which is equal to the anode surface area, i.e. 16 16 cm2. The applied charge densities were 1 106 C/m2 for the first treatment, and 4 106 C/m2 for the second treatment (for a total of 5 106 C/ m2). The duration of the treatments was approximately 139 and 556 h, respectively. After finishing the first treatment the electrolyte was removed and the blocks stored for 7 days in the humidity chamber before starting the second treatment. In some cases this rest period was prolonged to two months in order to investigate the possibility of Cl redistributions between the CCP overlay and concrete after finishing the EEC treatment. 2.3. Determination of the efficiencies of the electrochemical extraction treatments To determine the efficiency of the EEC trials in decreasing the Cl concentration of the blocks, powdered samples were obtained
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at selected depths from the concrete surface subjected to EEC, or from the CCP overlay surface, and analysed for their Cl contents. These samples were obtained before EEC and after finishing the extraction (5 106 C/m2). In some cases we obtained also samples after the first EEC step (1 106 C/m2), and after the rest period in the humidity chamber. The sampling was performed by dry drilling [20] with a rotary hammer equipped with a 22 mm-diameter masonry bit. The sampling positions were at the geometrical centre of the upper face of the blocks, or at the centres of the quadrants defined by the graphite electrodes, see Fig. 1. Previous tests had indicated that all these sampling positions were equivalent in terms of chloride content reduction efficiency [21]. The drilling step length was set at 5 mm, in order to get detailed chloride content profiles, and the drilling was performed up to levels near the steel reinforcement. The Cl profiles through the steel concrete cover were composed of 11 points, and those through the CCP overlay were composed of two or four points. The determination of acid-soluble Cl contents of the concrete samples was performed by potentiometric titration [22,23]. All Cl concentrations of concrete or CCP in this work refer to acid-soluble chlorides, and are expressed as percentages, relative to total mass. Electrolyte samples were also taken after each EEC step, and analysed as well for their Cl concentrations, in order to know the amount of chlorides brought into solution. By integration of the Cl content profiles of concrete and CCP one obtains the amounts of chloride present in each one of these materials, before and after EEC. The Cl content distributions after EEC, in concrete, CCP and electrolyte, are calculated in terms of percentages referred to the initial chloride content of the concrete cover zone, i.e. the upper 60 mm of the concrete blocks, see Fig. 1B. This allows a direct calculation of the efficiency in decreasing the Cl content of concrete.
Fig. 2. Chloride contents before (d), and after EEC tests performed on concrete type A blocks. Applied charge density: 1 106 C/m2. () Ti–RuO2 anode. (s) CCP anode (20 mm width layer). Vertical solid line indicates separation between CCP anode and concrete. Dashed vertical line indicates position of the steel reinforcement (cathode).
3. Results and discussion Figs. 2–4 show the chloride content profiles before and after EEC trials performed on the tested concrete blocks. The profiles have been drawn considering the origin of depths located at the surface of the CCP overlays, since the sampling has been performed through the CCP anode width, and through the concrete, up to levels near the steel reinforcement, located at half the height (60 mm) of the concrete blocks, see Fig. 1. The vertical solid line located at depths of 20 mm for Figs. 2 and 3, and at 7 mm for Fig. 4, indicates the separation between the CCP anode and the concrete. Typical initial profiles are shown in Figs. 2–4. The mean initial Cl contents are 0.24% and 0.31%, relative to total mass, for concretes A and B, respectively, which correspond approximately to dosages of 2% Cl referred to cement mass in each case, see Table 1. The abovementioned Figures also show the profiles corresponding to concrete blocks subjected to EEC trials. Table 2 contains the relevant experimental details of the tests, together with the Cl content distributions, calculated by integration of the Cl profiles, and the EEC efficiencies. All these contents and efficiencies are expressed as percentages of the initial chloride present in the concrete cover zone of the steel, i.e. the upper 60 mm of the concrete blocks (Fig. 1B). The balance is calculated as the difference between the total initial Cl content in the concrete cover zone, and the sum of the Cl amounts found in each phase after EEC. The physical meaning of the balance value is the difference between the Cl ions eliminated by electrochemical oxidation, (with evolution of Cl2 gas), at the anode [4–6], and the Cl ions migrated from the concrete zone behind the steel reinforcement to the concrete cover, due to the electric field [3,14]. This can explain the negative value of this balance obtained for one of the tests, see Table 2.
Fig. 3. Chloride contents after EEC tests performed on concrete type A blocks, using CCP anodes. Applied charge density: 5 106 C/m2. () CCP anodic layer of 7 mm width. (h) CCP anodic layer of 20 mm width. Vertical solid line at 13 mm depth indicates separation between CCP anode and electrolyte, refer only to () data. Vertical solid line at 20 mm depth indicates separation between CCP anodes and concrete. Dashed vertical line indicates position of the steel reinforcement (cathode). The initial chloride profile (j) corresponds to the test with the CCP anodic layer of 20 mm width.
For one of the tested specimens complete duplicate sets of powdered concrete samples were taken after finishing the EEC trial, from equivalent sampling positions, in order to ascertain if the determined Cl contents corresponding to the same depth, and re-
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Fig. 4. Chloride contents before (N), and after (4) EEC tests performed on concrete type B blocks, using CCP anode (7 mm width layer). Applied charge density: 5 106 C/m2. Vertical solid line indicates separation between CCP anode and concrete. Dashed vertical line indicates position of the steel reinforcement (cathode).
lated calculations of final chloride distributions and EEC efficiencies, were repetitive. This specimen was a concrete A block tested with a 7 mm thick CCP overlay, by passing 1 106 C/m2. The coefficients of variation of the measured chloride concentrations of the samples obtained for the same depth intervals, ranged from 0% to 24%, with a mean value of 11.7%. These coefficients of variation are higher than those associated only with the chloride analysis procedure (repeated analysis of the same sample), which range from 0.8% to 7%, with a mean value of 3.2% [22]. This difference can be attributed to the different amounts of aggregate in the samples obtained by dry drilling. From both sets of chloride content data the final Cl distributions (in concrete and CCP) and the EEC efficiencies were calculated. Regarding the dispersion of these calculations, due to using one or the other set of Cl content data, the coefficients of variation of the integrated Cl contents of concrete and CCP anode (after EEC), and of the extraction efficiency were: 7.0%, 9.5% and 9.0%, respectively. The found dispersion is not very high, and allows to consider the calculated chloride distributions and EEC efficiencies shown in Table 2, as reasonably repetitive and precise.
