Véronique Bouteiller
email: veronique.bouteiller@univ-eiffel.fr

Yolaine Tissier

Elisabeth Marie-Victoire
email: elisabeth.marie-victoire@culture.gouv.fr

Thierry Chaussadent
email: thierry.chaussadent@univ-eiffel.fr

Suzanne Joiret

The application of electrochemical chloride extraction to reinforced concretea review

Keywords: Reinforced Concrete, Electrochemical chloride extraction, Corrosion mitigation, pH, Long-term performance Highlights

In a context of ageing reinforced concrete structures, electrochemical chloride extraction (ECE) is an interesting treatment for structures damaged by chloride-induced corrosion. This paper summarises the state of the art by presenting the concept of ECE, assessing available measurement techniques and discussing influencing factors. The determination of ECE efficacy requires the consideration of different results, and particularly those pertaining to chloride extraction, corrosion mitigation and evolution of pH.

This literature review also highlights the potential side effects and risks to the concrete structure. Further work is needed to ascertain the long-term performance of ECE.

INTRODUCTION

Since the second half of the last century, steel corrosion has been identified as the main cause of deterioration in reinforced concrete structures of civil engineering structures (bridges, tunnels, dams, etc.), buildings (houses, car parks, etc.), and historical monuments (churches, sculptures, etc.) [START_REF] Broomfield | Corrosion of steel in concrete -Understanding, investigation and repair[END_REF][START_REF] Bentur | Steel corrosion in concrete -Fundamentals and civil engineering practice[END_REF][START_REF] Woodward | Bridge management in Europe (BRIME)-Deliverable D14-Final Report[END_REF][4][5][6]. From the moment the concrete is cast, the steel rebar it contains is passivated by the high alkaline pH levels (close to 13). During the service life of reinforced concrete structures, the presence of oxygen and water can lead steel rebar to corrode through the contamination of the rebar/concrete interface by chloride ions and/or the complete carbonation of the concrete cover. The anodic reaction occurring in the steel rebar generates corrosion products in a larger volume than that of the initial iron element, resulting in the formation of cracks in the concrete when stress exceeds its tensile strength. In terms of structural effects, the reduced diameter of the rebar and the debonding of the rebar/concrete interface can lead to decreased loading capacity. The spalling or delamination of the concrete can be a source of danger for the users. Finally, the structure may collapse.

This paper focuses in chloride-induced corrosion [START_REF] Broomfield | Corrosion of steel in concrete -Understanding, investigation and repair[END_REF][START_REF] Bentur | Steel corrosion in concrete -Fundamentals and civil engineering practice[END_REF]4,[START_REF] Angst | Critical chloride content in reinforced concrete -A review[END_REF][8][START_REF] Liang | Chloride transport and induced steel corrosion in recycled aggregate concrete: A review[END_REF], which is particularly common when the reinforced concrete structure is exposed to marine environment or de-icing salts. In these cases, chlorides penetrate the concrete cover from the outer surface and reach the inner steel rebar/concrete interface. Chloride ions may also have been added to the mix and therefore be more homogeneously distributed within the concrete cover; for example, chloridebased admixtures were sometimes used as setting accelerators (a method that is now prohibited), or marine sand was used for coastal structures.

A number of approaches to avoid steel rebar corrosion have been explored in the literature [START_REF] Cox | COST 509 -Corrosion and protection of metals in contact with concrete -Final Report[END_REF][START_REF] Cigna | COST 521 -Corrosion of steel in reinforced concrete structures -Final Report[END_REF]. Preventive solutions include (i) the use of a protective coating to insulate the steel rebar (epoxy, electroplating or galvanisation) and the use of stainless steel rebar, (ii) the use of anti-corrosion cement, the adapting of the concrete formulation or an increase in the thickness of the concrete cover in order to delay contamination by chlorides or carbonation, and (iii) the use of corrosion inhibitors or the application of concrete cover coatings.

Maintenance and/or rehabilitation repairs must also be provided for reinforced concrete structures affected by corrosion induced by chloride ions [4,[START_REF] Cox | COST 509 -Corrosion and protection of metals in contact with concrete -Final Report[END_REF][START_REF] Cigna | COST 521 -Corrosion of steel in reinforced concrete structures -Final Report[END_REF][START_REF] Matthews | Achieving Durable Repaired Concrete Structures: Adopting a Performance-based Intervention Strategy[END_REF][START_REF] Matthews | CONREPNET: Performance-based approach to the remediation of reinforced concrete structures: Achieving durable repaired concrete structures[END_REF].

The extensively used conventional "patch repair" consists of replacing all the contaminated concrete and also the rebar when necessary. However, performance of patching through the European CONREPNET project [5] was poor, with a 50% failure rate that was attributed to cracking (30%), debonding (25%), continued corrosion (25%) and other causes (20%). Patch repair becomes impractical for the maintenance of infrastructures such as bridges when contamination reaches beyond the rebar depth. Moreover, the replacement of original materials is not in accordance with restoration principles for historical monuments, as defined by the Venice Charter.

When electrochemical treatments such as cathodic protection or electrochemical chloride extraction are applied to chloride-contaminated reinforced structures, it is only necessary to remove and reconstruct the concrete that no longer bonds with the rebar as polarization ensures the protection of steel rebar, and chloride ions migrate from the steel to the outside of the concrete due to the electrical field.

-Cathodic Protection (CP) of steel in concrete has been demonstrated to be successful in appropriate conditions [START_REF] Afnor | ISO 12696 -Cathodic protection of steel in concrete[END_REF]. CP is designed to decrease the corrosion rate of the steel from significant to negligible values. For this purpose, the steel/concrete potential should be lowered (made more negative) to values in the range of the protection potential. Typical levels of applied current density (with respect to the steel surface) range from 0.2 to 2 mA/m² for prevention and from 2 to 20 mA/m² for protection. CP is a lifelong treatment and requires specific monitoring to assess its efficacy according to several criteria.

-Electrochemical Chloride Extraction (ECE) is also known as desalination, electrochemical chloride removal or electrochemical chloride mitigation. A current is applied between the steel rebar and an external anode in order to remove chloride ions from the concrete cover, and particularly from the rebar/concrete interface. The current density is usually 1 A/m² of steel rebar surface (approximately 100 times higher than the density used in CP) and is applied for a limited time (4 to 8 weeks) [START_REF] Afnor | Electrochemical realkalization and chloride extraction treatments for reinforced concrete -Part 2: Chloride extraction[END_REF].

The aim of this literature review is to gather all available data about the application of electrochemical chloride extraction (ECE) to reinforced concrete. The following section of this paper describes ECE parameters. We then move on to presenting the different ways in which the efficacy of the treatment can be defined. The results of studies based on different parameters pertaining to specificities such as chloride extraction efficacy, corrosion mitigation and pH evolution are discussed in the fourth part of this paper. The fifth section focuses on the potential side-effects and risks of the ECE treatment. The long-term efficacy of ECE is then discussed, followed by ECE specifications and modelling. Finally, this review ends with our conclusions, recommendations for laboratory studies and practical cases, and suggestions of future research avenues.

ECE PRINCIPLE

History

The first trials of electrochemical chloride extraction (ECE) treatment with high DC voltages of up to 220 V were led in the United States of America by the Federal Highway Administration in the mid-70s [START_REF] Slater | Electrochemical removal of chlorides from concrete bridge decks[END_REF][START_REF] Morrison | Chloride removal and monomer impregnation of bridge deck concrete by electro-osmosis[END_REF][START_REF] Lankard | Neutralization of chloride in concrete -Final report FHWA-RD-78-60[END_REF], but this procedure was rarely used over the next 20 years. An invention was patented in Europe during the 80s by Oystein Vennesland, a researcher at Norwegian

Concrete Technologies [START_REF] Vennesland | Removal of chlorides from concrete[END_REF]. Knowledge of ECE was developed further in the 90s through research carried out in the United States of America Strategic Highways Research Program (SHRP) [START_REF] Bennett | Evaluation of NORCURE Process for electrochemical chloride removal from steel reinforced concrete bridge components[END_REF][START_REF] Bennett | Chloride removal implementation[END_REF][START_REF] Bennett | Electrochemical chloride removal and protection of concrete bridge components : laboratory studies[END_REF][START_REF] Bennett | Electrochemical chloride removal and protection of concrete bridge components : field trials[END_REF]. Pocock's 1994 publication [START_REF] Pocock | Chloride extraction and realkalisation -six years on Corrosion and protection of reinforced concrete Dubai[END_REF] recognized the interest of this treatment for the repairing of reinforced concrete contaminated by chloride ions. Different publications, stateof-the-art report, chapters on ECE treatment were published [START_REF] Mietz | Electrochemical rehabilitation methods for reinforced concrete structuresa state of the art report[END_REF][START_REF] Elsener | Repair of reinforced concrete structures by electrochemical techniques -Field experience[END_REF][START_REF] Raupach | Corrosion of reinforcement in concrete -Mechanisms, monitoring, inhibitors and rehabilitation techniques[END_REF]. ECE treatment was also part of three European Cooperation in Science and Technology (COST) actions (COST 509 in 1997 [START_REF] Cox | COST 509 -Corrosion and protection of metals in contact with concrete -Final Report[END_REF], COST 521 in 2003 [START_REF] Cigna | COST 521 -Corrosion of steel in reinforced concrete structures -Final Report[END_REF] and COST 534 (2007) [START_REF] Polder | COST 534 -New materials, systems, methods and concepts for prestressed concrete structures-Final Report[END_REF]). In 2022, task group 8.1 from the federation Internationale du Béton published fib Bulletin 102 "Guide for protection and repair of concrete structures" [START_REF] Julio | Fib Bulletin 102 Guide for protection and repair of concrete structures -Guide to good practice Task Group 8[END_REF] and devoted a chapter on chloride extraction/desalination [START_REF] Kobayashi | Chloride extraction/Desalination[END_REF].

The present literature review seeks to gather the different results from the 1970s until 2022.

Mechanisms

Corrosion of the steel rebar occurs by two electrochemical reactions: the anodic dissolution of iron, as shown in Equation 1, and the corresponding oxygen reduction at the cathode that uses the electrons generated by the metal dissolution in its reaction, as shown in Equation 2.

Anodic dissolution of iron

Fe → Fe 2+ + 2e -

Equation 1

Oxygen reduction O 2 + 2H 2 0 +4e -→ 4OH -

Equation 2

Both of these reactions occur on the steel surface, resulting in the formation of corrosion products. The accumulation of corrosion products at the rebar/concrete interface (Equation 3) may induce cracks in the concrete cover.

Corrosion products Fe

2+ + 2OH -→ Fe(OH) 2 Fe(OH) 2 + nO 2 → Fe 2 O 3 .nH 2 O Equation 3
ECE is designed to arrest the corrosion of steel rebar, or even recover the passivity thereof, through the following phenomena: a lowering of the thermodynamic potential of the steel, a reduction of the chloride content and an increase in the pH levels at the rebar/concrete interface.

Figure 1 shows a schematic representation of the ECE set-up when a direct current source is used 1) . The rebar requiring protection acts as a cathode and is connected to the negative pole of the direct current source, while the anode (usually located on the surface of the concrete) is connected to the positive pole. During ECE, it is important to minimize circuit resistance between the cathode and the anode and to maintain a buffered pH. This is usually achieved with the joint use of a solution called electrolyte and one of three different methods: tanks, sprayed cellulose fibres, or synthetic felt mats. In alkaline reinforced concrete 2) , the difference in electrical potential between the cathode and anode triggers the transport of the ions present in the electrolyte and in the concrete [START_REF] Mietz | Electrochemical rehabilitation methods for reinforced concrete structuresa state of the art report[END_REF][START_REF] Hansson | Electrochemical extraction of chlorides from concrete part I -A qualitative model of the process[END_REF][START_REF] Tritthart | Electrochemical Removal of Chloride from Hardened Cement Paste[END_REF][START_REF] Andrade | Mathematical-Modeling of Electrochemical Chloride Extraction from Concrete[END_REF][START_REF] Tritthart | Electrochemical chloride removal -a case study and laboratory tests Fourth International Symposium on corrosion of reinforcement in concrete constructionThe royal society of chemistry[END_REF][START_REF] Buenfeld | Influence of electrochemical chloride extraction on the bond between steel and concrete[END_REF], and may cause the following reactions:

Oxygen reduction O 2 + 2H 2 0 +4e -→ 4OH - Equation 4
Water reduction

2H 2 0 + 2e -→ H 2 ↑ + 2OH - Equation 5
The low potential of the steel at the cathode protects the rebar from corrosion. The oxygen reduction reaction (Equation 4) takes place slowly due to the limited availability of oxygen in concrete: at the outset, there is very little oxygen available around the cathode within the concrete, and oxygen diffusion from the outside is slow. Thus the water reduction reaction 1) Sacrificial anodes are not considered in this paper. 2) Because concrete is an alkaline medium (sound concrete pH~13 and carbonated concrete pH~9), hydroxyl ions (OH -) >>> protons (H + ).

(Equation 5) predominates [START_REF] Tritthart | Electrochemical chloride removal -a case study and laboratory tests Fourth International Symposium on corrosion of reinforcement in concrete constructionThe royal society of chemistry[END_REF]. The production of hydrogen gas can increase local pressure, eventually leading to concrete cracking and hydrogen embrittlement of steel ( §5.2.2).

At the anode, the oxidation of hydroxyl ions forms oxygen and water (Equation 6). No dissolution of the anode occurs if the metal is inert (Equation 7). In the case of electrolyte acidification, the oxidation of chloride ions in chlorine gas (Equation 8) should be avoided for safety and environmental concerns.

