Re-alkalization
Technical PapersNon-destructive electrochemical treatment to halt ongoing and prevent future reinforcement corrosion in carbonated concreteDescription Norcure realkalisation is an electrochemical treatment to halt ongoing and prevent future reinforcement corrosion in carbonated concrete. Instead of breaking out carbonated but otherwise mechanically sound concrete, realkalisation provides a means to increase to pH of the carbonated concrete initially to more than 12, which favours passivation, with a final value greater than 10.5, which is sufficient to maintain passivity of the reinforcement. Realkalisation is performed by applying an electric field between the reinforcement in the concrete and an anode system consisting of an anode mesh embedded in an electrolytic reservoir and temporarily placed on the concrete surface. The principle is schematically presented below. During treatment, the electrolyte, which is a sodium carbonate solution, is transported into the carbonated concrete as symbolized by the penetrating front. The dominant transport mechanism may vary , but electro-osmosis and migration of ions are the two main contributors. Simultaneously, electrolysis at the reinforcement surface produces a very alkaline environment, symbolized by the ring around the reinforcement. In cases where electro-osmosis cannot be used, a variation of the treatment allows realkalisation by electrolysis alone. This situation may arise when carbonated concrete is also chloride contaminated, or if certain surface coatings or impregnations of the concrete have been used. Provided the treatment is allowed to continue for an extended period, that is, typically for some two to three weeks, sufficient realkalisation is achieved. Durability Realkalisation is an electrochemical process, as indeed is reinforcement corrosion. The corrosion reaction consists of an anode and a cathode reaction. The anode reaction causes the steel reinforcement to be dissolved and occurs at sites where the surrounding concrete has lost its corrosion protective nature. Simultaneously, the cathode reaction produces hydroxyl ions at other sites along the steel reinforcement. While the anode reaction has a harmful nature, the cathode reaction improves the corrosion protection of the steel. During realkalisation, the anode reaction - the harmful part - is forced to take place on the anode mesh temporarily placed in the electrolytic reservoir on the concrete surface, while the cathode reaction - the protective part - now takes place over the entire reinforcement surface. Immediately, all reinforcement corrosion is forced to stop, and during treatment, all corrosion sites become deactivated by the cathode reaction. Realkalisation is completed once the corrosion protective nature is re-established. The installation is then dismantled. The electrochemical nature of realkalisation facilitates the deactivation of all ongoing reinforcement corrosion. Thus, even corrosion at undetected sites is arrested. Corrosion, once started, tends to continue until actively halted, which is of utmost importance in order to achieve a long lasting repair. The durability then depends up[on the protective nature of the surrounding concrete. The electrolyte introduced is a sodium carbonate solution with an initial pH of about 12. Being a carbonate, it can to only a minor extent react with the CO² of the atmosphere. The final pH of the carbonated, realkalised concrete is greater than 10.5, which is sufficiently alkaline to sustain the corrosion protection properties of the concrete. Concrete survey An inspection of the concrete should be performed by approved personnel to determine the extent and cause of damage. If the structure suffers from carbonation, and realkalisation is to be considered, a detailed survey should be performed to assess rebar continuity, position of rebars, concrete cover, visual damage and delamination. In addition, the carbonation front should be recorded for future documentation of the effect of realkalisation. Petrographic analyses to determine the likelihood of alkali aggregate reactions (AAR) should be performed if there is reason to believe that reactive aggregates have been used in the concrete mix. If reactive aggregates are present, an evaluation of the danger of provoking AAR and its consequences must be outlined in cooperation with experts on AAR. A repair specification should be prepared and, where possible, as-built reinforcement drawings should be made available to the inspector and, subsequently, to the contractor. Preparation prior to realkalisation Only loose, spalled and delaminated concrete needs to be removed before treatment. Any exposed and corroding rebars should be cleaned, preferably by wet sandblasting to SA 2, and repaired with an approved cement based mortar. Concrete surface coatings should be removed prior to realkalisation since coatings may increase the treatment time or limit the effect of the treatment. If for any reason the removal of the surface finish must be avoided, a realkalisation trial is required to establish necessary treatment time. Step-by-step application guide Realkalisation can be performed under all weather conditions as long as the electrolyte