3.1. Comparison of results obtained with traditional and new anodes The performances of both anodes used, the Ti–RuO2 mesh and the CCP, can be compared in Fig. 2 and Table 2. The Cl profiles ob-
tained after applying charge densities of 1 106 C/m2, indicate that the Cl contents through the concrete cover are slightly higher for the Ti–RuO2 anode than those corresponding to the CCP anode, and a considerable accumulation of Cl ions is observed in the profile obtained through the 20 mm thick CCP layer (Fig. 2). The efficiency of the CCP anode in extracting the Cl from concrete is about 51%, a slightly higher value than the 41% value obtained with the Ti–RuO2 anode (Table 2). The final distributions of Cl after these EEC tests indicate that both anodes have eliminated high amounts of Cl by electrolysis, being the final Cl concentrations at the electrolytes negligible, but the CCP anode has accumulated an elevated percentage of the initial Cl (32%) after the EEC trial. These results confirm the possibility of using a CCP anode for EEC, with an efficiency in removing chloride ions from concrete, which may be similar or slightly higher than that obtained with a classic Ti–RuO2 mesh anode. Nevertheless, the big accumulation of Cl at CCP and the presence of a strong Cl concentration gradient at the CCP-concrete interface, lead to the possibility of a concentration redistribution by diffusion [24], especially if the concrete is exposed to a humid environment after EEC [25,26]. This redistribution could increase the Cl contents at the concrete cover zone and at the steel depth. The possibility of this redistribution, for the particular conditions of this work, has been confirmed with a companion specimen subjected to EEC in the same conditions and later allowed to rest during two months in the humidity chamber. In this case, (not included in Table 2), the Cl contents, after EEC, at the concrete cover and at the CCP anode were 54% and 24% of the initial Cl present in the concrete cover zone, respectively; while after the two months rest these Cl concentrations changed to 70% and 18%, respectively. The difference between the absolute values of the variations of Cl content by redistribution, at concrete and CCP, may be adequately explained by the contribution of Cl diffusion from the concrete layers behind the steel to the concrete cover zone. 3.2. Influence of the thickness of the conductive cement paste anode The abovementioned redistribution of Cl after EEC is a negative effect to be avoided, since it may reactivate the corrosion of steel. It involves the need to remove the CCP overlay after the treatment, thus losing one of its most interesting characteristics, i.e. the possibility of re-use. A modification of the experimental procedure was tried in order to avoid the accumulation of chlorides at the CCP overlay: the decrease of its width, from 20 mm to 7 mm. Fig. 3 shows the Cl profiles at the end of EEC treatments (5 106 C/m2) performed with CCP anodes of the mentioned widths. The profiles through the concrete cover zones are almost identical, but the profiles through the CCP layers differ markedly: the 7 mm thick anode shows Cl contents similar to those found in the concrete cover, while the Cl concentrations at the 20 mm thick CCP anode are as high as 1% approximately for a significant part of its width. The Cl accumulations for the 20 mm and 7 mm width CCP anodes are 61% and 5%, respectively (Table 2), while the EEC efficiencies are similar for the 7 mm anode (65%)
Table 2 Final chloride content distributions and net efficiencies of the EEC tests. The chloride contents and EEC efficiencies are expressed as percentages of the initial chloride present in the concrete cover zone. The estimated uncertainties have been calculated on the basis of the mean values obtained for the coefficients of variation of the integrated chloride contents in concrete and CCP anode (after EEC) and that of the efficiencies. See text in Section 3 for details. Type of concrete
A A A A B
Anode
Ti–RuO2 CCP CCP CCP CCP
Applied charge density (C/m2)
1 106 1 106 5 106 5 106 5 106
CCP layer width (mm)
– 20 20 7 7
Chloride content distribution after EEC
EEC efficiency (%)
Concrete (%)
Anode (%)
Electrolyte (%)
Balance (%)
59 ± 4.1 49 ± 3.4 40 ± 2.8 35 ± 2.5 59 ± 4.1
– 32 ± 3 61 ± 5.8 5.4 ± 0.5 6.6 ± 0.6
2.2 0.02 4.7 26 33
38.8 19.0 5.7 33.6 1.4
41 ± 3.7 51 ± 4.6 60 ± 5.4 65 ± 5.9 41 ± 3.7
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and for the 20 mm anode (60%). The amount of Cl ions brought into the electrolyte is high for the 7 mm anode (26%), and the amount of chlorides probably eliminated by electrolysis is also considerable for the 7 mm anode (balance of 33%). These data suggest that reducing the CCP thickness is a viable way of decreasing its capacity to accumulate chlorides, making it easier for Cl ions to pass through to electrolyte. The EEC tests were also performed with a 7 mm thick CCP overlay put on a block of concrete B, see Table 1. Fig. 4 shows the initial and final (after passing 5 106 C/m2) Cl profiles corresponding to these EEC treatments. It is interesting to note that the accumulation of chlorides at the CCP layer is also low (6.6%), and practically equal to that obtained in the test performed with a block of concrete A (5.4%), see Table 2. The overall efficiency for EEC on concrete B is 41%. This is, as expected, a lower value than that obtained for concrete A (65%), but it is within the usual range of values of overall efficiencies found for EEC treatments [6]. These results show the viability of the application of a CCP anode for EEC treatments on specimens or structures fabricated with a concrete mix similar to those used in usual construction practice [27]. 3.3. Stability of the conductive cement paste anodes Bertolini et al. [11] studied the damage produced on a cementitious conductive mortar anode used for cathodic protection, due to the acidity produced by the anodic reaction which led to dissolution of the cement matrix. This brought about the loss of electrical continuity between the primary anode and the conductive fibres, and an uneven current distribution on the reinforcement steel bars. These authors demonstrated that the damage was produced for current densities higher than 20 mA/m2, and that a sharp increase of the feeding voltage can be used as a simple parameter for monitoring the damage of the anode [11]. In the present study, the current density used for the EEC tests has been 2 A/m2, and some dissolution of the paste, especially around the graphite electrodes, was visually observed for the 7 mm thick CCP anode, after the 5 106 C/m2 EEC treatment. Nevertheless, for the trials performed in this work no cracking or delamination of the CCP overlay was observed after