Reinforced concrete is a versatile and widely used constructionmaterial. Its excellent performance and durability rely on thecompatibility of the steel with the concrete surrounding it and theability of the concrete to protect the steel from corrosion in mostcircumstances. Corrosion of the steel reinforcement does not occur,despite the presence of moisture and oxygen in the concrete pores,because of the alkalinity of the concrete pore water creating apassive oxide film on the reinforcing steel. Unfortunately,corrosion protection is not guaranteed and can fail if sufficientchlorides (usually in the form of sea salt, deicing salt, orchloride contamination of the original mix) or atmospheric carbondioxide (CO2) penetrate the concrete. This leads to thebreakdown of the passive layer that protects the steel. Thisbreakdown of the passive oxide layer leads to corrosion of thereinforcing steel if sufficient oxygen and water are available.
Regardless of the cause of depassivation (chlorides orcarbonation), corrosion occurs by the movement of electrical chargefrom an anode (a positively charged area of steel where steel isdissolving) to the cathode (a negatively charged area of steelwhere a charge-balancing reaction occurs, turning oxygen and waterinto hydroxyl ions).
One solution to carbonation-induced reinforcement corrosioninvolves applying an electrochemical treatment that suppressescorrosion. Figure 1 shows the basic components of anelectrochemical treatment system for realkalyzing concrete. Thecomponents are a direct-current (DC) power source and a temporaryanode distributed across the surface of the concrete encased in aconductive medium or electrolyte.
Electrochemical methods work by applying an external anode andpassing current from it to the reinforcing steel so that all of thesteel becomes a cathode.
Three electrochemical techniques are used to counter corrosionof steel in concrete. Cathodic protection can be applied byimpressed current or galvanic anodes. Electrochemical chlorideextraction (ECE) uses a temporary anode and high current over aperiod of 4 to 6 weeks (see NACE Publication 011011).Realkalization is a method for treating carbonated concrete. It issimilar to ECE but takes approximately one week and is gainingrapid acceptance as a rehabilitation method for carbonation inbuildings and other structures. Both ECE and realkalization usecurrents up to about 1 A/m2 (0.1 A/ft2) ofsteel surface area.