does not freeze. Realkalisation is suitable for most types of reinforced concrete, but is not necessarily universally applicable. Pre-stressed and post-tensioned structures and any concrete with unusual characteristics should be referred to Fosroc NCT for further guidance. Rebar connections The number of rebar connections needed depends upon rebar continuity, but there should be at least one for each 50 m² of concrete. The continuity is determined by means of resistance measurements performed during installation. The resistance between two rebar connection points should ideally be less than 1 ohm, but up to 10 ohms is generally acceptable. Sampling and testing The effect of realkalisation is determined by on-site analysis of concrete samples taken before, during and after treatment. The pH indicator phenolphtalein is used for qualitative measures, while sodium and sometimes potassium analyses are used if quantitative measures are required. Prior to installation of the system, test locations are selected and marked at and between rebars. Installation of the anode system The anode system consists of an anode mesh embedded in an electrolytic reservoir on the concrete surface. The anode mesh may be of steel or platinized titanium, and the electrolyte is usually a sodium carbonate solution. The purpose of the electrolyte is to conduct electricity and to provide alkalis to the carbonated concrete. The reservoir keeps the electrolyte in contact with the concrete surface and the anode mesh, and it may be sprayed-on cellulose fibre, felt cloth or coffer tanks, as described below. (i) Cellulose fibre, saturated with the electrolyte, is sprayed directly onto the concrete surface. Prior to spraying, wooden battens are attached to the concrete surface using plastic plugs. The anode mesh is fixed to the wooden battens, which act as spacers between the concrete and the mesh. The cellulose fibre and electrolyte are sprayed on to ensure electrolytic continuity between the anode mesh and the concrete surface, and applied to a thickness that encapsulates the mesh. Its adhesion properties make the cellulose fibre suitable for most concrete surfaces. During treatment, regular wetting is required. (ii) Felt cloth may be used for decks. The felt cloth is rolled onto the concrete surface in two layers, with the anode mesh placed between them. For most felt cloth application systems, constant wetting is required. (iii) Coffer tanks, consisting of plastic sheets with sealing edge strips of compressible expanded plastic and with built-in anode mesh, may be fastened directly to the concrete surface and filled with electrolyte. Coffer tanks are particularly suitable for the treatment of smaller, separated areas, provided that the concrete surface is relatively smooth and even. During treatment, the coffer tanks are topped up if required. General technical specification Anode: A metallic mesh temporarily placed onto the concrete surface The power supply is an AC/DC rectifier that may be either current of voltage controlled. The leads from the rebar are connected to the negative pole and the leads form the anode mesh are connected to the positive pole of the rectifier. The power supply is then switched on and adjusted to give a current density of ideally 1 A/m² of concrete surface. A Norcure RD2 unit has been specially developed for realkalisation and desalination purposes. Performance monitoring: control and documentation During treatment, the anode system must be kept moist at all times by wetting as appropriate. The process is monitored by means of current and voltage readings, and, if necessary, adjusted to maintain 1 A/m² of concrete surface. The Norcure RD2 units provide easy current control and process monitoring. After a few days of treatment, cores are drilled at the previously marked test locations. The cores are sprayed with phenolphthalein in accordance with the Fosroc NCT test procedure to determine the extent of realkalisation. (If a quantitative measure is required, sodium analysis of the concrete is recommended.) The treatment continues until satisfactory realkalisation is achieved, as proven by the test results. Dismantling When the carbonated concrete has been satisfactorily realkalised, the treatment is terminated. All leads are disconnected, and the anode system is removed. The surface is washed with lean water and remaining cavities and core drill holes are repaired. Post treatment monitoring and maintenance On structures in severe environments, it is recommended to install reference electrodes (e.g. Fosroc NCT's Corrocheck electrodes) to monitor the stability of the treated concrete. Where the application of a subsequent decorative and/or protective coating is required, the concrete surface must be allowed to dry and a compatible material must be chosen. Refer to Fosroc NCT. Approved applicators Realkalisation is a patent protected technique which may only be performed by approved and trained personnel operating under Fosroc NCT licence. Advantages Norcure realkalisation offers specifiers and end-clients major potential advantages over other methods of concrete repair. The cause of corrosion is addressed and removed |
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