EEC. Furthermore, no sharp increase of the feeding voltage was observed during the treatments. The typical increases of the feeding voltages were as follows: for concrete A the initial feeding voltage was about 5 V, increasing to about 6 V at the end of the first treatment (1 106 C/m2), and reaching values of about 10 V at the end of the test (5 106 C/m2); while for concrete B the initial feeding voltage was about 8 V, increasing to about 9.5 V at the end of the first treatment (1 106 C/m2), and reaching values of about 17 V at the end of the test (5 106 C/m2). These increases, which were always slow and gradual variations without sharp steps, may be considered as usual values for classic EEC treatments performed with external anodes and liquid electrolytes [28,29], and suggest that no important damage of the CCP overlay has been produced during EEC, or at least the damage has not led to a malfunction of the anode, even though the applied current density is two orders of magnitude higher than the safe value indicated by Bertolini et al. (20 mA/m2). This different behaviour may be adequately explained taking into account that the EEC tests are performed under continuous ponding conditions (anode immersed in electrolyte), and their maximum duration is about 30 days. Bertolini et al. [11] observed that the damages to the conductive cement mortar were quickly produced under dry conditions (most prevalent in cathodic protection applications), but were largely delayed in tests performed in solution. Furthermore, even being damaged, the anode can function satisfactorily if it is immersed in a concentrated saline solution [11]. Nevertheless, more research is needed to ascertain if the CCP overlays used for EEC may be seriously damaged during repeated treatments. It is also necessary to
investigate the influence of substituting the continuous immersion condition during the EEC test (the case studied in this work), by other possibilities such as the use of solid electrolytes (cellulosic pulp mixtures or others), or the use of wetted polymeric layers fixed to the CCP overlay, on the performance and possible malfunctions of the electrochemical system. 4. Conclusions The results of this work show the viability of using a conductive cement paste as an anode for electrochemical chloride extraction from reinforced concrete specimens, with an efficiency similar to that obtained with a classic Ti–RuO2 mesh anode. When the CCP anode is used for EEC treatments on specimens fabricated with a concrete mix similar to those used in usual construction practice, the efficiency is within the typical range of values of overall efficiencies found for EEC treatments. When a 20 mm thick CCP anode is used for an EEC treatment, a high portion of the Cl initially present in the concrete cover zone (up to 61% for a 5 106 C/m2 treatment), is accumulated at the anode. These Cl ions could, in turn, re-diffuse into the concrete cover, and eventually could reactivate the steel corrosion. The reduction of the width of the CCP layer to 7 mm decreases its capacity to accumulate chlorides to values of about 6% of the Cl initially present in the concrete cover zone. The reduction of the thickness of the CCP anode to 7 mm does not decrease the EEC efficiency, which is similar to that obtained with a 20 mm thick CCP anode. A CCP anodic overlay can experience some damage during electrochemical treatments, due to the acidity produced by the anodic reaction. Nevertheless, for the EEC trials performed in this work, no sharp increase of the feeding voltage was observed, indicating that no important damage was suffered by the CCP during the tests, or at least the damage did not lead to a malfunction of the anodic system. More research is needed to ascertain if the CCP overlays used for EEC may be seriously damaged during repeated treatments, and if this damage can produce malfunctions of the electrochemical system. Acknowledgements This work has been financially supported by the Ministerio de Educación y Ciencia of Spain and Fondo Europeo de Desarrollo Regional (FEDER) through projects BIA2006-05961 and BIA200610703, and by the Ministerio de Fomento of Spain (C63/2006). We thank Dr. C. Axness for help with the English version of the text. References [1] D.R. Lankard, J.E. Slater, W.A. Hedden, D.E. Niesz, Neutralization of chloride in concrete, Battelle Columbus Laboratories, Federal Highway Administration (USA), Report No. FHWA-RD-76-60, 1975, 143 pp. [2] J.E. Slater, D.R. Lankard, P.J. Moreland, Electrochemical removal of chlorides from concrete bridge decks, Mater. Perform. 15 (1976) 21–26. [3] G.L. Morrison, Y.P. Virmani, F.W. Stratton, W.J. Gilliland, Chloride removal and monomer impregnation of bridge deck concrete by electro-osmosis, Kansas Department of Transportation, Federal Highway Administration (USA), Report No. FHWA-Ks-RD-74-1, 1976, 38 pp. [4] C. Andrade, M. Castellote, C. Alonso, An overview of electrochemical realkalisation and chloride extraction, in: D.W.S. Ho, I. Godson, F. Collins, (Eds.), Proceedings of 2nd Int. RILEM/CSIRO/ACRA Conference on Rehabilitation of Structures, Melbourne, Australia, RILEM Publications, Cachan, France, 1998, pp. 1–12. [5] J. Tritthart, Electrochemical chloride removal: an overview and scientific aspects, in: J. Skalny, S. Mindess (Eds.), Materials Science of Concrete V, American Ceramic Society, Westerville, Ohio, USA, 1998, pp. 401–441. [6] Electrochemical rehabilitation methods for reinforced concrete structures. A state of the art report, in: J. Mietz, (Ed.), Publication Number 24 of the
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[20] M.A. Climent, G. de Vera, E. Viqueira, Bit shape geometric considerations when sampling by dry drilling for obtaining chloride profiles in concrete, Mater. Struct. 