Water oxidation

4OH -→ O 2 + 2H 2 O +4e - Equation 6
Anode dissolution Me → Me n+ + ne -

Equation 7

Chlorides 2 Cl -→ Cl 2 + 2e -

Equation 8

In addition to these oxidation-reduction reactions, at typical current densities for ECE, migration dominates with the transport of ions contained in the concrete and in the electrolyte.

The transport of ions involves different processes based on concentration gradients, electrical migration, lixiviation and diffusion. Due to the electrical field of the rebar and the anode, anions such as free chlorides (Cl -), hydroxyls (OH -), carbonates (CO 3 2-), and sulfates (SO 4 2-), are mostly repelled from the vicinity of the rebar and migrate away from the cathode region towards the anode. Similarly, cations such as calcium (Ca 2+ ), potassium (K + ), sodium (Na + ) and lithium (Li+) ions migrate towards the cathodic steel region. Electrical neutrality is maintained on a macroscale despite the modified distribution of specific ions within the concrete. It is important to note that the accumulation of alkali ions can cause an alkali-silicate reaction if the concrete contains reactive aggregates ( §5.2.4).

Figure 2 provides a schematic representation of the electrical field obtained during the treatment, and shows that the maximal extraction efficacy is obtained in the area in between the rebar and the anode. Moreover, chloride ions present at the deepest levels close to the rebar cannot be easily extracted [START_REF] Mietz | Electrochemical rehabilitation methods for reinforced concrete structuresa state of the art report[END_REF].

Process parameters

In this section, each component of the ECE treatment (Figure 1) is presented and some important features are explained.

Cathode

In almost all cases, the cathode is the rebar (simple or deformed rebar, mild steel plate, rebar plus stirrups or structural reinforcement).

Arya et al. [START_REF] Arya | Factors influencing electrochemical removal of chloride from concrete[END_REF] used an additional external cathode for cases in which chloride was present at a deeper level than the rebar. Setup design affected chloride extraction, which increased from 40% (obtained with the conventional setup) to 55% when both the rebar and external platinised titanium mesh were connected as cathodes.

Anode

In the context of ECE treatment, the anode has to satisfy various criteria to be fully functional [4]. It must distribute the chosen current throughout the area requiring protection for the entire duration of the treatment. This element must also be corrosion resistant and should not affect the appearance of the concrete surface. Finally, its installation should be inexpensive.

During their preliminary study in 1976, Slater et al. [START_REF] Slater | Electrochemical removal of chlorides from concrete bridge decks[END_REF] tested platinized titanium, titanium and graphite anodes in a 0.1 N calcium hydroxide solution. Platinized titanium was found to be the most appropriate anode material; it satisfied the test criteria, unlike graphite (which disintegrated at high current densities) and titanium (which had poor electro catalytic activity).

The most reliable and widely used anode today is composed of titanium that has been activated with oxides or coated with different metals. This anode can take the form of mesh, wire or strip. It is suitable for the protection of different concrete structures, and can be used to treat vertical and horizontal structures and both sides of curved surfaces. Ueda et al. [START_REF] Ueda | A new electrochemical rehabilitation method for reinforced concrete employing the DFRCC anode system[END_REF] used a titanium mesh cast in a ductile fibre (PE or PVA)-reinforced, cementitious composite (DFRCC), which improved its adhesion to a prepared concrete surface. Although chloride extraction efficacy at the rebar level was close to 75%, chlorides were accumulating in the DFRCC layer and could therefore spread back into the concrete cover.

Steel was tested given its low price, however its use is not recommended because it is rapidly consumed during the treatment (Equation 9) and causes rust spots on the concrete surface [START_REF] Bennett | Evaluation of NORCURE Process for electrochemical chloride removal from steel reinforced concrete bridge components[END_REF].

Different anode systems to replace titanium anodes have been studied and referenced in the literature. Pérez et al. [START_REF] Pérez | Electrochemical extraction of chlorides from reinforced concrete using a conductive cement paste as the anode[END_REF] worked on the viability of replacing the classical Ti-RuO 2 mesh anode with a graphite anode and a conductive cement paste (CCP) combination, applied superficially as a plaster. Results showed that although the chloride extraction was similar for both types of anodes, an accumulation of chloride ions appeared in the CCP layer that could eventually spread back into the concrete cover. Canon et al. [START_REF] Cañón | Feasibility of electrochemical chloride extraction from structural reinforced concrete using a sprayed conductive graphite powder-cement paste as anode[END_REF] and Carmona et al. [START_REF] Carmona | Efficiency of a conductive cement-based anodic system for the application of cathodic protection, cathodic prevention and electrochemical chloride extraction to control corrosion in reinforced concrete structures[END_REF] studied the feasibility of using a sprayed conductive graphite powder cement paste as an anode, which would make it possible to work on various surface shapes and inclinations and therefore facilitate the application of ECE. The results showed that the chloride extraction efficacy was similar to that observed in the titanium-based anode. Moreover, unlike the anode suggested by Pérez et al. [START_REF] Pérez | Electrochemical extraction of chlorides from reinforced concrete using a conductive cement paste as the anode[END_REF] this sprayed layer retained moisture for up to 10-12 h but failed to retain chlorides. The durability of this anode layer has been questioned following damage by anodic reactions. Finally, Zhu et al. [START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF] studied the application of a conductive carbon-fibre-reinforced polymer (CFRP) anode in the form of a mesh with good mechanical properties, durability, conductivity and electrochemical stability. The results for chloride extraction were similar to those obtained with an activated titanium mesh anode. It should be noted that values are homogenized when a conductive anode layer is used meaning that half-cell potential mapping measurements cannot be carried out for inspection.

Electrolyte

The electrolyte plays three main roles in ECE treatment:

-First, it decreases the electrical resistance by wetting between concrete and reinforcement (hydration and dissolution of the ions in the concrete), thus lowering the voltages used and improving ionic migration, -Second, the pH of the electrolyte should prevent any modification of the concrete pH, -Lastly, it should trap chlorine gas, which is hazardous to health.

Usually, renewal or circulation of the electrolyte is needed. Tanks or sprayed cellulose fibre can be used for the treatment when applied on horizontal or vertical surfaces.

The two most widespread electrolytes for ECE are water [START_REF] Tritthart | Electrochemical chloride removal -a case study and laboratory tests Fourth International Symposium on corrosion of reinforcement in concrete constructionThe royal society of chemistry[END_REF][START_REF] Cañón | Feasibility of electrochemical chloride extraction from structural reinforced concrete using a sprayed conductive graphite powder-cement paste as anode[END_REF][START_REF] Carmona | Efficiency of a conductive cement-based anodic system for the application of cathodic protection, cathodic prevention and electrochemical chloride extraction to control corrosion in reinforced concrete structures[END_REF][START_REF] Elsener | The Electrochemical Removal of Chlorides from Reinforced-Concrete[END_REF][START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Abdelaziz | Evaluation of the short and long-term efficiencies of electro-chemical chloride extraction[END_REF][START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Garcés | Effect of the reinforcement bar arrangement on the efficiency of electrochemical chloride removal technique applied to reinforced concrete structures[END_REF][START_REF] Siegwart | Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions[END_REF][START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF][START_REF] Sánchez | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of calcium nitrite[END_REF][START_REF] Vennesland | Electrochemical removal of chlorides from concrete -Effect of bond strength and removal efficiency[END_REF][START_REF] Stoop | Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms[END_REF][START_REF] Ihekwaba | Aspects of electochemical rehabilitation of chloride contaminated concrete structures[END_REF] and saturated calcium hydroxide (Ca(OH) 2 ) solution [START_REF] Slater | Electrochemical removal of chlorides from concrete bridge decks[END_REF][START_REF] Bennett | Evaluation of NORCURE Process for electrochemical chloride removal from steel reinforced concrete bridge components[END_REF][START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF][START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Abdelaziz | Evaluation of the short and long-term efficiencies of electro-chemical chloride extraction[END_REF][START_REF] Stoop | Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms[END_REF][START_REF] Ihekwaba | Aspects of electochemical rehabilitation of chloride contaminated concrete structures[END_REF][START_REF] Green | Electrochemical Changes in Chloride-Contaminated Reinforced-Concrete Following Cathodic Polarization[END_REF][START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF][START_REF] Ihekwaba | Carbonation and electrochemical chloride extraction from concrete[END_REF][START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Chang | Bond degradation due to the desalination process[END_REF][START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF][START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF][START_REF] Bertolini | Effects of electrochemical chloride extraction on chemical and mechanical properties of hydrated cement paste[END_REF][START_REF] Armstrong | The trial repair of Victoria Pier, St Helier, Jersey using electrochemical desalination[END_REF][START_REF] Polder | Eletrochemical desalination of cores from a reinforced concrete coastal structure[END_REF][START_REF] Chatterji | Simultaneous Chloride Removal and Realkalinization of Old Concrete Structures[END_REF][START_REF] Polder | Electrochemical chloride tests of concrete cores from a coastal structure[END_REF][START_REF] Page | Potential Effects of Electrochemical Desalination of Concrete on Alkali-Silica Reaction[END_REF]. Saturated calcium hydroxide solution with a pH equal to 12.4 is consistent with the requirement for a buffered medium and the need for a relatively high pH value to prevent acidification and chlorine gas.

Other electrolyte solutions containing sodium ions were studied, namely sodium hydroxide (NaOH) [START_REF] Arya | Factors influencing electrochemical removal of chloride from concrete[END_REF][START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF], sodium carbonate (Na 2 CO 3 ) [START_REF] Polder | Electrochemical chloride tests of concrete cores from a coastal structure[END_REF] and sodium borate solutions (Na 3 BO 3 [START_REF] Ihekwaba | Carbonation and electrochemical chloride extraction from concrete[END_REF],

Na 2 B 4 O 7 [START_REF] Bouteiller | Electrochemical chloride extraction of a beam polluted by chlorides after 40 years in the sea[END_REF] and Na 2 B 4 O 7 10H 2 O [START_REF] Tissier | Etude des processus de corrosion et de la réparation par traitement électrochimique d'extraction des ions chlorure dans les bétons armés doublement contaminés carbonatation et chlorures)[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF]. Sodium ions can sometimes be detrimental in terms of Alkali-Silica Reaction (ASR) ( §5.2.4). Finally, lithium borate solution was also used as an electrolyte, as lithium ions have already demonstrated their ability to limit ASR [START_REF] Bennett | Electrochemical chloride removal and protection of concrete bridge components : field trials[END_REF][START_REF] Ueda | A new electrochemical rehabilitation method for reinforced concrete employing the DFRCC anode system[END_REF][START_REF] Ueda | Influence of electrolyte temperature on efficiency of electrochemical chloride removal from concrete[END_REF][START_REF] Ueda | Influence of temperature on electrochemical remedial measures and complex deterioration due to chloride attack and ASR[END_REF].

Moreover, Ueda [START_REF] Ueda | Influence of electrolyte temperature on efficiency of electrochemical chloride removal from concrete[END_REF] demonstrated that increasing the temperature of Li 3 BO 3 electrolyte solution resulted in a higher percentage of chloride extraction.

Current flow

The current flow between the rebar and the anode must ensure the cathodic protection of the steel rebar. Current flow can be obtained in two modes: constant voltage (voltage control) and constant current (current control) in a continuous or intermittent ways. In the voltage control mode, the current flow is expressed in voltage [16-18, 20-23, 36, 49, 58, 72-75]. In the current control mode, the current flow is expressed in current density in A/m² of steel surface or concrete surface that must be indicated. Current density is expressed in A/m² of steel rebar surface in references [35, 40, 41, 43, 45, 47, 48, 51, 54, 55, 57-60, 63, 65, 67, 69, 76-85], and in A/m² of concrete surface in references [21, 34, 37-39, 44, 46, 53, 62, 70, 71, 86, 87]. No surface information is clearly indicated in references [START_REF] Bertolini | Effects of electrochemical chloride extraction on chemical and mechanical properties of hydrated cement paste[END_REF][START_REF] Siegwart | The effect of electrochemical chloride extraction on pre-stressed concrete[END_REF][START_REF] Banfill | Electrochemical re-alkalisation, chloride extraction and alkali-silica reaction in concrete Concrete Solutions Thessaloniki[END_REF].

Moreover, some authors attempted to increase the efficacy of the treatment through the use of intermittent rather than continuous current flow, via voltage control mode [START_REF] Lankard | Neutralization of chloride in concrete -Final report FHWA-RD-78-60[END_REF] or current control mode [START_REF] Tritthart | Electrochemical chloride removal -a case study and laboratory tests Fourth International Symposium on corrosion of reinforcement in concrete constructionThe royal society of chemistry[END_REF][START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Garcés | Effect of the reinforcement bar arrangement on the efficiency of electrochemical chloride removal technique applied to reinforced concrete structures[END_REF][START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF][START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF][START_REF] Ihekwaba | Structural shape effect on rehabilitation of vertical concrete structures by ECE technique[END_REF][START_REF] Tang | Effect of an electrical field on the removal of chloride ions from concrete slabs[END_REF][START_REF] Climent Llorca | Effect of type of anodic arrangements on efficiency of electrochemical chloride removal from concrete[END_REF].

It is important to note that it is very difficult if not impossible to compare the results if the ECE modes are not the same or if the current density units are different.

