Direct electrical current (DC) is used in several ways for its healing effect in reinforced concrete structures. In long-term application, it is used for cathodic protection (CP), both in preventive and remedial mode. DC applied for a short time is used for electrochemical chloride extraction. In all cases, the DC current induces migration of chlorides from the cathodic reinforcement to the surface anode. The passage of current through wet concrete induces the transport not only of chloride but also of other ions such as hydroxides or calcium and sodium ions. This can cause local concentration changes, dissolution and changes in the microstructure of the cement binder and affect its sorption properties towards chlorides. Thus, concrete after chloride extraction may be less resistant to future chloride penetration. Series of concrete samples based on Portland cement (OPC) and samples containing supplementary materials (micromilled limestone, microsilica) were prepared, treated with DC current and tested for diffusion and migration resistance and microstructural changes afterward. The results showed significant impact of DC treatment on all the properties under study.
The direct current electrical treatments are applied with the aim of improving corrosion resistance of steel embedded in concrete. It is the impressed current cathodic protection in both widely used modes – preventive or remedial, electrochemical chloride extraction, realkalization of carbonated concrete and electrochemical injection of protective agents [1-6]. All the treatments are similar to each other in its principle and arrangement. In all the cases, an external source of DC current is needed which is connected with its minus pole to the reinforcement, an external or embedded anode that is connected to the plus pole and direct current or direct voltage that is applied between them through concrete. The charge between the electrodes is transferred by migration of ions; positively charged cations (Ca2+, Na+, K+) are moving towards cathode; negatively charged ions (Cl−, OH−) are moving away from cathode. But the hydroxides are being produced at the same time by accelerated reduction of oxygen on cathode which makes the electrolyte around cathode more alkaline. Hydroxides can be produced by water reduction, too, if the potential is negative enough, but this is definitely undesirable because of hydrogen production at the same time. The DC treatments are different in duration of application and current density flowing through the concrete and position of anode and environment around it.