34 (237) (2001) 150–154. [21] A. Pérez, Extracción electroquímica de cloruros del hormigón armado utilizando una pasta de cemento conductora (electrochemical extraction of chlorides from reinforced concrete using a conductive cement paste), (only available in Spanish), Civil Eng., Degree Thesis, University of Alacant, Alacant, Spain, 2004. [22] M.A. Climent, E. Viqueira, G. de Vera, M.M. López, Analysis of acid-soluble chloride in cement, mortar and concrete by potentiometric titration without filtration steps, Cem. Concr. Res. 29 (6) (1999) 893–898. [23] M.A. Climent, G. de Vera, E. Viqueira, M.M. López, Generalization of the possibility of eliminating the filtration step in the determination of acidsoluble chloride content in cement and concrete by potentiometric titration, Cem. Concr. Res. 34 (12) (2004) 2291–2295. [24] B.T.J. Stoop, R.B. Polder, Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms, in: C.L. Page, P.B. Bamforth, J.W. Figg (Eds.), Proc. of the 4th Int. Symposium on Corrosion of Reinforcement in Concrete Construction, The Royal Society of Chemistry, Cambridge, UK, 1996, pp. 456–465. [25] M.A. Climent, E. Viqueira, G. de Vera, M.M. López, Chloride contamination of concrete by interaction with PVC combustion gases, Cem. Concr. Res. 28 (2) (1998) 209–219. [26] M.A. Climent, G. de Vera, J.F. López, E. Viqueira, C. Andrade, A test method for measuring chloride diffusion coefficients through non-saturated concrete. Part I: the instantaneous plane source diffusion case, Cem. Concr. Res. 32 (7) (2002) 1113–1123. [27] P. Garcés, M.A. Climent, A. Pérez, Procedimiento de extracción electroquímica de cloruros de hormigón armado utilizando como ánodo una pasta de cemento conductora, (only available in Spanish), Patent application number P200601977, Oficina Española de Patentes y Marcas, Ministerio de Educación y Ciencia, Madrid, Spain, 10 pp. [28] W.K. Green, S.B. Lyon, J.D. Scantlebury, Electrochemical changes in chloride contaminated reinforced concrete following cathodic polarisation, Corros. Sci. 35 (5–8) (1993) 1627–1631. [29] P. Garcés, M.J. Sánchez de Rojas, M.A. Climent, Effect of reinforcement bar arrangement on the efficiency of electrochemical chloride removal technique applied to reinforced concrete structures, Corros. Sci. 48 (3) (2006) 531–545.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Electrochemical extraction of chlorides from reinforced concrete using a conductive cement paste as the anode A. Pérez a, M.A. Climent b,*, P. Garcés b a b
Structural Design Engineering (SDE), C/. Santa Hortensia, 15, 28002 Madrid, Spain Departament d’Enginyeria de la Construcció, Obres Públiques i Infrastructura Urbana, Universitat d’Alacant, Ap. Correus 99, 03080 Alacant/Alicante, Spain
a r t i c l e
i n f o
Article history: Received 5 December 2008 Accepted 16 January 2010 Available online 22 January 2010 Keywords: A. Concrete A. Steel A. Conductive cement paste C. Cathodic protection C. Electrochemical extraction of chloride
a b s t r a c t The results of this research show the viability of using a conductive cement paste anode for the electrochemical extraction of chlorides from reinforced concrete with an efficiency similar to that obtained with a classic Ti–RuO2 anode. The obtained efficiencies are within the typical range of values of overall efficiencies found for such treatments. The thickness of the conductive cement paste anode has a great influence on the capacity of the anode to retain an important part of the extracted chlorides after finishing the electrochemical treatments. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The electrochemical extraction of chlorides (EEC) is a nondestructive methodology to prevent corrosion of steel rebar, a principle problem in structural concrete. There is an extensive bibliography on this technique beginning with its initial utilization in the 1970s [1–3]. The method basically consists in applying an electric field between the steel rebar (the negative pole or cathode) and an externally deposited electrode at the concrete surface (the positive pole or anode). Since chlorides (Cl) are negatively charged ions, the imposed force field causes them to migrate from the rebar to the exterior electrode through the concrete pores [4–6]. Nevertheless, some doubts have been expressed concerning features such as end-point determination, side effects and durability of structures submitted to EEC trials [7]. The main crux of research on cement materials used in public works and building has traditionally concentrated on mechanical properties in line with its principal structural function. However, a new tendency has recently emerged along this line of research; the integration of materials with multiple functions [8]. Included among these multifunctional materials are cementitious conductors. They obtain their electrical properties through the addition of conductors like carbonaceous materials [9], for example, graphite powder or carbon fibers. Some potential applications of electrically conducting cementitious materials are: electromagnetic interference shielding (to protect electronics from external radia* Corresponding author. Tel.: +34 965903707; fax: +34 965903678. E-mail address: [email protected] (M.A. Climent). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.01.016
tion interference or avoid electromagnetic forms of spying or terrorism), self heating for deicing of concrete surfaces, and the possible function of strain sensitivity of cement matrix composites under the action of internal structural forces (without embedded sensors). These last applications can include structural vibration control, traffic monitoring, room occupancy monitoring, etc. [8]. Cementitious conductors have been used as anodes for cathodic protection in research [9–12]. A commercial conductive, polymer-modified, cementitious mortar has been successfully used as the anode for applying cathodic protection to more than 40,000 m2 of reinforced concrete [13]. The principal objective of this research is to study the viability of the application of a conductive cement paste (CCP) as an anode for the EEC technique because of the potential advantages it has when compared to the usual anodic systems [14]. Some of these advantages include the possibility of the application as a fine layer, the adaptation to various types of surfaces, and the possibility of reutilization for repeated EEC treatments, which may be necessary for certain structures and conditions [15]. A single EEC trial may be unable to eliminate enough Cl ions, in cases of heavy or progressive chloride contamination of concrete, to reduce permanently the steel corrosion rate below its threshold value of about 0.1–0.2 lA/cm2 [16]. Another interesting possibility is the combination of an initial EEC treatment, followed eventually by cathodic protection applied using the same CCP anode. The second of these actions may be deemed necessary or convenient when analysing the corrosion state of the structure after the EEC trial. Furthermore, the current density needed for the effective cathodic protection of steel, may be significantly lowered as a consequence of the reduction of Cl content of concrete
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near the steel [17], produced by the initial EEC treatment. A disadvantage of using CCP anodes is the impossibility of using the half cell potential mapping technique for assessing the corrosion state of reinforcement after the EEC treatments, since the conductive character of the overlay homogenizes the potential values. A number of secondary objectives arose during our research: the analysis of the effect of the applied charge density, the thickness of the anodic conductive cement paste and the quality of the concrete.