Duration

ECE is a short-term treatment (vs Cathodic Protection). It is often applied for periods ranging from 2 to 8 weeks [23, 32, 34, 36-39, 41-44, 46-48, 51-55, 57-61, 65-67, 70, 71, 75, 79, 80, 82, 84, 87, 88, 92-95]. Larger duration than 15 weeks [START_REF] Elsener | The Electrochemical Removal of Chlorides from Reinforced-Concrete[END_REF][START_REF] Martinez | Chloride Electroremediation in reinforced structures: preliminary electrochemical tests to detect the steel repassivation during the treatment[END_REF][START_REF] Buenfeld | Effect of chloride removal on rebar bond strength and concrete properties[END_REF][START_REF] Siegwart | The effect of electrochemical chloride extraction on pre-stressed concrete[END_REF][START_REF] Buenfeld | Influence of electrochemical chloride extraction fon the bond between steel and concrete[END_REF] has also been considered particularly when using lower current density or for testing prestressed concretes.

TECHNIQUES TO ASSESS ECE EFFICACY AND POTENTIAL SIDE EFFECTS

This third section describes the different non-destructive and destructive techniques used by researchers to characterize the efficacy of ECE, and seeks to verify whether the ECE treatment induces chemical, microstructural and/or mechanical changes that could be detrimental to the reinforced structure.

Visual inspections

A visual inspection of the appearance of the rebar and its surrounding concrete can provide general information such as corrosion product colours, layer of deposits, cracks, etc. [45, 49, 57, 59, 71, 76-79, 83, 87, 96].

Chloride analyses

The efficacy of the treatment varies according to the chloride content before, during, after and long after ECE (in case of re-diffusion). In concrete, chlorides are either freely dissolved in the pore solution or exist in a bound form with cement hydration products such as hydrated C3A

or C4AF phases and CHS gel. Moreover, penetrated chloride content varies according to the depth at which it is located in the concrete. Usually specimens (from the laboratory or from cores obtained on the field) are sliced, crushed, sieved and dissolved in water where the chloride content profile is determined as water soluble (free chlorides) or acid soluble (total chlorides) based on potentiometric titration using silver nitrate or spectrophotometric analysis or chemical analysis [43, 48, 49, 53-55, 59, 70, 97-99].

It is important to note that the measurement of free chloride ions in the interstitial solution is only possible for pastes [START_REF] Tritthart | Electrochemical Removal of Chloride from Hardened Cement Paste[END_REF][START_REF] Tritthart | Electrochemical chloride removal -a case study and laboratory tests Fourth International Symposium on corrosion of reinforcement in concrete constructionThe royal society of chemistry[END_REF] where pressure needed for extraction can be achieved. Finally, Elsener et al. [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF] developed a chloride ion-selective electrode as an embedded sensor in concrete.

The concentration of removed chloride ions can also be determined from electrolyte analysis [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Stoop | Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF][START_REF] Ueda | Influence of electrolyte temperature on efficiency of electrochemical chloride removal from concrete[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF]. Polder et al. [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF] found a good general correlation between the amount of chloride calculated from electrolyte analysis and concrete analysis, respectively.

Electrochemical characterizations

Non-destructive electrochemical measurements such as half-cell potential, linear polarization resistance and corrosion rate can be used to characterise the corrosion state of the rebar when the latter is in a steady state (no polarization) [16, 20, 23, 37, 42-46, 48, 49, 51-54, 59, 60, 67, 70, 73-75, 79, 82, 84, 100]. According to the literature, the efficacy of the treatment that will eventually lead to a passive state can be assessed by performing electrochemical measurements before ECE and after ECE once the polarization of the rebar is over. It is important to note that the time at which depolarization of the steel occurs cannot be known in advance. During or immediately after the treatment, these electrochemical measurements should confirm that the rebar is polarized (i.e. high electronegative half-cell potential and high current densities values are observed). This is an interesting means to verify that different treated specimens are reacting in the same way, or to prove that no area has been forgotten

when the treatment is applied on-site, for example.

Half-cell potential measurements are the most commonly used method because they are very easy to conduct [20, 37, 43, 45, 46, 48, 49, 51-54, 59, 60, 67, 70, 73-75, 79, 82, 84, 100].

However, it should be noted that the results are strongly dependent on parameters such as concrete resistivity (moisture content), chloride content, pH, temperature or climatic conditions.

Half-cell potential measurements aim to evaluate the probability of rebar corrosion (when all the other parameters remain constant) and/or to locate corroding zones in larger specimens and in on-site structures, where climatic conditions are not controlled. For reinforced concretes in atmospheric conditions, test procedures and guidelines are described in the ASTM C-876 standard (1991 and 2009) [START_REF] Astm | Standard test method for half-cell potentials of uncoated reinforcing steel in concrete[END_REF][START_REF] Astm | Standard test method for half-cell potentials of uncoated reinforcing steel in concrete[END_REF] and in the RILEM TC 154 recommendation (2003) [START_REF] Andrade | RILEM TC 154-EMC:Electrochemical Techniques for Measuring Metallic Corrosion -Recommendations -Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF].

After ECE, more positive potentials and smaller gradients are expected (versus before ECE).

Despite the availability of these tools, result analysis for reinforced concrete structures in a marine environment can still be difficult because of the tide that modifies humidity and oxygen content.

The measurement of polarization resistance (Rp) before, during and after treatment with a potentiostat [START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF][START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF] permits the evaluation of the corrosion rate at a given time. Corrosion rate can also be measured on site using a GECOR 6 [START_REF] Tritthart | Electrochemical Removal of Chloride from Hardened Cement Paste[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF] or a GPM-5000 [START_REF] Chang | Effects of stirrups on electrochemical chloride removal efficiency[END_REF]. The results obtained can be compared to the established corrosion rate classification [START_REF] Orellan | Efficacité et effets secondaires des techniques électrochimiques de maintenance des structures en béton armé[END_REF]. The RILEM TC 154-EMC document provides important recommendations for on-site corrosion rate measurements [START_REF] Andrade | RILEM TC 154-EMC:Electrochemical Techniques for Measuring Metallic Corrosion -Recommendations -Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF]. It is important to note that corrosion rate measurements using Rp do not

give realistic values at short timescales after current interruption. This is likely to be due to the chemical changes induced close to the rebar (i.e. high alkalinity and low O 2 activity) [START_REF] Green | Electrochemical Changes in Chloride-Contaminated Reinforced-Concrete Following Cathodic Polarization[END_REF].

Miranda et al. [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF] and Marcotte et al. [START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF] used a non-stationary method by performing full polarization curves to assess the corrosion rate of reinforced concrete slabs before and after ECE (with ECE performed on both sides of the slabs).

pH determinations

During the ECE process, pH evolution at the rebar/concrete interface, in the concrete and in the electrolyte can be analysed. This analysis can involve the use of phenolphthalein, a colour acid/base indicator that will turn purple when sprayed on concrete with a pH level of 9-10 or above [START_REF] Buenfeld | Influence of electrochemical chloride extraction on the bond between steel and concrete[END_REF]. Other authors titrated the pH of concrete from concrete samples dissolved in water [START_REF] Chang | Bond degradation due to the desalination process[END_REF][START_REF] Chang | Effects of stirrups on electrochemical chloride removal efficiency[END_REF][START_REF] Yeih | Electrochemical chloride removal for reinforced concrete with steel rebar cage using auxiliary electrodes[END_REF][START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF]. Finally, pH values for the electrolyte were reported in [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF].

Concrete characterizations

Ion migration

The performance of atomic absorption spectrophotometry on solutions obtained from dissolved concrete provides quantitative information on the presence of ions such as potassium, sodium, calcium and lithium and their migration through the electrical field during ECE, and thus provides information about the evolution of the concrete composition [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Ihekwaba | Aspects of electochemical rehabilitation of chloride contaminated concrete structures[END_REF][START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Ueda | Influence of temperature on electrochemical remedial measures and complex deterioration due to chloride attack and ASR[END_REF].

Microstructure transformation

Petrographic examination can be performed [START_REF] Bennett | Electrochemical chloride removal and protection of concrete bridge components : field trials[END_REF][START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF] to study the microstructural evolution of the concrete (i.e. micro cracks, pores, etc.).

Scanning electron microscopy can be used to observe the microstructure of concrete and identify any new phases (petrographic) formed during ECE by comparing treated or untreated specimens [START_REF] Buenfeld | Influence of electrochemical chloride extraction on the bond between steel and concrete[END_REF][START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF][START_REF] Siegwart | Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions[END_REF][START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part II: Microstructural characterization[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF][START_REF] Siegwart | The effect of electrochemical chloride extraction on pre-stressed concrete[END_REF].

X-ray diffraction is also used to analyse additional crystalline materials present in the concrete or in concrete surface deposits [START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF][START_REF] Sharp | Electrochemical chloride extraction: influence of concrete surface on treatment[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF].

The Mercury Intrusion Porosimetry technique can be used to determine the pore size distribution within the concrete cover [START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF] and evaluate ECE impact.

Steel characterizations

Microstructural observations of the rebar/concrete interface by scanning electron microscopy coupled with dispersive X-ray spectroscopy are used to identify the corrosion products and their location at the rebar/concrete interface after the corrosion process and after the ECE treatment. X-ray diffraction was used by Miranda [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF] to determine the nature of the corrosion products.

Raman microspectroscopy can identify the iron corrosion products during and after treatment and help to ascertain the corrosion state of the rebar. Tissier et al [START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF] developed a specific electrochemical cell set-up without any oxygen contact to perform in-situ observations in the laboratory.

Mechanical tests

Compressive strengths were studied in [START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part II: Microstructural characterization[END_REF]. Pull-out tests were performed [START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF][START_REF] Vennesland | Electrochemical removal of chlorides from concrete -Effect of bond strength and removal efficiency[END_REF][START_REF] Chang | Bond degradation due to the desalination process[END_REF][START_REF] Buenfeld | Effect of chloride removal on rebar bond strength and concrete properties[END_REF][START_REF] Ihekwaba | Pull-out and bond degradation of steel rebars in ECE concrete[END_REF][START_REF] Buenfeld | Influence of electrochemical chloride extraction fon the bond between steel and concrete[END_REF] to determine bond stress changes. For bigger specimens like beams, flexural strength tests were conducted [START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF].

LABORATORY RESULTS

All the published studies considered in the present evaluation are presented in three tables (Table 2, Table 3 andTable 4) according to the electrochemical chloride extraction mode used.

All the abbreviations can be found in Table 1.

Table 2 summarises all the published studies regarding voltage control mode. The voltages are in the range of 5 to 100 V. High voltage (100 V) led the temperature of the concrete to increase drastically (from 25 °C to 50-84° C) and resulted in concrete cracks in some cases.

The major disadvantage when using a voltage control mode is that the current received by the rebar will decrease over time as the resistance of the medium increases (due to the extraction of chloride).

Using current control mode allows a better repeatability of the experiment. Table 3 summarises all the published studies where the current density is based on the area of the concrete surface (A/m² CS), whereas Table 4 concerns studies where the current density is based on the area of the steel rebar surface (A/m² RS). The type of surface was not clearly indicated in papers [START_REF] Bertolini | Effects of electrochemical chloride extraction on chemical and mechanical properties of hydrated cement paste[END_REF][START_REF] Siegwart | The effect of electrochemical chloride extraction on pre-stressed concrete[END_REF].

Current densities based on the area of the concrete surface (Table 3), are calculated on the assumption that the ratio of the rebar area to concrete area in the treated structure is equal to one [START_REF] Siegwart | The effect of electrochemical chloride extraction on pre-stressed concrete[END_REF]. However, this is not always correct. For example, 1 A/m² of CS can be equal to 5

A/m² RS [START_REF] Siegwart | The effect of electrochemical chloride extraction on pre-stressed concrete[END_REF] or to 9.5 A/m² RS [START_REF] Ihekwaba | Pull-out and bond degradation of steel rebars in ECE concrete[END_REF]. Unnecessarily high current densities can lead to detrimental side effects including increased temperature and concrete cracking. The aim of ECE is to protect the steel from corrosion in concrete; the current density must therefore be based on the area of the steel rebar surface.

Elsener et al. [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF] proposed to use an intermittent current control mode in which bound chlorides were released as free chlorides during the OFF period. This led to increased efficacy of chloride extraction during the next ON period. The authors even reached 100% efficacy in this experimental program.

Several papers studied the use of an intermittent voltage control to increase the efficacy of ECE [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF][START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF][START_REF] Tang | Effect of an electrical field on the removal of chloride ions from concrete slabs[END_REF]]. An interruption of current [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF] may have the following advantages:

-Bound chlorides may become free chlorides.

-A redistribution of ions (Cl -, OH -) may occur within the concrete. The current interruption period should be long enough to achieve the depolarization of the rebar and therefore permit the expected re-distribution of ions.

-Risks of undesirable side effects arising from the application of electrochemical techniques can be reduced. were mainly used to study the influence of the ECE treatment on the mechanical properties of reinforced concretes and particularly the loss of bond between the rebar and the concrete.

Treatment most often lasts 28 or 56 days, but other periods in the range (3-112 days) were also reported.

From here on, our discussion will solely concern the ECE results obtained with a current control mode and a current density expressed in A/m² of steel rebar surface (RS), described in Table 4. Since these data were obtained from a variety of experimental setups, the corresponding details must be considered when comparing them.

The determination of ECE treatment efficacy requires the simultaneous consideration of different results, particularly those pertaining to chloride extraction ( §4.1), corrosion mitigation ( §4.2), evolution of pH (4.3) and visual aspects ( §4.4). Miranda et al. [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF] argued that the determination of chloride profiles, the use of the phenolphthalein test, or the simple measurement of potentials before and after the application of ECE cannot guarantee its efficacy, and underlined the importance of using techniques that provide quantitative determinations of corrosion kinetics. Orellan et al. [START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF] added that a polarization resistance technique is needed to confirm that the steel has re-passivated after treatment.

