Electrochemical realkalization

Electrochemical realkalization. This method is based on the application of direct... 

Electrochemical realkalization (RE) and electrochemical chloride removal (CE) can be applied to structures in which corrosion has not or has already initiated. They are techniques aimed at modifying the composition of concrete that is carbonated or contains chlorides, in order to restore its original protective characteristics. 

J. Mietz, B. Isecke, Investigation on electrochemical realkalization for carbonated concrete , Corrosion 94, NACE, paper 297, 1994. 

R. B. Polder, H. J. van den Hondel, Electrochemical realkalization and chloride removal of concrete , Proc. of the Int. RILEM/CSIRO/ACRA Conference Rehabilitation of Concrete Structures, D. W. S. Ho,... 

Chapters 6 and 7 are then concerned with repair and rehabilitation options, first the conventional physical intervention of concrete repair, patching, overlaying and coatings. The electrochemical techniques of cathodic protection, chloride extraction and realkalization are dealt with in Chapter 7. 

Several factors in the explanation given in this section are important and will be used later to explain how we measure and stop corrosion. The electrical current flow, and the generation and consumption of electrons in the anode and cathode reactions are used in half-cell potential measurements and cathodic protection. The formation of protective, alkaline hydroxyl ions is used in cathodic protection, electrochemical chloride removal and realkalization. The fact that the cathodic and anodic reactions must balance... 

Electrochemical treatments - cathodic protection, electrochemical chloride extraction and electrochemical realkalization are designed to shift the potential of the steel. This effect may be permanent in the case of cathodic protection or temporary but quite long term in the case of the other two techniques. 

In one case a current density of 0.3-0.5 A-m" was applied (at 12 V) to 2,000 m of a building in Norway with a treatment time of 3-5 days. In another case 10-22 V was applied to give a current density of 0.4-1.5 A-m in 12 days on 300 m of a bridge control tower in Belgium. A further section of 140 m was treated in nine days with a current of l-2A m . All figures are for concrete surface area. The steel to concrete surface ratio was not given. The CEN Technical Specification for electrochemical realkalization (CEN/TS 14038-1, 2004) states that a minimum of 100 A h m shall be applied. 

Standards and guidance for electrochemical chloride extraction and realkalization... 

The phenomenon of electro-osmosis has already been mentioned in connection with electrochemical realkalization (Section 7.8). It is well known that when a porous medium like concrete contains a solution, then an electric current applied between an anode and a cathode will move the water from the anode to the cathode. This leads to drying of anodes for pipelines in soils. The basis of the phenomenon is that when a compound dissolves, water molecules attach themselves to it. This happens more for positively charged metal ions than for negatively charged ions. Therefore more water is carried by the positive ions towards the negative cathode. 

BSEN/TS 14038-1 (2004). Electrochemical Realkalization and Chloride Extraction Treatments for Reinforced Concrete - Part 1 Realkalization. British Standards Institute, London. 

One of the major issues facing any consultant or owner of a structure suffering from chloride or carbonation induced corrosion is what form of repair to undertake. As we have seen from the previous sections there are coatings, sealants, membranes and enclosures, specialized patch repair materials, options for total or partial replacement, impressed current and galvanic cathodic protection, electrochemical chloride removal, realkalization, electro-osmosis and corrosion inhibitors. These can be applied to structures suffering different degrees of corrosion due to chloride attack or carbonation or a combination of these two. Each treatment will have implications for the future maintenance requirements, time to next major intervention and ultimate service life of the structure. 

For carbonation the usual options are patch repairing usually with a suitable anticarbonation coating applied afterwards (Section 6.3.1) and realkalization (Section 7.11). Realkalization is very difficult in the presence of prestressing. The latest evidence is that ASR is not a significant problem. Realkalization (and all electrochemical techniques) becomes less cost effective if there are large number of unconnected rebars. Realkalization is most... 

Until recently membranes were not considered compatible with cathodic protection as the anode would have to be under the membrane to pass current, and the anode generates gases that must escape. Membranes and cathodic protection have been used in car parks on thin slabs with some success. Some of the newer probe anodes have venting tubes and so they can be used under membranes. Galvanic anodes can be used as they do not generate gasses electrochemical chloride removal (or realkalization) could be done before the replacement of the membrane. 

It may be very risky to apply electrochemical treatments to galvanized reinforcing steel. Very severe pitting can result. NCT, the patent holders on the realkalization and desalination techniques, do not recommend their use on structures containing galvanized rebar (Miller, 1995). 

Meitz, J. and Isecke, B. (1994) investigatioirs iJii Electrochemical Realkalization for Carbonated Concrete, Paper No. 297, Corrosion 94, NACE International, Houston, TX. 

In all electrochemical restoration techniques a direct current is applied between the reinforcement (cathode) and an external anode in electrolytic contact with the concrete (Fig. 8-22). Cathodic protection (CP) is a permanent installation with design currents below 10 mA m , electrochemical chloride removal (ECR) and electrochemical realkalization (ER) are applied only on a temporary basis and use currents up to 2 A m. Both the electrochemical reactions at the rebars and ion migration in concrete are important for the treatments. 

The feasibility of electrochemical restoration has been demonstrated in the laboratory for both techniques, chloride removal and realkalization. For electrochemical chloride removal all studies show a reduction in the total chloride content of between 40 and 60% (SHRP, 1993 a Tritthart, 1996), the most pronounced decrease in chloride content being found in the cover concrete (Fig. 8-23). Chlorides beneath the first layer of the reinforcement are removed more slowly. As is further shown in Fig. 8-23, the expected increase in OH" content at the rebars does actually take place. Chloride removal is most efficient in the early stages of the treatment (Bennet, 1990).Prolonging the treatment to more than 1500 A h m" is found to be ineffective because nearly all of the current flow is transported by hydroxyl... 

Extensive laboratory tests on electrochemical realkalization have been conducted at BAM Berlin (Mietz and Isecke, 1994 Mietz et al., 1994). It has been found that the realkalized zone around the rebar increases with time and current density, i.e. proportionately to the electric charge passed (Mietz et al., 1994). On the other hand the observed realkalization from the concrete surface is independent of current density and thus only due to capillary suction of... 

After electrochemical realkalization, half cell potential values of about-0.2 V CSE were measured, thus the potentials shifted to more negative values by about 250 mV compared with the values of the untreated control field. This behaviour can be rationally explained bearing in mind that (a) the concrete cover after the ER treatment has a much lower resistivity (sodium bicarbonate in the pore solution) and (b) passive steel in concrete acts as a pH electrode. The potential values measured indicate that the steel/ concrete interface has become more alkaline. 

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