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chloride penetration has occurred. By using both the Type 1 within the patch areas and the Type 2 in the parent concrete, you have provided global protection for your structure. Anodes work by corroding preferentially to the reinforcing steel. The anode is electrically connected to the reinforcing steel and provides a protective current through that connection. To do that properly, the anode needs to be placed at a spacing that provides the needed amount of current necessary to adequately protect the steel. The needed information for spacing requirements are reinforcing bar size, density, and steel distribution. Chloride content in the existing concrete is also very useful. Once the needed information is considered, the spacing can be calculated using this formula: Surface area of steel/surface area of concrete = steel density ratio With the higher resistivity of shotcrete material, anode spacing is important. Shotcrete (both wet- and dry-mix) inherently has higher resistivity than most form-and-pour concrete due to higher cement contents, often using microsilica or fly ash supplemental cementitious materials and a lower water-cementitious materials ratio (w/cm). Thus, these shotcrete-specific mixture designs will slow the migration of chlorides along with increasing the resistivity of the mixture. The normal recommendation is to have a resistivity of less than 5900 ohm-in. (15,000 ohm-cm). This allows sufficient protective current to flow from anode to steel. This number also relates to approximately 1500 coulombs using the Rapid Chloride Permeability testing (ASTM C1202). If the concrete mixture used will have a higher than 5900 ohm-in. (15,000 ohm-cm) resistivity, it is suggested to use a bridging or embedding mortar. The conductive bridge is then created with this mortar between the anode and the parent concrete. This also prevents the risk of shadowing or voids behind the anode, as shotcrete may have difficulty wrapping around the backside of the anode. You may use this bridging mortar even with a lower resistivity concrete to help prevent shadowing. The decision for using a bridging mortar is based on several factors: • What is resistivity of the parent concrete? • Should more anodes be used to generate more current for a higher resistivity mixture? Fig.3: Type 2 anodes are placed in series into drilled in holes in the existing concrete prior to shotcrete application • How close can the anodes be placed to the patch edge? • Can the anodes be tied on the steel well enough to not be damaged by the shotcrete application or to prevent shadowing? Once these questions are reviewed and answered, the decision to use the bridging mortar can be made. There are several anode sizes available today. So, once the steel density ratio is calculated, the decision can be made for proper anode size and zinc content use. In summary, the use of embedded galvanic anodes in shotcrete repairs gives an additional level of protection from future corrosion. Use of shotcrete in conjunction with this type of cathodic protection is being used on a more regular basis, as more owners and specifiers realize the savings and long-term performance produced with this type of repair process. Rachel Stiffler received her BS in biology and chemistry from California University of Pennsylvania, California, PA. She is currently the Business Development Manager for Vector Corrosion Technologies. Stiffler has been with Vector for almost 12 years and has 30-plus years of experience in the concrete industry. She is responsible for business development for the northeastern United States, working closely with DOT agencies, structural and civil engineering firms, owners, and university groups introducing corrosion mitigation concepts for reinforced concrete structures. Stiffler’s professional affiliations consist of being a cathodic protection tester certified by the National Association of Corrosion Engineers. She is a member of the International Concrete Repair Institute, greatly involved both locally in the Pittsburgh Chapter and the national organization. She is also involved with the American Concrete Institute, and currently most actively involved as a member of the executive committee of the International Bridge Conference, having been General Chairman in 2015. SUPERIOR GUNITE STRUCTURAL SHOTCRETE & ENGINEERING Steven J. Crawford, AIC, CPC President 12306 Van Nuys Blvd. Lakeview Terrace, CA 91342 www.shotcrete.com steven.crawford@shotcrete.com Main (818) 896-9199 Direct (818) 485- 8812 Cell (503) 709-3204 CA Lic # 370034 www.shotcrete.org Summer 2017 | Shotcrete 25


2017SummerShotcreteEMag
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