1.7%bwce] R Ti [NaOH] / [tank] [G] / [RS 1, 2 , 4, 8] 7, 14, 21, 28, 35, 42 Carmona [40]

Ion migration

The current flowing from the rebar to the anode allows the migration of ions (anions Cl -, OH - migrate from the rebar to the external anode and cations Na + and K + migrate towards the rebar). The extraction of chloride ions from the vicinity of the rebar and from the concrete cover is expected to decrease corrosion. The efficacy of chloride extraction depends on the chloride transport number t Cl that depends on the chloride concentration and on the amount of other ions present in the medium. The transport numbers of the different ions [START_REF] Elsener | Repair of reinforced concrete structures by electrochemical techniques -Field experience[END_REF] are indicated in Table 5. The quantity of chloride ions extracted (Cl ext ) per m² per second is obtained using Equation 9.

Cl ext = i.t Cl. G/F

Equation 9

Where i is the current density, t Cl is the transport number of chlorides, G is a geometrical constant (f (rebar-concrete interface)) and F is the Faraday constant.

Assuming that the current remains constant, the quantity of chloride ions extracted will vary according to the transport number of the different ions present in the concrete pore solution [START_REF] Polder | Eletrochemical desalination of cores from a reinforced concrete coastal structure[END_REF][START_REF] Ihekwaba | Structural shape effect on rehabilitation of vertical concrete structures by ECE technique[END_REF].

Efficacy of ECE

The authors calculated the efficacy of chloride extraction (Cl eff ) according to Equation 10 based on the available published results using current control mode.

𝐶𝑙 𝐸𝑓𝑓𝑖𝑐𝑎𝑐𝑦 = 100 𝑥 𝐶ℎ𝑙𝑜𝑟𝑖𝑑𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑏𝑒𝑓𝑜𝑟𝑒 𝐸𝐶𝐸 -𝐶ℎ𝑙𝑜𝑟𝑖𝑑𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑎𝑓𝑡𝑒𝑟 𝐸𝐶𝐸 𝐶ℎ𝑙𝑜𝑟𝑖𝑑𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑏𝑒𝑓𝑜𝑟𝑒 𝐸𝐶𝐸 Equation 10

Figure 3 presents the efficacy of ECE using a current control mode and current densities expressed in surface of steel rebar (RS) applied to reinforced concretes or mortars contaminated by chloride ions from different sources (studies from Table 4). The authors calculated the chloride extraction efficacy close to the rebar (Re) based on free (f) or total (t) chloride ions. In some papers, it was not clear if the chloride concentration after ECE was determined close to the rebar or averaged from the concrete cover. We therefore considered the data as the average chloride extraction in the concrete (Av). Figure 3 plots chloride efficacy versus total charge in the range of 168 to 39 782 A.h/m² RS. The total charge is indicated on the abscissa axis, as well as duration and applied current density, which are noted in brackets.

The authors highlight the need to include these three types of information (applied current density, duration and total charge) as part of the design. When the efficacy is related to a parameter of the ECE set-up, it is indicated in Figure 3 as f (parameter). Finally, results

pertaining to OPC binder are presented on the right-hand side of the graph, whereas the results for the other binders are presented on the left.

For OPC binder, which is the main cement studied in literature, chloride extraction efficacy in areas located close to the rebar increases from 25 to 97% as the applied total charge increases from 168 to 1344 A.h/m² RS. The application of higher total charges ranging from 2016-39 782

A.h/m² RS with current densities above 2A/m² RS can cause detrimental mechanical degradations [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF].

Kim et al. [START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF] found that an increase in the total charge resulted in a decrease in the chloride concentration around the steel rebar (i.e., at a depth of 27-30 mm) for current densities that varied from 0.25 to 0.75 A/m² RS and were applied for 28 days, whatever the binder concerned.

Yeih et al. [START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF] studied the efficacy of chloride extraction using different current densities varying between 1 to 8 A/m² RS and different durations (7, 14, 21, 28, 35, 42 days) applied on different reinforced concretes (w/c ratio (0.4 or 0.66)) and concrete covers (25 or 50 mm). They proposed a unified regression curve describing the evolution of the chloride content near the rebar versus the total charge [START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF]. Looking at the overview of many data (Figure 3), the authors can recommend for ordinary Portland cement a total charge of 1 000 -2 000 A.h/m² RS for achieving a 80% efficacy.

On the left-hand side of Figure 3, chloride extraction is also shown to increase with total charge for non-OPC binders such as BFSC [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Stoop | Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms[END_REF], binary (40%OPC+60%GGBS) or ternary binders (40%OPC+40%GGBS+20%FA) [START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF] and CEM II/A-S (6-20% slag) [START_REF] Bouteiller | Electrochemical chloride extraction of a beam polluted by chlorides after 40 years in the sea[END_REF].

Finally, the results of Tissier et al [START_REF] Tissier | Etude des processus de corrosion et de la réparation par traitement électrochimique d'extraction des ions chlorure dans les bétons armés doublement contaminés carbonatation et chlorures)[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF][START_REF] Tissier | Study of an electrochemical chloride extraction treatment on a both carbonated and chlorinated reinforced concrete[END_REF][START_REF] Tissier | Corrosion processes of carbonated chloride-contaminated reinforced concrete and electrochemical chloride extraction[END_REF] are original because the reinforced concretes made with OPC or CEM III/B binders were contaminated by both chloride ions and carbonation prior to ECE. The excellent extraction efficacy reported was due to the free form of most chloride ions after carbonation and the thinness of the concrete cover (17 mm).

From a practical perspective, clear criteria are required to ensure that a satisfactory chloride extraction has been achieved. The literature proposes the following criteria: (i) attaining a certain percentage of extracted chlorides and/or (ii) obtaining chloride content values in the vicinity of the rebar that are below a threshold content, such as 0.4% bwce.

Figure 3: Efficacy of chloride extraction applied to reinforced concrete specimens (chloride concentration close to the rebar (Re for rebar (red)) or chloride concentration of concrete cover (Av fo r Average, (blue)). Efficacy was calculated based on total or free chloride ions (shown in dark or pale colours, respectively ). A star following the reference (top axis) indicates that the reinforced specimens were contaminated by both chlorides and carbonation prior to ECE.

Influencing factors on chloride extraction

The authors used the published results to analyse the factors influencing chloride extraction efficacy, i.e. concrete properties, chloride characteristics, electrolyte type and duration of treatment.

Concrete properties

Concrete properties such as w/c ratio, concrete cover, binder type and contamination can have an impact on ECE efficacy.

Water/cement ratio

Polder et al. [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Polder | Laboratory investigation of electro-chemical chloride extraction from concrete with penetrated chloride[END_REF] found no impact on the extraction efficacy when ECE was applied to concretes with different w/c ratios (0.4 up to 0.65). Zhu et al. [START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF] did not find a clear trend between w/c ratios (0.32, 0.4 and 0.5) and chloride extraction efficacy. Other authors [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF][START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF][START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF][START_REF] Orellan | Efficacité et effets secondaires des techniques électrochimiques de maintenance des structures en béton armé[END_REF] found that specimens with high water/cement ratio exhibit a higher chloride extraction efficacy due to their greater porosity. In his thesis, Orellan [START_REF] Orellan | Efficacité et effets secondaires des techniques électrochimiques de maintenance des structures en béton armé[END_REF] showed that the w/c ratio (0.45 or 0.7) influences the efficacy of the average chloride extraction value but does not influence chloride extraction efficacy in the vicinity of the rebar. Swamy et al. [START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF] observed that the efficacy of chloride removal from reinforced concrete beams was neither consistent nor concurrent with the w/c ratio (0.6 or 0.75).

Concrete cover

Several authors found that a thinner concrete cover permits a more effective chloride extraction due to lower electrical resistance and shorter chloride transport distances [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF][START_REF] Orellan | Efficacité et effets secondaires des techniques électrochimiques de maintenance des structures en béton armé[END_REF]. As an example, Fajardo et al. [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Fajardo San | Limites d'utilisation des techniques électrochimiques de maintenance non permanentes pour les ouvrages en béton armé[END_REF] obtained a chloride extraction efficacy that varied between 75% and 30% respectively for specimens with a 20 mm and 50 mm concrete cover after applying a current density of 1 A/m 2 RS for 90 days.

Binder type

When Polder [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Polder | Eletrochemical desalination of cores from a reinforced concrete coastal structure[END_REF][START_REF] Polder | Laboratory investigation of electro-chemical chloride extraction from concrete with penetrated chloride[END_REF] applied ECE to reinforced concretes cast with OPC or BFSC, no impact on chloride extraction efficacy was observed. However, it is important to note that the driving voltages needed to obtain the required current density were significantly different given the resistivity of each binder: whereas OPC specimens with resistivity in the range of 100-200 ohm.m required 3-5 V to obtain a current density of 1 A/m² RS and 16-30 V for 4 A/m² RS, BFSC specimens with resistivity in the range of 400-1000 ohm.m needed 16 V (1A/m² RS) and 38 V (4A/m² RS) to obtain the same density. Kim et al. [START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF] found that it was easier to remove chlorides from concretes made with Portland cement than from cements containing GGBS (40%OPC+60%GGBS) or ternary mixtures (40%OPC+40%GGBS+20%FA). A higher binding capacity of chlorides in concrete may therefore result in a lower chloride extraction efficacy.

Concrete contamination

Tissier et al. [START_REF] Tissier | Etude des processus de corrosion et de la réparation par traitement électrochimique d'extraction des ions chlorure dans les bétons armés doublement contaminés carbonatation et chlorures)[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF] obtained a chloride efficacy of above 90 % for specimens that had been carbonated and chloride contaminated (via different methods) prior to ECE. The high efficacy was obtained for all binder types (OPC or CEM III/A) This very good result was explained by two factors, namely the thinness of the concrete cover (17.5 mm), and the fact that carbonation tends to release the bound chlorides into the matrix as free chlorides.

Chloride characteristics

The efficacy of ECE treatment is affected by chloride characteristics, namely the method of contamination, the initial content and the type of chlorides analysed (total, free).

Method of chloride contamination

Chloride contamination can result from different methods:

-Mix addition is the most common means of contamination, because the chloride contamination is immediate. Studies have used NaCl (Cl -range 1 % -8 % bwce) [START_REF] Buenfeld | Influence of electrochemical chloride extraction on the bond between steel and concrete[END_REF][START_REF] Carmona | Efficiency of a conductive cement-based anodic system for the application of cathodic protection, cathodic prevention and electrochemical chloride extraction to control corrosion in reinforced concrete structures[END_REF][START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF][START_REF] Garcés | Effect of the reinforcement bar arrangement on the efficiency of electrochemical chloride removal technique applied to reinforced concrete structures[END_REF][START_REF] Siegwart | Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF][START_REF] Bertolini | Effects of electrochemical chloride extraction on chemical and mechanical properties of hydrated cement paste[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF][START_REF] Yeih | Electrochemical chloride removal for reinforced concrete with steel rebar cage using auxiliary electrodes[END_REF][START_REF] Vennesland | Electrochemical removal of chlorides from concrete -Effect of bond strength and removal efficiency[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part II: Microstructural characterization[END_REF][START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF][START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF] or CaCl 2 (Cl -range 2.35 % -3 %bwce) [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF][START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF],

-Wetting/drying cycles in NaCl solution [START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF][START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF] or in artificial sea water [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF],

-Submersion in NaCl [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF],

-Salt spray chamber [START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF],

-Outdoor exposure conditions or real structures: splash zone (8 years) [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF], sea water (16 years) [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Stoop | Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms[END_REF], sea water tide (40 years) [START_REF] Bouteiller | Electrochemical chloride extraction of a beam polluted by chlorides after 40 years in the sea[END_REF].

Marcotte et al. [START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF] used rather strong ECE conditions (8.37 A/m² RS for 56 days) and found that admixed chlorides are more difficult to extract than ingressed chlorides: chloride extraction efficacies in the vicinity of the rebar were respectively 79 %, 74 % and 3 % when chloride contamination was achieved by mix + ingress additions (initial Cl -0.36 % bwmo), ingress (initial Cl -0.38 % bwmo) or mix (initial Cl -0.25 % bwmo). The study did not clearly indicate whether the chloride was free or total.

Initial chloride content

Several authors stated that chloride extraction is greater for concretes with a higher initial chloride content [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF][START_REF] Ihekwaba | Pull-out and bond degradation of steel rebars in ECE concrete[END_REF][START_REF] Tang | Effect of an electrical field on the removal of chloride ions from concrete slabs[END_REF][START_REF] Castellote | Electrochemical removal of chlorides -Modelling of the extraction, resulting profiles and determination of the efficient time of treatment[END_REF]109]. Tang [START_REF] Tang | Effect of an electrical field on the removal of chloride ions from concrete slabs[END_REF] concluded that ECE treatment has a limit beyond which the further removal chloride becomes difficult. Finally, no correlation was found between the initial chloride and the remaining chloride after treatment.

Type of chloride analysed (free, total or bound)

Elsener [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF] described the equilibria between chemically bound, physically adsorbed and free chlorides in the ECE treatment (Figure 4). Part of the remaining chloride measured was in an insoluble form after one or more ECE treatment cycles [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF]. On the other hand, Orellan et al. [START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF] found that ECE efficacy on concrete was slightly affected by the C 3 A content (4.3 or 9.5%) in OPC cement because only a part of the bound chloride ions was released. (from [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF]).

Electrolyte

A study by Monteiro et al. [START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF] reported that the use of tap water as the electrolyte caused an increase in the resistance and thus also in the polarization of the anode.