2. Experimental 2.1. Reinforced concrete specimens and conductive cement paste anodes Two concrete mixes were produced, whose dosages and characteristics are shown in Table 1. Concrete mixes A and B were purposefully designed with different cement contents and different water/cement (w/c) ratios in order to study the effect of different concrete quality on the EEC tests performed with the conductive paste anode. An ordinary Portland cement with limestone, CEM II/B-L 32,5 N following Ref. [18], was used for both mixes. The concretes were admixed with NaCl at a dosage of 2% Cl referred to cement mass. The mix used for the conductive cement paste to be used as the anode had a mass composition of 1/3 cement (same cement used for the concretes), 1/3 graphite powder and 1/3 distilled water. The hardened paste showed a porosity of 50.4%, as determined by the hydrostatic balance method [19]; and an electrolytic resistivity of 6 (Xm), as measured on a 40 40 160 mm paste specimen equilibrated with a 100% relative humidity (RH) atmosphere. The concrete blocks fabricated for the EEC tests, had dimensions of 180 180 120 mm (see Fig. 1). They were cast in appropriate wooden moulds. A steel mesh was embedded within each block at medium height, i.e. at 60 mm from the bottom. The square shaped mesh was obtained by welding 12 plain steel bars (six in each perpendicular direction), of 2 mm diameter, and 160 mm length, so that the square shaped openings of the mesh had approximate dimension of 30 mm. The electric contact to the mesh was provided by copper wires welded to the outermost bars of the mesh. These weldings were protected with coatings of epoxy resin. After pouring and compacting the fresh concrete into the moulds, a bottomless bucket made of PVC was partially sunk in the upper surface of each block, to form a reservoir in order to introduce later the CCP anode and the electrolyte for the EEC tests. The concrete blocks were cured for 28 days in a humidity chamber at 20 °C and 95% RH. After curing, the CCP anode was applied superficially like a plaster, with four embedded graphite primary anodes (6 mm diameter, 100 mm length), which provided the current leads for connecting the CCP anode to the current source during the EEC
Table 1 Dosages and characteristics of the concrete mixes tested. Materials or properties
Dosages expressed in kg/m3 Cementa Distilled water Limestone sand Limestone gravel (5–20 mm) Plasticizerb NaCl (2% Cl ref. cement mass) Water/cement ratio Mean compressive strength @ 28 days (MPa) a b
Concrete A
B
195.0 195.0 1083.2 1050.0 – 6.43 1.0 7.6
341.6 170.8 950.0 921.6 1.37 11.26 0.5 43.0
Ordinary Portland cement with limestone, CEM II/B-L 32,5 N following Ref. [14]. Plastiment HP-1, Sika.
Fig. 1. Upper (A) and side (B) views of the concrete blocks and experimental setup for the EEC tests performed with the conductive cement paste anode.
