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2017SummerShotcreteEMag

Overcoming Existing Corrosion When Using Shotcrete for Repair By Rachel Stiffler For over 100 years, shotcrete has been used in a variety of new construction applications, but one of its major uses is in repair, rehabilitation, and repurposing. The shotcrete repair process is increasing in use, both for transportation and building applications. It is ideal for repairing both vertical and overhead surfaces at most thicknesses. On many rehabilitation projects, there are challenges with corrosion of the existing reinforcing steel. Once the existing concrete has been removed, we can easily evaluate the condition of the reinforcement. The corrosion of the steel has usually been caused by penetration of chlorides into the concrete. When sufficient chloride reaches the reinforcement— typically 0.03% by weight of concrete in non-carbonated concrete—the natural protection of the steel created in the highly alkaline environment of the concrete becomes compromised and corrosion begins. The iron oxide (rust), created from the expansion of the reinforcement, once corrosion begins, occupies a greater volume than the steel itself, thus causing cracking that can lead to delamination and spalling of the concrete. When these concrete structures are repaired, the original concrete material remaining is contaminated with residual chloride. When the repair concrete is placed, there is an electrochemical incompatibility that occurs between the new and existing concrete. When this incompatibility is present, new corrosion sites will begin to appear. The electrical potential of the reinforcing steel within the existing contaminated concrete becomes more electronegative, shifting current to the repaired area. When this happens, concrete deterioration is noticeable in the parent concrete a short time after the repairs are completed. This is commonly referred to as the “halo effect,” “patch accelerated corrosion,” or the “ring anode effect” (refer to Fig. 1). Galvanic zinc-based anodes are used to prevent this from occurring. These anodes can be tied onto the reinforcing steel around the perimeter of the patch areas, or on a grid pattern (refer to Fig. 2). As the anode is connected directly to the cleaned reinforcing steel, the generated current protects the steel. This prevention or control of corrosion is effective within the area defined by the spacing of the anodes. A good reference for anode installation is the American Concrete Institute’s RAP-8 Bulletin, “Installation of Embedded Galvanic Anodes” (reprinted in this issue on page 37). It is available in PDF format as a free download from ACI (www.concrete.org). This guideline references installation guidelines and anode nomenclature. The nomenclature refers to a Type 1 anode as being attached and embedded inside the patch area while Type 2 anodes are placed into holes drilled into sound areas. Occasionally, the Type 2 anodes are used in a shotcrete application (refer to Fig. 3). This application is preferred when the concrete cannot be completely removed to expose all reinforcing steel, either for structural reasons or due to very little concrete cover. This is also a proactive approach of anode use when the concrete has not delaminated or spalled, but the testing shows Fig.1: Example of the halo effect or ring anode effect Fig.2: Anodes placed on a grid pattern 24 Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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