Duration

Although most ECE treatments last 28 or 56 days, other periods were reported ranging from 3-112 days. The extraction of chlorides was reported to be at its highest at the beginning of the treatment [START_REF] Buenfeld | Influence of electrochemical chloride extraction on the bond between steel and concrete[END_REF][START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF][START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF][START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF][START_REF] Tang | Effect of an electrical field on the removal of chloride ions from concrete slabs[END_REF]. This phenomenon is easily explained: at the beginning of the process, chloride ions are the main negative ions that migrate through the current flow. After some time, the chloride transport number decreases (due to the extraction) and the hydroxyl ions that are formed at the cathode compete as negative charge carriers.

Tang et al [START_REF] Tang | Effect of an electrical field on the removal of chloride ions from concrete slabs[END_REF] suggested that there is a limit beyond which the further removal of chloride ions becomes difficult.

Steel corrosion mitigation

The characterization of ECE efficacy requires evidence that steel rebar corrosion has been mitigated following the extraction of chloride ions (Table 4). Table 6 presents published results on resistivity, half-cell potential and corrosion current density.

The resistivity of the concrete is a key parameter that affects both corrosion behaviour (a concrete resistivity below 10 kohm.cm is linked to a high corrosion probability [START_REF] Polder | RILEM TC 154-EMC:Electrochemical Techniques for Measuring Metallic Corrosion -Recommendations -Test methods for on-site measurements of resistivity of concrete[END_REF]) and ECE treatment conditions (the voltage increases with resistivity).

Half-Cell Potential (Ecorr) indicates the probability of corrosion. When the measured potential is more positive than -250 mV CSE, there is a 90 % probability that rebar corrosion is absent [START_REF] Astm | Standard test method for half-cell potentials of uncoated reinforcing steel in concrete[END_REF]. An increase of potential values after treatment indicates a lower probability of corrosion [START_REF] Elsener | RILEM TC 154-EMC:Electrochemical Techniques for Measuring Metallic Corrosion -Recommendations -Half-cell potential measurements -Potential mapping on reinforced concrete structures[END_REF]. Miranda et al. [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF][START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF] warned, however, that this approach might give rise to serious errors when evaluating the active or passive state of the rebar if the saturation degree of the pores is not identical. A significant ennobling of potentials after the application of ECE cannot therefore be deemed sufficient evidence to demonstrate the efficacy of the treatment; evidence of a decrease in corrosion current density is also needed.

Corrosion current density (Icorr) is an instantaneous value. A value in the range of above 1, 1-0.5, 0.5-0.1 or less than 0.1 µA/cm² respectively indicates a high, moderate, low or negligible corrosion level [START_REF] Andrade | RILEM TC 154-EMC:Electrochemical Techniques for Measuring Metallic Corrosion -Recommendations -Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF]. A decrease in corrosion current density after treatment will therefore highlight mitigation of corrosion.

The efficacy of ECE treatment is dependent on the mitigation of corrosion. The latter must be evaluated by comparing electrochemical characterizations before ECE, immediately after ECE and after the depolarization of the steel rebar.

It is important to highlight the following conditions:

a) The comparison of electrochemical results is only valid if the "steel rebar in concrete" system is at its equilibrium (i.e.; not polarized) and b) The comparison of electrochemical results should only be carried out for the same sample if conditions of temperature, humidity and oxygen availability remain identical.

Finally, it is important to note that increasing Ecorr and decreasing Icorr can indicate a mitigation of corrosion, but this in itself does not suffice to conclude that the rebar is passivated.

No clear demonstration of re-passivation has been published to date.

Electrical resistivity

Polder [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF] states that no major changes occurred during the ECE process, resulting in the relatively stable driving voltages needed to obtain the fixed currents. In contrast, Miranda [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF] found that the voltage increased from 7 to 40 V due to the progressive elimination of the chlorides and the increase in mortar resistivity. Buenfeld [START_REF] Buenfeld | Effect of chloride removal on rebar bond strength and concrete properties[END_REF] and Yeih [START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF] also reported an increase in resistivity due to the extraction of chlorides. Despite obvious differences between studies, resistivity is not conclusive with regard to corrosion mitigation.

Half-cell potentials

Half-cell potential is an interesting measurement for evaluating ECE. Immediately after the treatment, potential values are highly electronegative due to the polarization of the steel rebar [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF][START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF][START_REF] Bouteiller | Electrochemical chloride extraction of a beam polluted by chlorides after 40 years in the sea[END_REF][START_REF] Tissier | Etude des processus de corrosion et de la réparation par traitement électrochimique d'extraction des ions chlorure dans les bétons armés doublement contaminés carbonatation et chlorures)[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF][START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF][START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF]. This confirms that the treatment has been applied correctly. It is impossible to predict how long it will take for the rebar to depolarize, and the results from Table 6 show this process to take 1 to 4 months. As depolarization occurs over time, the halfcell potential gradually becomes nobler [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF][START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF][START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF][START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF][START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF] and reaches a stable value.

If the latter is less electronegative than the value obtained prior to ECE, it can indicate a decrease in corrosion activity that is consistent with the reduction in chloride ions around the rebar.

Corrosion current density

Before ECE, corrosion current densities are in the range of 5 to 27 µA/cm² (Table 6), with the highest density reported at 100 µA/cm² in reference [START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF]. Higher corrosion current densities immediately after ECE can be explained by (i) the polarization of the steel, (ii) the accumulation of hydroxyl ions at the rebar (iii) a decrease in oxygen at the rebar/mortar interface [START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF]. After depolarization (over 1 to 4 months), decreases in corrosion current density ranging from fifty percent [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF] to even 1 or 2 orders of magnitude were obtained [START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF][START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF].

Influencing factors on steel corrosion mitigation

ECE conditions

At a chosen ECE current flow, the increase in duration (and thus an increase in the total charge) increases the efficacy of corrosion mitigation [START_REF] Tissier | Etude des processus de corrosion et de la réparation par traitement électrochimique d'extraction des ions chlorure dans les bétons armés doublement contaminés carbonatation et chlorures)[END_REF][START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF]. Kim et al. [START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF] studied the optimisation of current density applied during ECE (varying from 0.25 to 0.75 A/m² RS). They observed that an increase in current density resulted in an increase in the half-cell potential and a decrease in the corrosion rate.

Binder types

Kim [START_REF] Vennesland | Electrochemical removal of chlorides from concrete -Effect of bond strength and removal efficiency[END_REF] reported that in ECE treatment carried out at 0.75 A/m² RS, the corrosion rates were 2.5 and 4.3 µA/cm² for GGBS and ternary mixes respectively. These rates are substantially lower than those found for OPC mix (8 µA/cm²), presumably due to a lower supply of water and oxygen (i.e. cathodic reactions) through concretes that contain pozzolanic materials. Tissier et al. [START_REF] Tissier | Etude des processus de corrosion et de la réparation par traitement électrochimique d'extraction des ions chlorure dans les bétons armés doublement contaminés carbonatation et chlorures)[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF] found that corrosion rates decreased by two orders of magnitude for micro-concrete specimens that had been contaminated by both carbonation and chloride ions prior to ECE, whatever the binder used (OPC or CEM III/A (max 68% slag)).

Evolution of pH

The evolution of pH has been studied in two main contexts, namely the pH of the electrolyte, which is central to determining a suitable treatment, and the pH of the rebar/concrete interface, which should provide evidence of improved treatment efficacy.

pH of the electrolyte

The pH of the electrolyte has been studied throughout the ECE treatment to ensure that the electrolyte does not become acid or differ too greatly from the concrete pH. Maintaining a high pH also allows monitoring of Cl in the electrolyte; at low neutral to pH it is oxidised and disappears as Cl 2 (Equation 8).

Water has a pH of 7 and is not a suitable electrolyte; during ECE, its pH will decrease to 3.5 (and produce a chlorine gas smell) without circulation, whereas the use of circulation will lead pH to decrease to 4-6 [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF], 6.5 [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF] and 5-5.5 [START_REF] Carmona | Efficiency of a conductive cement-based anodic system for the application of cathodic protection, cathodic prevention and electrochemical chloride extraction to control corrosion in reinforced concrete structures[END_REF]. Using calcium hydroxide (Ca(OH) 2 ) electrolyte (pH=12.5), Polder et al. [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF] observed a local acidification in which the pH dropped to about 5.5. While performing ECE on beams, Swamy et al. [START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF] also observed the acidification of the calcium hydroxide electrolyte and chlorine smell in the laboratory for 2-8 weeks. A renewal of the calcium hydroxide electrolyte prevented these reactions (stable 12.5 pH and no chlorine smell).

pH of the rebar/concrete interface

During ECE, pH values increase at the rebar/concrete interface because of the formation of hydroxyl ions at the cathode (Equation 5). This should play a role in corrosion mitigation and durable protection. In an ECE set-up with water electrolyte, Buenfeld [START_REF] Buenfeld | Influence of electrochemical chloride extraction on the bond between steel and concrete[END_REF] used a phenolphthalein indicator and found that a uniform purple staining was obtained 48 h after the spraying on areas that included the concrete adjacent to the rebar. Zhu et al. [START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF] used calcium hydroxide electrolyte and found an increase in pH close to the rebar. The use of sodium hydroxide (NaOH 0.1M) solution in a study by Yeih et al [START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF] led pH values to increase to 12.4, with the total charge attaining 8064 A.h/m². These authors suggested that a pH value of 11.5 in the vicinity of the rebar might be one of the performance criteria for determining ECE efficacy. Finally, in the case of a realkalisation treatment, an increase in pH at the rebar was also consistent with halting the corrosion [START_REF] Yeih | A study on the efficiency of electrochemical realkalisation of carbonated concrete[END_REF][START_REF] Tong | Efficiency investigations of electrochemical realkalisation treatment applied to carbonated reinforced concrete -Part 1: Sacrificial anode process[END_REF]. Corrosion products are observed before treatment: they do not usually cover the entire surface of the steel rebar and they do not always appear as pits, although corrosion is induced by chloride ions (Table 7). After ECE, whitish deposits [START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Ihekwaba | Pull-out and bond degradation of steel rebars in ECE concrete[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF] or black-brown products [START_REF] Buenfeld | Influence of electrochemical chloride extraction on the bond between steel and concrete[END_REF][START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF][START_REF] Buenfeld | Effect of chloride removal on rebar bond strength and concrete properties[END_REF][START_REF] Vennesland | Electrochemical removal of chlorides from concrete -Effect of bond strength and removal efficiency[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF] were observed on the surface of the steel rebar (Table 7). Ihekwaba [START_REF] Ihekwaba | Pull-out and bond degradation of steel rebars in ECE concrete[END_REF] reported that the whitish deposits are due to the alkali hydroxides formed as a result of cathodic action during ECE application. Vennesland [START_REF] Vennesland | Electrochemical removal of chlorides from concrete -Effect of bond strength and removal efficiency[END_REF] studied the blackish products and found that ECE induced an accumulation of cations (sodium and potassium) close to the rebar. Marcotte [START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF] explained the black products by the formation of magnetite: although HFeO 2 -and FeO 2 2- ions are likely to have been formed in the high pH/ low potential conditions of the treatment, the steel would become more noble as the effect of the cathodic polarization decreased over time and, with the redistribution of the OH -ions, magnetite (Fe 3 O 4 ) would become the dominant product formed. Miranda [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF][START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF] considered that the changing appearance of the corrosion products from reddish to dark-brown was due to the ECE-induced reduction of ferric compounds in magnetite (very small portion of goethite), as determined by X-ray diffraction.

Visual observations and gravimetric analysis performed on the rebar

Finally, based on gravimetric analysis, Kim [START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF] found that the mass loss of corrosion products on samples with no treatment was in the range of 0.79 to 0.88 % whereas this value was significantly reduced in samples treated with different strengths of cathodic current.

Loss of bond at the steel rebar/concrete interface

The bond strength between steel rebar and concrete occurs through adhesion, friction and the mechanical interlocking of surface deformation. Several authors have studied the impact of ECE on bond strength [35, 59, 76-78, 80, 82]. Marcotte et al. [START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part II: Microstructural characterization[END_REF] observed a change in the microstructure of the mortar at the rebar/concrete interface (no C-S-H, an increase in the proportion of Ca(OH)2 and the formation of new cementitious phases) that may explain a loss of bond. Miranda et al. [START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF] suggested that a treatment-related reduction of certain ferric compounds in magnetite probably weakens the bond at the interface.

Buenfeld et al. [START_REF] Buenfeld | Influence of electrochemical chloride extraction on the bond between steel and concrete[END_REF][START_REF] Buenfeld | Effect of chloride removal on rebar bond strength and concrete properties[END_REF] studied plain rebar treated with a current density of between 0.02 and 0.75 A/m² RS for periods of up to 32 weeks. They found that prior to ECE the bond strength of specimens containing chlorides was around 57 % higher than that of specimens with no chlorides. This was attributed to corrosion enhancing the bond through a combination of prestressing and mechanical interlock. After 2 weeks of ECE, the enhanced bond strength was eliminated. The authors explained this result by the removal of corrosion products, as confirmed by visual examination. They also noted that the bond strength was heavily dependent upon the time interval between the ECE current being turned off and the bond test being carried out. Vennesland et al. [START_REF] Vennesland | Electrochemical removal of chlorides from concrete -Effect of bond strength and removal efficiency[END_REF] monitored bond stress evolution for samples subjected to ECE for 26 days and for different applied currents (1.6, 4 and 8 A/m² RS) after 1, 2, 7, 17 and 27 days. They noted extreme decreases in bond strength when higher currents were applied. After the end of ECE, they first observed a drop in bond stress between 7 and 17 days post treatment, followed by an increase when compared to the level of the reference specimen. Chang [START_REF] Chang | A study of the bond degradation of rebar due to cathodic protection current[END_REF] reported significant bond loss and softening around rebars due to applied charge at ECE levels and proposed a parameter based on the impressed current density and polarization time. Ihekwaba et al. [START_REF] Ihekwaba | Pull-out and bond degradation of steel rebars in ECE concrete[END_REF] performed pull-out tests and found that the specimens that were electrochemically treated with a very high impressed current density of 28.6 A/m² RS consistently showed increased degradation in bond strength and rebar slippage compared with specimens that had been impressed with a lower current density of 9.5 A/m² RS. Yeih et al. [START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF] based their research on the "pushing-away concept", which does not aim to completely remove free chloride but rather seeks a compromise between corrosion prevention and bond degradation.