experiments, see Fig. 1. The ratio between the surface of concrete covered by the CCP overlay and the surface of the primary anodes was 5.1. The thickness of the CCP layer was 20 mm or alternatively 7 mm. The lateral and bottom faces of the blocks were covered with a sealing mortar (Thoroseal Grey, Construction Chemicals). The cement paste and the sealing mortar were allowed to set for an additional 7 days in the humidity chamber. Some concrete blocks were left without the CCP anode in order to perform reference EEC tests with a standard anode: a Ti–RuO2 wire mesh previously used with satisfactory results [14]. 2.2. Electrochemical extraction treatments The starting electrolyte used for the EEC treatments was distilled water, which was placed on the CCP anode. In the case of the reference tests the Ti–RuO2 mesh anode was immersed in the electrolyte placed on the concrete surface. The electrolyte level was kept constant during the current passing periods. All EEC trials were performed galvanostatically, at a constant current density of 2 A/m2. This area relates to the exposed concrete surface, which is equal to the anode surface area, i.e. 16 16 cm2. The applied charge densities were 1 106 C/m2 for the first treatment, and 4 106 C/m2 for the second treatment (for a total of 5 106 C/ m2). The duration of the treatments was approximately 139 and 556 h, respectively. After finishing the first treatment the electrolyte was removed and the blocks stored for 7 days in the humidity chamber before starting the second treatment. In some cases this rest period was prolonged to two months in order to investigate the possibility of Cl redistributions between the CCP overlay and concrete after finishing the EEC treatment. 2.3. Determination of the efficiencies of the electrochemical extraction treatments To determine the efficiency of the EEC trials in decreasing the Cl concentration of the blocks, powdered samples were obtained
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at selected depths from the concrete surface subjected to EEC, or from the CCP overlay surface, and analysed for their Cl contents. These samples were obtained before EEC and after finishing the extraction (5 106 C/m2). In some cases we obtained also samples after the first EEC step (1 106 C/m2), and after the rest period in the humidity chamber. The sampling was performed by dry drilling [20] with a rotary hammer equipped with a 22 mm-diameter masonry bit. The sampling positions were at the geometrical centre of the upper face of the blocks, or at the centres of the quadrants defined by the graphite electrodes, see Fig. 1. Previous tests had indicated that all these sampling positions were equivalent in terms of chloride content reduction efficiency [21]. The drilling step length was set at 5 mm, in order to get detailed chloride content profiles, and the drilling was performed up to levels near the steel reinforcement. The Cl profiles through the steel concrete cover were composed of 11 points, and those through the CCP overlay were composed of two or four points. The determination of acid-soluble Cl contents of the concrete samples was performed by potentiometric titration [22,23]. All Cl concentrations of concrete or CCP in this work refer to acid-soluble chlorides, and are expressed as percentages, relative to total mass. Electrolyte samples were also taken after each EEC step, and analysed as well for their Cl concentrations, in order to know the amount of chlorides brought into solution. By integration of the Cl content profiles of concrete and CCP one obtains the amounts of chloride present in each one of these materials, before and after EEC. The Cl content distributions after EEC, in concrete, CCP and electrolyte, are calculated in terms of percentages referred to the initial chloride content of the concrete cover zone, i.e. the upper 60 mm of the concrete blocks, see Fig. 1B. This allows a direct calculation of the efficiency in decreasing the Cl content of concrete.
Fig. 2. Chloride contents before (d), and after EEC tests performed on concrete type A blocks. Applied charge density: 1 106 C/m2. () Ti–RuO2 anode. (s) CCP anode (20 mm width layer). Vertical solid line indicates separation between CCP anode and concrete. Dashed vertical line indicates position of the steel reinforcement (cathode).
3. Results and discussion Figs. 2–4 show the chloride content profiles before and after EEC trials performed on the tested concrete blocks. The profiles have been drawn considering the origin of depths located at the surface of the CCP overlays, since the sampling has been performed through the CCP anode width, and through the concrete, up to levels near the steel reinforcement, located at half the height (60 mm) of the concrete blocks, see Fig. 1. The vertical solid line located at depths of 20 mm for Figs. 2 and 3, and at 7 mm for Fig. 4, indicates the separation between the CCP anode and the concrete. Typical initial profiles are shown in Figs. 2–4. The mean initial Cl contents are 0.24% and 0.31%, relative to total mass, for concretes A and B, respectively, which correspond approximately to dosages of 2% Cl referred to cement mass in each case, see Table 1. The abovementioned Figures also show the profiles corresponding to concrete blocks subjected to EEC trials. Table 2 contains the relevant experimental details of the tests, together with the Cl content distributions, calculated by integration of the Cl profiles, and the EEC efficiencies. All these contents and efficiencies are expressed as percentages of the initial chloride present in the concrete cover zone of the steel, i.e. the upper 60 mm of the concrete blocks (Fig. 1B). The balance is calculated as the difference between the total initial Cl content in the concrete cover zone, and the sum of the Cl amounts found in each phase after EEC. The physical meaning of the balance value is the difference between the Cl ions eliminated by electrochemical oxidation, (with evolution of Cl2 gas), at the anode [4–6], and the Cl ions migrated from the concrete zone behind the steel reinforcement to the concrete cover, due to the electric field [3,14]. This can explain the negative value of this balance obtained for one of the tests, see Table 2.
Fig. 3. Chloride contents after EEC tests performed on concrete type A blocks, using CCP anodes. Applied charge density: 5 106 C/m2. () CCP anodic layer of 7 mm width. (h) CCP anodic layer of 20 mm width. Vertical solid line at 13 mm depth indicates separation between CCP anode and electrolyte, refer only to () data. Vertical solid line at 20 mm depth indicates separation between CCP anodes and concrete. Dashed vertical line indicates position of the steel reinforcement (cathode). The initial chloride profile (j) corresponds to the test with the CCP anodic layer of 20 mm width.
For one of the tested specimens complete duplicate sets of powdered concrete samples were taken after finishing the EEC trial, from equivalent sampling positions, in order to ascertain if the determined Cl contents corresponding to the same depth, and re-
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Fig. 4. Chloride contents before (N), and after (4) EEC tests performed on concrete type B blocks, using CCP anode (7 mm width layer). Applied charge density: 5 106 C/m2. Vertical solid line indicates separation between CCP anode and concrete. Dashed vertical line indicates position of the steel reinforcement (cathode).
lated calculations of final chloride distributions and EEC efficiencies, were repetitive. This specimen was a concrete A block tested with a 7 mm thick CCP overlay, by passing 1 106 C/m2. The coefficients of variation of the measured chloride concentrations of the samples obtained for the same depth intervals, ranged from 0% to 24%, with a mean value of 11.7%. These coefficients of variation are higher than those associated only with the chloride analysis procedure (repeated analysis of the same sample), which range from 0.8% to 7%, with a mean value of 3.2% [22]. This difference can be attributed to the different amounts of aggregate in the samples obtained by dry drilling. From both sets of chloride content data the final Cl distributions (in concrete and CCP) and the EEC efficiencies were calculated. Regarding the dispersion of these calculations, due to using one or the other set of Cl content data, the coefficients of variation of the integrated Cl contents of concrete and CCP anode (after EEC), and of the extraction efficiency were: 7.0%, 9.5% and 9.0%, respectively. The found dispersion is not very high, and allows to consider the calculated chloride distributions and EEC efficiencies shown in Table 2, as reasonably repetitive and precise.
3.1. Comparison of results obtained with traditional and new anodes The performances of both anodes used, the Ti–RuO2 mesh and the CCP, can be compared in Fig. 2 and Table 2. The Cl profiles ob-
tained after applying charge densities of 1 106 C/m2, indicate that the Cl contents through the concrete cover are slightly higher for the Ti–RuO2 anode than those corresponding to the CCP anode, and a considerable accumulation of Cl ions is observed in the profile obtained through the 20 mm thick CCP layer (Fig. 2). The efficiency of the CCP anode in extracting the Cl from concrete is about 51%, a slightly higher value than the 41% value obtained with the Ti–RuO2 anode (Table 2). The final distributions of Cl after these EEC tests indicate that both anodes have eliminated high amounts of Cl by electrolysis, being the final Cl concentrations at the electrolytes negligible, but the CCP anode has accumulated an elevated percentage of the initial Cl (32%) after the EEC trial. These results confirm the possibility of using a CCP anode for EEC, with an efficiency in removing chloride ions from concrete, which may be similar or slightly higher than that obtained with a classic Ti–RuO2 mesh anode. Nevertheless, the big accumulation of Cl at CCP and the presence of a strong Cl concentration gradient at the CCP-concrete interface, lead to the possibility of a concentration redistribution by diffusion [24], especially if the concrete is exposed to a humid environment after EEC [25,26]. This redistribution could increase the Cl contents at the concrete cover zone and at the steel depth. The possibility of this redistribution, for the particular conditions of this work, has been confirmed with a companion specimen subjected to EEC in the same conditions and later allowed to rest during two months in the humidity chamber. In this case, (not included in Table 2), the Cl contents, after EEC, at the concrete cover and at the CCP anode were 54% and 24% of the initial Cl present in the concrete cover zone, respectively; while after the two months rest these Cl concentrations changed to 70% and 18%, respectively. The difference between the absolute values of the variations of Cl content by redistribution, at concrete and CCP, may be adequately explained by the contribution of Cl diffusion from the concrete layers behind the steel to the concrete cover zone. 3.2. Influence of the thickness of the conductive cement paste anode The abovementioned redistribution of Cl after EEC is a negative effect to be avoided, since it may reactivate the corrosion of steel. It involves the need to remove the CCP overlay after the treatment, thus losing one of its most interesting characteristics, i.e. the possibility of re-use. A modification of the experimental procedure was tried in order to avoid the accumulation of chlorides at the CCP overlay: the decrease of its width, from 20 mm to 7 mm. Fig. 3 shows the Cl profiles at the end of EEC treatments (5 106 C/m2) performed with CCP anodes of the mentioned widths. The profiles through the concrete cover zones are almost identical, but the profiles through the CCP layers differ markedly: the 7 mm thick anode shows Cl contents similar to those found in the concrete cover, while the Cl concentrations at the 20 mm thick CCP anode are as high as 1% approximately for a significant part of its width. The Cl accumulations for the 20 mm and 7 mm width CCP anodes are 61% and 5%, respectively (Table 2), while the EEC efficiencies are similar for the 7 mm anode (65%)
Table 2 Final chloride content distributions and net efficiencies of the EEC tests. The chloride contents and EEC efficiencies are expressed as percentages of the initial chloride present in the concrete cover zone. The estimated uncertainties have been calculated on the basis of the mean values obtained for the coefficients of variation of the integrated chloride contents in concrete and CCP anode (after EEC) and that of the efficiencies. See text in Section 3 for details. Type of concrete
A A A A B
Anode
Ti–RuO2 CCP CCP CCP CCP
Applied charge density (C/m2)
1 106 1 106 5 106 5 106 5 106
CCP layer width (mm)
– 20 20 7 7
Chloride content distribution after EEC
EEC efficiency (%)
Concrete (%)
Anode (%)
Electrolyte (%)
Balance (%)
59 ± 4.1 49 ± 3.4 40 ± 2.8 35 ± 2.5 59 ± 4.1
– 32 ± 3 61 ± 5.8 5.4 ± 0.5 6.6 ± 0.6
2.2 0.02 4.7 26 33
38.8 19.0 5.7 33.6 1.4
41 ± 3.7 51 ± 4.6 60 ± 5.4 65 ± 5.9 41 ± 3.7
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and for the 20 mm anode (60%). The amount of Cl ions brought into the electrolyte is high for the 7 mm anode (26%), and the amount of chlorides probably eliminated by electrolysis is also considerable for the 7 mm anode (balance of 33%). These data suggest that reducing the CCP thickness is a viable way of decreasing its capacity to accumulate chlorides, making it easier for Cl ions to pass through to electrolyte. The EEC tests were also performed with a 7 mm thick CCP overlay put on a block of concrete B, see Table 1. Fig. 4 shows the initial and final (after passing 5 106 C/m2) Cl profiles corresponding to these EEC treatments. It is interesting to note that the accumulation of chlorides at the CCP layer is also low (6.6%), and practically equal to that obtained in the test performed with a block of concrete A (5.4%), see Table 2. The overall efficiency for EEC on concrete B is 41%. This is, as expected, a lower value than that obtained for concrete A (65%), but it is within the usual range of values of overall efficiencies found for EEC treatments [6]. These results show the viability of the application of a CCP anode for EEC treatments on specimens or structures fabricated with a concrete mix similar to those used in usual construction practice [27]. 