Loss of compressive strength

Marcotte et al. [START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part II: Microstructural characterization[END_REF] found that a reduction of compressive strength is one of the consequences of the cement phase alteration (composition and morphology) at the concrete/steel interface.

Swamy et al. [START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF] also noted a loss in compressive strength and attributed it to ASR.

Alkali-silica reaction (ASR)

Sodium or potassium cations migrate towards the steel rebar during ECE ( §5.1.2). If the concrete contains reactive aggregates, this increase in alkaline content may lead to alkali silica reaction (ASR). To avoid adding sodium or potassium ions to concrete, a Ca(OH) 2 solution is usually chosen as the electrolyte. Work by Swamy et al [START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF] demonstrated that ECE could trigger ASR in concretes that contain a reactive aggregate, i.e. synthetic fused silica (5 % or 15 % by mass of total aggregates). The concrete beams studied in the aforementioned article exhibited significant cracks after an 8-week treatment at a current density of 1 A/m² RS.

Miller [START_REF] Miller | The perception of the ASR problem with particular reference to electrochemical treatments of reinforced concrete in EFC[END_REF] states that ECE will not cause ASR in the field mainly due to ECE being a short term treatment and ASR development is slow. On the other hand, Banfill [START_REF] Banfill | Electrochemical re-alkalisation, chloride extraction and alkali-silica reaction in concrete Concrete Solutions Thessaloniki[END_REF] based on a mechanistic model for ASR found ECE to enhance the reaction, depending on the presence of chloride. It is therefore important to consider aggregate ASR reactivity before performing ECE.

In the case of structures contaminated by chlorides and already suffering from ASR, the literature reported that lithium-based electrolytes could be suitable for use in ECE treatment.

In Ueda's studies [START_REF] Ueda | Influence of electrolyte temperature on efficiency of electrochemical chloride removal from concrete[END_REF][START_REF] Ueda | Influence of temperature on electrochemical remedial measures and complex deterioration due to chloride attack and ASR[END_REF], the current flow concerns the concrete surface and not the steel surface, it was found that ECE treatment could inject Li + into the concrete from the electrolyte solution (Li 3 BO 3 , 40° C) and produce an accumulation of Li + around the rebar. Indeed, Li + ions competed with the alkali ions (Na + and K + ) and would therefore limit or suppress/prevent the ASR-induced expansion of concrete. Ueda [START_REF] Ueda | Influence of temperature on electrochemical remedial measures and complex deterioration due to chloride attack and ASR[END_REF] studied the penetration of Li + ions using different electrolyte solutions (Li Marcotte et al. [START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF] observed significant corrosion rates one month after treatment, and attributed this to the reduction of the passive film along with any existing corrosion products during the treatment. This would lead to general corrosion, and the reduction of dissolved oxygen in the concrete pore solution would prevent repassivation. They added that the increase in pH at the steel surface puts it in the alkaline corrosion region of the Pourbaix diagram (HFeO 2 -and FeO 2 2-formation). However, it is important to note that samples were maintained in solution in this study, thus limiting the oxygen supply and preventing repassivation.

Elsener et al. [START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF] used chloride sensors to confirm that only a negligible amount of bound chloride was released two years after the end of ECE treatment.

Bouteiller et al. [START_REF] Bouteiller | Electrochemical chloride extraction of a beam polluted by chlorides after 40 years in the sea[END_REF] reported no difference between steel potential values obtained 2 months after ECE and those obtained after 17 months of immersion of the beam in sea water. They also noticed that the free chloride concentration remained stable close to the steel rebar for both the treated area and the control area.

Carmona et al. [START_REF] Carmona | Efficiency of a conductive cement-based anodic system for the application of cathodic protection, cathodic prevention and electrochemical chloride extraction to control corrosion in reinforced concrete structures[END_REF] tested chloride penetration sensitivity after treatment. They found that the capacity of chloride absorption during the 24 weeks of contamination was 17.4 % lower in the treated specimens than in non-treated samples.

Shan et al. [START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF] immersed the samples in a salt solution after ECE treatment. EIS, Ecorr and Icorr values showed that corrosion of the rebar resumed after 50 days.

Finally, Stoop et al. [START_REF] Stoop | Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms[END_REF] studied the chloride ion redistribution within the cover over a one-year period, then developed a model to predict redistribution and thus risks of corrosion re-initiation over a 10-year period.

Swamy et al. [START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF] studied the effect of ECE on structural strength and bond for 400 days following the end of treatment. They did not report any adverse effect, although some of the treated specimens did contain fused silica (which is ASR reactive).

REGULATIONS

Different projects completed in North America have followed guidelines developed by the Strategic Highway Research Program (SHRP) [START_REF] Bennett | Evaluation of NORCURE Process for electrochemical chloride removal from steel reinforced concrete bridge components[END_REF][START_REF] Bennett | Chloride removal implementation[END_REF][START_REF] Bennett | Electrochemical chloride removal and protection of concrete bridge components : laboratory studies[END_REF][START_REF] Bennett | Electrochemical chloride removal and protection of concrete bridge components : field trials[END_REF] and by the Federal Highway Administration [START_REF] Clemena | Pilot applications of electrochemical chloride extraction on concrete piers in Virginia[END_REF][START_REF] Sharp | Electrochemical chloride extraction: influence of concrete surface on treatment[END_REF]. A NACE international publication on electrochemical chloride extraction from reinforced concretes was published in 2001 [START_REF] Nace | Electrochemical chloride extraction from steel reinforced concrete-A State-ofthe-art report[END_REF] and re-approved in 2018 [START_REF] Nace | Electrochemical chloride extraction from steel reinforced concrete -A state of the art report[END_REF].

In

MODELLING

Although experiments are a relevant approach when investigating ECE, this type of treatment time or chloride concentration and found that the quantity of free chloride removed from behind the rebar can increase with treatment duration.

Bound chlorides were not considered in the models in the second and third studies presented above [START_REF] Mietz | Electrochemical rehabilitation methods for reinforced concrete structuresa state of the art report[END_REF][START_REF] Yeih | A study on the efficiency of electrochemical realkalisation of carbonated concrete[END_REF], yet this phenomenon could have an influence on mechanisms. Li et al. [START_REF] Li | Modelling of electrochemical chloride extraction from concrete: Influence of ionic activity coefficients[END_REF][START_REF] Li | Finite element modelling of chloride removal from concrete by an electrochemical method[END_REF] investigated the effect of ionic concentrations on diffusion and migration of ions in a concentrated pore solution when a constant current is applied. They revealed that the higher the current density applied or the concentration of the species, the more the "activity coefficient" (chemical potential change arising from ion-ion and ion-solvent interaction) can affect ionic diffusion and migration. The authors then presented a finite element model and performed numerical investigations in order to show the effects of applied current density, treatment duration, the diffusion coefficient of different ions, ionic binding, boundary conditions and medium porosity. They highlighted that even if the removal of chloride ions increases with both applied current density and time, a long-term treatment is likely to be less effective in decontaminating the zone at the steel/concrete interface and would even be counterproductive: their results show that during the treatment, the potential gradient decreases near the steel but increases near the anode. They also show the influence of chloride binding on chloride removal. The same authors (Wang et al. [START_REF] Wang | A two-dimensional model of electrochemical chloride removal from concrete[END_REF]) presented a twodimensional computer model that considered the electrostatic coupling of ions and other factors (ionic adsorption, porosity and tortuosity of pore structure). Based on their results, they underlined that when possible, the use of anodes on two opposite sides of the structure improves the removal of chloride in the area behind the rebar. This is in accordance with results obtained during experimental studies [START_REF] Arya | Factors influencing electrochemical removal of chloride from concrete[END_REF][START_REF] Ihekwaba | Aspects of electochemical rehabilitation of chloride contaminated concrete structures[END_REF]. Castellote et al. [START_REF] Castellote | Electrochemical removal of chlorides -Modelling of the extraction, resulting profiles and determination of the efficient time of treatment[END_REF], sought to identify an optimum treatment duration beyond which no further chloride is extracted due to the decrease in chloride transfer number through the increase of hydroxyl ions near the rebar. This theoretical optimum duration was defined as a function of the electrical charge passed, and was calculated using an empirical equation that considered the chloride diffusion coefficient decrease during the treatment. They also predicted chloride profiles after treatment according to the current applied.

While the previous works considered electro-neutrality condition, works conducted at the University of Plymouth [START_REF] Liu | Multi-phase modelling of ionic transport in concrete when subjected to an externally applied electric field[END_REF][START_REF] Liu | Three-phase modelling of electrochemical chloride removal from corroded steel-reinforced concrete[END_REF][START_REF] Xia | Numerical simulation of ionic transport in cement paste under the action of externally applied electric field[END_REF] used Poisson's equation to determine the electrostatic potential. However, Poisson's equation is theoretically applicable when no electrical field is applied. When ECE treatment is applied, the electrolysis of water ensures the electro neutrality. In the study [START_REF] Liu | Multi-phase modelling of ionic transport in concrete when subjected to an externally applied electric field[END_REF], the concrete was treated as a heterogeneous composite structure with two phases (mortar and aggregates that were assumed to be impermeable). The finite element model was developed for a voltage control mode and provided information about the migration of four ionic species (Cl -, Na + , OH -, K + ) during the treatment. The analysis of the results highlighted the variation and the influence of migration speed, distribution and total amounts of ionic species. They also found that local diffusion could have a significant influence on the evolution of migration speed. In the study [START_REF] Liu | Three-phase modelling of electrochemical chloride removal from corroded steel-reinforced concrete[END_REF], the concrete was presented as a threeconstituent composite (i.e. a mortar matrix, aggregates and the interfacial zone between them).

This two-dimensional model was also used to study how current flow pattern and the evolution of chloride concentration are affected by the use of different configurations of multiple rebars as cathodes. The authors underlined that the configuration of the rebars (cathode) in ECE is very important; a single well-located rebar provided a better extraction than the same rebar coupled with another rebar that was located closer to the anode. As described in numerous other experimental and numerical studies, although the amount of removed chloride increased with applied current density and treatment time, the daily removal rate decreased with treatment duration. Finally, they found that the two opposite effects on chloride extraction that resulted from the presence of aggregates, namely an increase in current and a change in porosity and tortuosity, almost cancelled each other out. Liu et al. [START_REF] Liu | Multi-phase modelling of ionic transport in concrete when subjected to an externally applied electric field[END_REF] also studied different parameters (aggregates and their effect on interfacial zone effect of rebar position, etc.) using a finite element model based on inverse parameterization of chloride diffusion coefficients.

They notably found that Na + and OH -were mainly located around the rebar at the end of the treatment, while K + was widely distributed between the rebar and the anode. They also studied the influence of cracks and found that chloride extraction was slower when cracks were perpendicular to the electric field (the study excluded aggregates for this analysis).These recent types of modelling were developed to facilitate the study of structures with complicated geometries and of numerous physical parameters, but have not always been validated with experimental data.

An experimental analysis by Cheng et al. [START_REF] Cheng | Numerical modeling of the effect of concrete porosity evolution on electrochemical chloride removal from concrete structures[END_REF] of how ECE affects the evolution of porosity in concrete structure observed a decrease in porosity with treatment time close to the cathode and anode regions of the concrete, with a higher decrease noted near the anode region. They proposed a numerical simulation of this aspect and highlighted that taking the evolution of porosity into account led to a better accuracy in ECE treatment prediction.

Following this analysis of some of the attempts to model the ECE treatment of concrete, we can conclude that there are a great number of parameters which can (or should) be considered.

Some authors proposed interesting models but did not always validate them by a comparison with experimental data. The use of some of the models can currently be considered to define or validate certain issues that are specific to concrete or inherent to the application of the treatment, but much work remains to be done. In particular, the type of used cement has a major influence on the porosity and the characteristics of the porous network (pore sizes and connectivity) and this aspect must be better defined in future models. Moreover, still in relation to the nature of the cement, the quantity of bound-free-total chlorides can be very different depending on cement and the kinetics of these chloride forms must be better evaluated.

Finally, models involving electrochemical data would be a step forward.

CONCLUSIONS

Based on this literature review the following conclusions and recommendations can be drawn:

1. Electrochemical chloride extraction has proven to be an efficient treatment for the extraction of chloride ions from the concrete cover. In order to continuously protect the rebar, constant current mode is preferred (when using a constant voltage mode the current received by the rebar will decrease over time as the resistance of the medium increases).

Moreover, as ECE seeks to protect the steel rebar from corrosion in concrete, the current density must be calculated according to the area of the steel rebar surface (A/m² RS) and not the concrete surface. Finally, it is important to specify the duration of the treatment in order to calculate the total charge (in A.h/m²), which is useful data when comparing results.

Figure 1 :

 1 Figure 1: Schematic representation of ECE set-up.

Figure 2 :

 2 Figure 2: Schematic representation of streamlines in an inhomogeneous field between rebar (cathode) and anode at the concrete surface.