3.3. Stability of the conductive cement paste anodes Bertolini et al. [11] studied the damage produced on a cementitious conductive mortar anode used for cathodic protection, due to the acidity produced by the anodic reaction which led to dissolution of the cement matrix. This brought about the loss of electrical continuity between the primary anode and the conductive fibres, and an uneven current distribution on the reinforcement steel bars. These authors demonstrated that the damage was produced for current densities higher than 20 mA/m2, and that a sharp increase of the feeding voltage can be used as a simple parameter for monitoring the damage of the anode [11]. In the present study, the current density used for the EEC tests has been 2 A/m2, and some dissolution of the paste, especially around the graphite electrodes, was visually observed for the 7 mm thick CCP anode, after the 5 106 C/m2 EEC treatment. Nevertheless, for the trials performed in this work no cracking or delamination of the CCP overlay was observed after EEC. Furthermore, no sharp increase of the feeding voltage was observed during the treatments. The typical increases of the feeding voltages were as follows: for concrete A the initial feeding voltage was about 5 V, increasing to about 6 V at the end of the first treatment (1 106 C/m2), and reaching values of about 10 V at the end of the test (5 106 C/m2); while for concrete B the initial feeding voltage was about 8 V, increasing to about 9.5 V at the end of the first treatment (1 106 C/m2), and reaching values of about 17 V at the end of the test (5 106 C/m2). These increases, which were always slow and gradual variations without sharp steps, may be considered as usual values for classic EEC treatments performed with external anodes and liquid electrolytes [28,29], and suggest that no important damage of the CCP overlay has been produced during EEC, or at least the damage has not led to a malfunction of the anode, even though the applied current density is two orders of magnitude higher than the safe value indicated by Bertolini et al. (20 mA/m2). This different behaviour may be adequately explained taking into account that the EEC tests are performed under continuous ponding conditions (anode immersed in electrolyte), and their maximum duration is about 30 days. Bertolini et al. [11] observed that the damages to the conductive cement mortar were quickly produced under dry conditions (most prevalent in cathodic protection applications), but were largely delayed in tests performed in solution. Furthermore, even being damaged, the anode can function satisfactorily if it is immersed in a concentrated saline solution [11]. Nevertheless, more research is needed to ascertain if the CCP overlays used for EEC may be seriously damaged during repeated treatments. It is also necessary to
investigate the influence of substituting the continuous immersion condition during the EEC test (the case studied in this work), by other possibilities such as the use of solid electrolytes (cellulosic pulp mixtures or others), or the use of wetted polymeric layers fixed to the CCP overlay, on the performance and possible malfunctions of the electrochemical system. 4. Conclusions The results of this work show the viability of using a conductive cement paste as an anode for electrochemical chloride extraction from reinforced concrete specimens, with an efficiency similar to that obtained with a classic Ti–RuO2 mesh anode. When the CCP anode is used for EEC treatments on specimens fabricated with a concrete mix similar to those used in usual construction practice, the efficiency is within the typical range of values of overall efficiencies found for EEC treatments. When a 20 mm thick CCP anode is used for an EEC treatment, a high portion of the Cl initially present in the concrete cover zone (up to 61% for a 5 106 C/m2 treatment), is accumulated at the anode. These Cl ions could, in turn, re-diffuse into the concrete cover, and eventually could reactivate the steel corrosion. The reduction of the width of the CCP layer to 7 mm decreases its capacity to accumulate chlorides to values of about 6% of the Cl initially present in the concrete cover zone. The reduction of the thickness of the CCP anode to 7 mm does not decrease the EEC efficiency, which is similar to that obtained with a 20 mm thick CCP anode. A CCP anodic overlay can experience some damage during electrochemical treatments, due to the acidity produced by the anodic reaction. Nevertheless, for the EEC trials performed in this work, no sharp increase of the feeding voltage was observed, indicating that no important damage was suffered by the CCP during the tests, or at least the damage did not lead to a malfunction of the anodic system. More research is needed to ascertain if the CCP overlays used for EEC may be seriously damaged during repeated treatments, and if this damage can produce malfunctions of the electrochemical system. Acknowledgements This work has been financially supported by the Ministerio de Educación y Ciencia of Spain and Fondo Europeo de Desarrollo Regional (FEDER) through projects BIA2006-05961 and BIA200610703, and by the Ministerio de Fomento of Spain (C63/2006). We thank Dr. C. Axness for help with the English version of the text. References [1] D.R. Lankard, J.E. Slater, W.A. Hedden, D.E. Niesz, Neutralization of chloride in concrete, Battelle Columbus Laboratories, Federal Highway Administration (USA), Report No. FHWA-RD-76-60, 1975, 143 pp. [2] J.E. Slater, D.R. Lankard, P.J. Moreland, Electrochemical removal of chlorides from concrete bridge decks, Mater. Perform. 15 (1976) 21–26. [3] G.L. Morrison, Y.P. Virmani, F.W. Stratton, W.J. Gilliland, Chloride removal and monomer impregnation of bridge deck concrete by electro-osmosis, Kansas Department of Transportation, Federal Highway Administration (USA), Report No. FHWA-Ks-RD-74-1, 1976, 38 pp. [4] C. Andrade, M. Castellote, C. Alonso, An overview of electrochemical realkalisation and chloride extraction, in: D.W.S. Ho, I. Godson, F. Collins, (Eds.), Proceedings of 2nd Int. RILEM/CSIRO/ACRA Conference on Rehabilitation of Structures, Melbourne, Australia, RILEM Publications, Cachan, France, 1998, pp. 1–12. [5] J. Tritthart, Electrochemical chloride removal: an overview and scientific aspects, in: J. Skalny, S. Mindess (Eds.), Materials Science of Concrete V, American Ceramic Society, Westerville, Ohio, USA, 1998, pp. 401–441. [6] Electrochemical rehabilitation methods for reinforced concrete structures. A state of the art report, in: J. Mietz, (Ed.), Publication Number 24 of the
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