Figure 4 :

 4 Figure 4: Equilibria between bound, adsorbed and free chloride in the ECE treatment

O 7 )

 7 solution to extract chloride from a reinforced concrete beam (4 cm cover) that had been exposed to a marine environment in France for 40 years. The treatment allowed the removal of up to 70 % of the free chlorides located in the vicinity of the rebar, 50 % of those in the intermediate cover and 5 % of those located at the surface of the concrete. In his thesis, Orellan [104] used identical experimental conditions and ranked different electrolytes from the most to the least effective in terms of chloride extraction, namely tap water (60 %), sodium-based electrolyte (54 %), and calcium and lithium-based electrolytes (40 %).

--

  Fine white products cover the steel surface -All steel samples had large areas (50 to 100 % of the surface area) of black-Rust stain was still present on the surface rather than pit corrosion -Mass loss: 0.48-0.55 %, 0.27-0.32 % and 0.26-0.29 % respectively for 2.5, 5 and 7.5 µA/cm² RS (Mass loss 0.79-0.88 % without ECE treatment).

  Europe, the technical guidelines written in 2003 in CEN TS 14038-2 were replaced in 2018 by an AFNOR draft standard (PR NF EN 14038-2) entitled "Electrochemical realkalization and chloride extraction treatments for reinforced concrete -Part 2: Chloride extraction" [15].

  Hassanein et al.[START_REF] Hassanein | A mathematical model for electrochemical removal of chloride from concrete structures[END_REF] developed a model to predict chloride extraction during a constant current ECE, in a two-step process. The first step corresponds to the rapid removal of free chloride ions and the second concerns the release of bound chloride ions. In their experiments, they found that a significant amount of chloride was removed during the first 4 weeks of treatment and very little during the next 4 weeks. The rate of release of bound chloride was determined using the difference between the chloride concentration at 4 and 8 weeks of treatment, assuming that all chlorides were all either initially bound or free (an intermediate value for their model was chosen). Several other input parameters were also used for chloride ions (diffusion coefficient, initial concentration, percentage of initially bound, etc.), concrete (pore solution fraction, density, cover thickness, etc.) and the electric field. They validated their model based on 12 cases and obtained reliable accuracy despite the use of estimated values for many parameters.In 2007, Toumi et al.[START_REF] Toumi | Experimental and numerical study of electrochemical chloride removal from brick and concrete specimens[END_REF] also used the Nernst-Planck equation, with concrete porosity density, pore solution composition, chloride-binding isotherm, and diffusion coefficients as input data. They found good accordance between the model and the experimental results and highlighted the important role played by the binding isotherm in the prediction of chloride removal.

2 . 5 . 6 .ooo 9 .

 2569 Design is an important step to achieve successful ECE (steel rebar surface, choice of electrolyte, position of the anode, continuous or intermittent treatment, etc.) 3. ECE efficacy should be determined according to multiple criteria: extraction of chlorides (free and total), mitigation of corrosion (half-cell potentials, corrosion current density, resistivity, etc.), pH and visual inspection. 4. ECE efficacy has been demonstrated based on analyses performed before and immediately after the ECE, and after steel rebar depolarization, with full consideration of treated and control samples. Optimisation of ECE (current density and duration) is needed to successfully demonstrate its efficacy without causing detrimental effects (such as altered composition and morphology at the rebar/concrete interface, loss of bond, etc.) and to propose performance criteria. Limitations of ECE are: Chloride ions penetrated behind the rebar (in this case Cathodic Protection should be preferred to ECE). Prestressed concrete (hydrogen embrittlement) Reactive aggregates (ASR risk) 7. ECE-related rebar repassivation should be studied in greater depth using in-situ techniques such as micro-Raman spectroscopy in laboratory. The only evidence provided in the literature concerns corrosion mitigation; no evidence of repassivation has been presented to date. 8. Models still need to be validated with experimental data. Further work is needed to demonstrate the long-term efficiency of ECE 1, 2, 5 and 10 years after treatment. Finally, this literature review clearly highlights the urgent need to transfer technology from a state of laboratory knowledge (experimental and modelling) to functional use in the practical field.

Table 1 :

 1 Abbreviations used in this paper.

	-	not determined
	Add	addition of chlorides in the mix
	AFNOR	Association Française de Normalisation
	ASR	alkali-silica reaction
	ASTM	The American Society for Testing and Materials
	aSW	artificial sea water
	aTi	activated Ti
	Av	average (efficacy of chloride extraction)
	be	beam
	BFSC	blast furnace slag cement
	bl	block
	bwce	by weight of cement
	bwco	by weight of concrete
	bwmo	by weight of mortar
	C	concrete
	CCP	conductive cement paste
	CFRP	carbon fibre reinforced polymer
	co	column
	COST	Cooperation in Science and Technology
	CP	cathodic protection
	CS	concrete surface
	CSE	Copper-copper sulfate electrode
	CSH	Calcium-silicate hydrate
	cu	cube

Table 4

 4

describes the current control mode and applied current density based on the steel rebar surface. Here, 1A/m² RS is the most frequently used value

[START_REF] Zhu | Application of carbon-fiberreinforced polymer anode in electrochemical chloride extraction of steel-reinforced concrete[END_REF][START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF][START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF][START_REF] Siegwart | Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions[END_REF][START_REF] Stoop | Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms[END_REF][START_REF] Herrera | Electro-chemical chloride extraction: Influence of C3A of the cement on treatment efficiency[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF][START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Bertolini | Effects of electrochemical chloride extraction on chemical and mechanical properties of hydrated cement paste[END_REF][START_REF] Polder | Eletrochemical desalination of cores from a reinforced concrete coastal structure[END_REF][START_REF] Polder | Electrochemical chloride tests of concrete cores from a coastal structure[END_REF][START_REF] Bouteiller | Electrochemical chloride extraction of a beam polluted by chlorides after 40 years in the sea[END_REF][START_REF] Tissier | Electrochemical chloride extraction to repair combined carbonated and chloride contaminated reinforced concrete[END_REF][START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF][START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF][START_REF] Polder | Laboratory investigation of electro-chemical chloride extraction from concrete with penetrated chloride[END_REF][START_REF] Tang | Effect of an electrical field on the removal of chloride ions from concrete slabs[END_REF]

. Lower densities of applied current (0.25-0.75 /m² RS) were chosen to study the influence of cementitious binder on chloride extraction under electrochemical treatment in concrete

[START_REF] Polder | Eletrochemical desalination of cores from a reinforced concrete coastal structure[END_REF][START_REF] Polder | Electrochemical chloride tests of concrete cores from a coastal structure[END_REF][START_REF] Buenfeld | Effect of chloride removal on rebar bond strength and concrete properties[END_REF][START_REF] Kim | Influence of cementitious binder on chloride removal under electrochemical treatment in concrete[END_REF][START_REF] Tang | Effect of an electrical field on the removal of chloride ions from concrete slabs[END_REF]

. The application of higher current densities was also reported in ranges of >1-4 A/m² RS

[40, 41, 43, 48, 51, 59-61, 63, 65, 77, 82, 83, 85]

and ≥ 5-28.6 A/m² RS

[START_REF] Miranda | Several questions about electrochemical rehabilitation methods for reinforced concrete structures[END_REF][START_REF] Miranda | Limitations and advantages of electrochemical chloride removal in corroded reinforced concrete structures[END_REF][START_REF] Vennesland | Electrochemical removal of chlorides from concrete -Effect of bond strength and removal efficiency[END_REF][START_REF] Ihekwaba | Pull-out and bond degradation of steel rebars in ECE concrete[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part II: Microstructural characterization[END_REF][START_REF] Yeih | Selecting an adequate procedure for the electrochemical chloride removal[END_REF]

. The highest densities of applied current

Table 2 :

 2 Published laboratory studies on ECE treatment performed using a voltage control mode (continuous P or intermittent Pi).

	1
	2
	First author	Binder	Sample	w/c	Cement	Dimensions	Steel rebar	Cover	Initial chloride content	Cathode Anode	Electrolyte	Voltage control	Duration
	[Reference]			ratio	(kg/m3)	(mm)	(diameter	(mm)	[mode] /				mode	(days)
						[shape]	mm)		[content Cl -]
	Lankard	OPC	C	0.44	360	76dx150	10	50	[Mix, NaCl] /	Re	Ti-Pt	[Ca(OH) 2 +IER] /	[P] /	1-3
	[18]					[cy]			[(De) (t) 0.019-0.144%bwco]			[top pond]	[V 50, 100]
	Lankard	OPC	C	0.45	360	1170x2280x230	20	50	[Mix, NaCl] + [ponding 3%NaCl 5	Re	Ti/Pt	[Ca(OH) 2 +IER] /	[P, Pi] /	1-2
	[18]					[sl]			months]			[top pond]	[V 100]
	Chang	OPC	C	0.5	393	150x150x300	Re# 9.52,	20	[Mix, NaCl] /	Re#,	TiPt	[NaOH] /	[P] /	14, 28, 42,
	[73]					[pr]	12.30		[(Add), 1.28%bwce]	External		[tank]	[V 20]	56
										cathode
	Yeih	OPC	C	0.5	393	150x150x300	R# 9.52,	20	[Mix, NaCl] /	Re	Ti	[NaOH anode	[P] /	56
	[75]					[pr]	12.30		[(Add), 1.82% bwce], [(Av), (f),		mesh	cell, TW cathode	[V 20]
									1.79%bwce]			cell] / [acrylic
												cell]
	Arya	OPC	C	0.65	400	200x200x120	10	30, 50,	[Mix, NaCl] /	Re#,	iTi	[NaOH] /	[P] /	33, 83
	[36]					[pr]		80	[(Add) 2, 3, 4%bwce] / [2 layers:	External		[tank]	[V 10, 20, 30]
									bottom 0%bwce and top layer 75mm:	cathode
									1.87%bwce]
	Chang *	OPC	C	0.66	307	100dx200	12.3	40	[Mix, NaCl] /	Re	Ti	[Ca(OH) 2 ] /	[P] /	7, 14, 21, 28,
	[58]					[cy]			[(Add), 1.28 %bwce; (Re), 3800ppm			[tank]	[V 5, 10, 20, 30]	35, 42
									bwco]			[NaOH] /

*) Authors said that a voltage control of 5 V corresponds to a 18.8A/m² CS current control

Table 3 :

 3 Published laboratory studies on ECE treatment performed using a current control mode (continuous G or intermittent Gi) with current densities expressed as A/m² of concrete surface (CS).

	First author	Binder	Sample	w/c	Cement	Dimensions	Steel rebar	Cover	Initial chloride content	Cathode Anode	Electrolyte	Current control	Duration
	[Reference]			ratio	(kg/m 3 )	(mm)	(diameter	(mm)	[mode] /				mode	(days)
						[shape]	mm)		[content Cl -]
	Tritthart	-	C	-	-	50d, 200d	2 rebar	45-55	[Ingress, DS] /	Re	Ti	[W] /	[G, Gi] /	40, 83, 96,
	[34]					[cores]	layers	and	[(t), (De) 0-170mm: 15-0.5%bwce;			[paper pulp]	[A/m² CS 1]	interruption
								60-70	(De) 0-10mm: 12-15%bwce, 40-					98d, 139d
									60mm (overlapping bar depth): 7.8-
									9%bwce; 90-100mm: 3-4.5%bwce;
									150-160mm:0.5%bwce]
	Canon	-	C	0.6	350	200dx1000	Re# d8	40	[Mix, NaCl] /	Re	Ti-	[TW] /	[G] /	6, 8, 13.5,
	[39]					[co]	Stirrups d6		[(Add), 2%bwce] / [(t) 1.5%bwce]		RuO 2 ,	[irrigation or	[A/m² CS 5]	14.6
											CCP	dampening]
	Ihekwaba	OPC	C	0.3	432	300dx1200	16	50	[Mix, NaCl] /	Re#	iTi	[Ca(OH) 2 ] /	[G, Gi] /	56, 70
	[86]					[co]			[(Add), (De) 1.7%bwce inner 15cm,			[cellulose fiber]	[A/m² CS 1]	8w ON then
									3%bwce outer 15cm] / [(Add), (Av),					2w OFF,
									5000 (#1), 6000 (#2) ppm bwco]					then a
														second ECE
														on 1 of the
														specimens
														for 2w ON
	Ihekwaba	OPC	C	0.3	432	300x300x1200	16	50	[Mix, NaCl] /	Re#	iTi	[Ca(OH) 2 ] /	[G, Gi] /	56, 70
	[86]					[co]						[cellulose fiber]	[A/m² CS 1]	8w ON then
														2w OFF,

Table 4 :

 4 Published laboratory studies on ECE treatment performed using a current control mode (continuous G or intermittent Gi) with current densities expressed as A/m² of steel rebar surface (RS).

	First author	Binder	Sample	w/c	Cement	Dimensions	Steel rebar	Cover	chloride content	Cathode Anode	Electrolyte	Current control	Duration
	[Reference]			ratio	(kg/m 3 )	(mm)	(diameter	(mm)	before ECE				mode	(days)
						[shape]	mm)		[mode] /				(G or Gi] /
									[content Cl -]				[A/m² RS]
	Monteiro	-	C	0.45	500	1180x560x1200	6.3	20,	[SS, 4m] /		SS	[TW] / [tank]	[Gi] /	2w ON, 1w
	[81]			0.65	380	[pr]		40	[(De 0-40mm), (t), 1.5, 0.5%bwce]		mesh		[RS 1]	OFF, 2w ON,
						1220x960x1200								1w OFF, for
						[pr]								a total cycle
														of 6w
	Elsener	-	C	0.5	340	320x245x70	8	40,	[Cycles+I, NaCl] /	SSR#	SS	[TW] / [foam	[Gi] /	18 (7d
	[48]			0.6	303	[bl]		50	[(De 0-50mm), (t) 0.45-1.45%bwce,		plate	plastic]	[RS 2]	ON,12d
									(f) 0.02-0.15%bwce]					OFF, 4d ON,
														3d OFF, 7d
														ON)
	Miranda	-	M	0.5	-	1330x1330x70	8	31	[Mix, CaCl 2 3% bwce] /	R	Ti	[Ca(OH) 2 ] / [cloth	[G] /	42 (per side)
	[59, 60]					[sl]			[(Add), (De 0-60mm), 1000-3500			soaked]	[RS 5]
									ppm]				1 or 2 sides
	Zhu	OPC	C	0.32	480	100dx100	10	45	[Mix, NaCl] /	R	CFRP [Ca(OH) 2 ] / [tank]	[G] /	28
	[41]			0.4		[cy]			[(Add), 1.82%bwce]				[RS 0, 1, 2, 3]
				0.5
	Siegwart	OPC	C	0.4	460	1000x100x100	22	39	[Mix, NaCl] /	R+PSC	Ir	[W] / [bath]	[G] /
	[47]					[pr]			[(Add), 1.5%bwce]				[RS 1]

[(Add), 1.8%bwce] / [(Re), (f),

Table 5 :

 5 Transport numbers t of ions (from[START_REF] Elsener | Repair of reinforced concrete structures by electrochemical techniques -Field experience[END_REF]).

	Ion	OH -	Cl -	Na +	K +	Ca 2+
	C (mol/L)	0.5	0.2	0.035	0.58	10 -4
	t	0.62	0.1	0.01	0.27	0

  of chloride extracted over the same period with Na 2 CO 3 electrolyte was respectively only 35 % and 55 %, with current densities of 1 and 4A/m² RS respectively. Using an electrolyte with Na causes Na + migration into the pore solution, thus reducing the Cl - transport number. Bouteiller et al.[START_REF] Bouteiller | Electrochemical chloride extraction of a beam polluted by chlorides after 40 years in the sea[END_REF] used sodium tetraborate (Na 2 B 4

Polder found no difference in chloride extraction efficacy between tap water and saturated calcium hydroxide solutions [43]. Polder et al. [63] compared sodium carbonate (Na 2 CO 3 ) and calcium hydroxide (Ca(OH) 2 ) solutions, and found that extraction with calcium hydroxide was more efficient. The use of Ca(OH) 2 electrolyte produced a 67% extraction of total chlorides (0.7 A/m² RS, 42 days), whereas the proportion

Table 6 :

 6 Published results for electrochemical chloride extraction (italic values for non-treated samples)

	222
	223
	Reference	Condition	Resistivity	Half-cell potential

Table 7

 7 summarizes the published results of visual observations and gravimetric analysis before and after ECE treatment.

Table 7 :

 7 Visual observations and gravimetric analysis before and after ECE

	Reference	Before ECE	After ECE
	Buenfeld

  3 BO 3 , LiOH, Li 2 CO 3 , Li 2 SiO 4 , LiNO 3 ) at various temperatures(30 or 40 C). Results showed that the transference number of Li + increases with temperature and that Li 2 CO 3 and LiNO 3 would be the most effective in terms of concrete penetration. LiNO 3 at 40 °C was the most suitable electrolyte for the prevention of ASR-related expansion.6 LONG-TERM PERFORMANCERemaining chloride ions in concrete may redistribute and migrate back to the rebar and eventually result in the (re)initiation of corrosion. The protective effect of ECE treatment would thus be limited in time. Therefore, long-term performance of ECE treatment needs to be evaluated. Very few laboratory studies report the long-term durability of ECE treatment; the majority of publications describe periods ranging from 2 months to two years after the end of treatment[START_REF] Carmona | Efficiency of a conductive cement-based anodic system for the application of cathodic protection, cathodic prevention and electrochemical chloride extraction to control corrosion in reinforced concrete structures[END_REF][START_REF] Elsener | Mechanism of electrochemical chloride removal[END_REF][START_REF] Stoop | Redistribution of chloride after electrochemical chloride removal from reinforced concrete prisms[END_REF][START_REF] Swamy | Effectiveness and structural implications of electrochemical chloride extraction from reinforced concrete beams[END_REF][START_REF] Bouteiller | Electrochemical chloride extraction of a beam polluted by chlorides after 40 years in the sea[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part I: Electrochemical measurements[END_REF][START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part II: Microstructural characterization[END_REF][START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF][START_REF] Sohanghpurwala | Long-Term Effects of Electrochemical Chloride Extraction on Laboratory Specimens and Concrete Bridge Components[END_REF].

Acknowledgements .......................................................................................................... 66 References ........................................................................................................................ 67 Acknowledgements The authors acknowledge the support of University Gustave Eiffel (previously IFSTTAR French Institute of Science and Technology for Transport, Development and Networks from the French Ministries of Environment and Research), LRMH (Historical Monuments Research Laboratory from Ministry of Cultural Heritage) and CNRS -Sorbonne University.

SIDE EFFECTS

ECE treatment may also modify the characteristics of the reinforced concrete. It is therefore important to know to what extent this occurs, and whether such effects can be detrimental to the concrete or the steel rebar.

Modifications of concrete characteristics

Porosity Buenfeld [76] and Monteiro [START_REF] Monteiro | Influence of water-cement ratio and cover thickness on chloride extraction of reinforced concrete[END_REF] did not observe any change in the porosity induced by the treatment. In contrast, other authors found that ECE treatment modifies the concrete porosity:

Castellote [START_REF] Castellote | Changes in concrete pore size distribution dur to electrochemical chloride migration trials[END_REF] reported an increase in total porosity as the electrical current was applied through the concrete. Siegwart et al. [START_REF] Siegwart | Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions[END_REF]'s study based on visual observations and scanning electron microscopy images of concrete treated samples (1 A/m² RS 42 d) at four different locations found that the number of pores (2309-4296) was larger and the average pore size (0.35-0.47 µm) was smaller than those observed in the control sample (1479 and 0.7 µm).

Shan [START_REF] Shan | Electrochemical chloride removal in reinforced concrete structures: Improvement of effectiveness by simultaneous migration of silicate ion[END_REF] also found a modification in the pore size distribution in mortar samples after ECE: the number of small pores (<100 nm) increased whereas the number of larger, harmful pores (≥100 nm) decreased.

Ion migration

Polarization applied during ECE treatment induces an ionic redistribution. A decrease in chloride concentration is observed in regions close to the steel rebar, as well as an increase of hydroxyl ions. Moreover, the accumulation of alkali ions (K + , Na + ) is found to increase with time and/or current and varies according to the concrete cover. Orellan [START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Orellan | Efficacité et effets secondaires des techniques électrochimiques de maintenance des structures en béton armé[END_REF] observed an increase of alkali ions in Na 2 Oequiv/m 3 from 12 to 27 kgs of concrete located close to the steel rebar, and attributed this event to the introduction of Na+ during the immersion-drying cycles (NaCl was used to contaminate the specimens). This phenomenon was more pronounced when the concrete cover was thin and the alkali ion concentration levels were sufficiently high.

Fajardo [START_REF] Fajardo | Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by "artificial" sea-water[END_REF] also observed an increase compared to the control specimen, according to the thickness of the concrete cover (20 or 50 mm) for Na + (20 to 40%), for K + (42 to 30%) and for Ca 2+ (30 or 17%). Ihekwaba [START_REF] Ihekwaba | Carbonation and electrochemical chloride extraction from concrete[END_REF] found that the accumulation of alkali ions at the rebar/concrete interface is higher in carbonated than in non-carbonated concrete.

New phases formed

Marcotte et al. [START_REF] Marcotte | The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar -Part II: Microstructural characterization[END_REF] used environmental scanning electron microscopy and energy dispersive X-Ray microscopy to study the effect of ECE on the microstructure of concrete, and observed that chlorides were no longer detectable at the surface of the steel after treatment. They also noted the absence of C-S-H and suggested that it had decomposed into cationic (calcium-rich) and anionic (silicate-rich) species under the impressed current. The formation of new cementitious phases (i.e., calcium-aluminium-rich, sodium-rich, iron-rich and silicon-poor phases) and their type depended on the source and magnitude of the chloride exposure.

Orellan [START_REF] Orellan | Electrochemical chloride extraction: efficiency and side effects[END_REF][START_REF] Orellan | Efficacité et effets secondaires des techniques électrochimiques de maintenance des structures en béton armé[END_REF] monitored the evolution of concrete microstructure at different times after treatment. Immediately after treatment, he observed the systematic formation of a new thin, cementitious layer at the steel/concrete interface. This layer was composed of Ca, Si, Na, Al, and K. New compounds including Na, Ca and Si were observed, and the formation of secondary ettringite was reported in some cases. Two months after treatment, small amounts of alkali-silicate gel had formed near the steel rebar but no cracks were observed. Six months after treatment, large hexagonal crystals composed of Ca, Al and Cl had formed, as well as a large amount of alkali-silicate gel.

Potential risks to reinforced concrete

Although ECE is shown to be an effective treatment for extracting chlorides, mitigate corrosion and increase pH, there are a number of questions concerning potential harmful effects on the reinforced concrete.

Concrete cracking

Hydrogen gas produced during the treatment causes localised increases in concrete pressure and may promote concrete cracking. Polder et al. [START_REF] Polder | Electrochemical chloride removal from concrete prisms containing chloride penetrated from sea water[END_REF] observed a slight increase in micro crack density that appeared to be related to the material composition rather than the applied current density (1 and 4 A/m² RS). Indeed, it seemed that the high w/c ratio of OPC concrete made it sufficiently permeable to allow the gas to escape at a relatively low pressure without causing crack formation/expansion.

Steel embrittlement

Part of the hydrogen gas formed during the ECE process can be adsorbed on the metal surface as atomic hydrogen and may then enter the steel, contributing to its embrittlement. This risk can be monitored: no potential more negative than -1 100 mV with respect to Ag/AgCl/0,5 M KCl shall be permitted for plain reinforcing steel or -900 mV for prestressing steel for an alkaline concrete. Ordinary reinforcing steel does not suffer embrittlement [4] on the contrary to prestressed steel [START_REF] Polder | COST 534 -New materials, systems, methods and concepts for prestressed concrete structures-Final Report[END_REF][START_REF] Siegwart | The effect of electrochemical chloride extraction on pre-stressed concrete[END_REF][START_REF] Bertolini | Corrosion of Steel in Concrete: Prevention, Diagnosis[END_REF]. Siegwart et al. [START_REF] Siegwart | The effect of electrochemical chloride extraction on pre-stressed concrete[END_REF] observed hydrogen embrittlement in their study of prestressed concrete, using tendons stressed to the same degree. They investigated the role of stress in hydrogen-induced stress corrosion cracking (HISCC) on the central wire of the 7-wire strand of pre-stressed concrete samples which was post-tensioned to 90% of its proof stress and subjected to ECE treatment at 2 A/m² RS for 40 days. The test was repeated three times, but none of the samples failed prematurely due to HISCC. Other authors think that the risk of HISCC depends on the type of prestressed steel and on levels of prestressing [START_REF] Polder | COST 534 -New materials, systems, methods and concepts for prestressed concrete structures-Final Report[END_REF][START_REF] Bertolini | Corrosion of Steel in Concrete: Prevention, Diagnosis[END_REF]. Therefore, it is not advised to use ECE on prestressed concrete structures.

Modification of mechanical properties

The chemical changes occurring during electrochemical chloride extraction, and particularly those in the vicinity of the steel rebar, can lead to modifications of concrete mechanical properties. These changes deserve some attention. investigation comes at a significant cost and requires time. It therefore appears interesting to approach the processes through modelling. This approach requires a good knowledge of the materials and the chemical and electrochemical mechanisms involved in ECE, and makes it possible to determine influencing factors, carry out a first approach for the planned objectives and predict the evolution of the process over time.

Several authors used the Nernst-Planck equation to model the migration of chlorides during extraction. Yu et al. [START_REF] Yu | Computer simulation of ionic migration during electrochemical chloride extraction from hardened concrete[END_REF] presented a computer simulation of multicomponent ionic migration that did not take binding and desorption effects into account, leading to a description of experimental features that was qualitative but not quantitative. Andrade et al. [START_REF] Andrade | Mathematical-Modeling of Electrochemical Chloride Extraction from Concrete[END_REF] presented the different mathematical models that were available related to the treatment conditions. They first compared the mathematical solutions provided in the literature for constant initial chloride concentrations in order to define the chloride concentration gradient throughout ECE treatment under voltage or current control modes (easier mathematical calculation is possible with the former). They then suggested a new computer program that could calculate the treatment duration in the case of an initial gradient of chloride concentration (the model is based on diffusion coefficient determination as a function of electrolyte conductivity, and on Fick's second law). Only voltage control mode was considered. Andrade et al. [START_REF] Andrade | Mathematical-Modeling of Electrochemical Chloride Extraction from Concrete[END_REF] finally showed that mathematical modelling can be performed. However, few data were available to validate their proposal at the time of their study.

Polder [START_REF] Polder | Laboratory investigation of electro-chemical chloride extraction from concrete with penetrated chloride[END_REF] used diffusion modelling of chloride remaining after ECE to approach the time until corrosion might reappear. Sa'id-Shawqi et al. [122] used the Nernst-Planck and Laplace (potential and current distributions determination) equations to predict the amount of extracted chloride and the remaining chloride profile in a ECE treatment (voltage mode). The study solely considered free chloride removal from concrete. Despite some differences between experimental and modelling data, they highlighted that the chloride transport number decreases with increasing