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2014SumShotcreteEMag

Application Bulletin (RAP) 8—Installation of Embedded Galvanic Anodes,” which is a free download from the American Concrete Institute (ACI) (http://www.concrete.org/Store/Product Detail.aspx?ItemID=ERAP8). This guideline details anode nomenclature and installation guidelines. For example, Type 1 anodes are attached inside the patch repair and Type 2 anodes are placed into drilled holes in sound concrete. Resistivity of Repair Materials As detailed in the ACI RAP 8 document, con- crete repair material selection is an important consideration for repairs that include Type 1 galvanic anodes installed inside repairs. “Resistivity of repair materials or concrete for use with embedded galvanic anodes Fig. 4: Approximate relationship between resistivity and rapid should be less than 15,000 ohm-cm. High- chloride permeability resistivity materials such as epoxies or highly polymer-modified repair mortars greatly reduce the available galvanic current or prevent the anodes from functioning prop- erly. If a low-resistivity material is not suit- able for the full repair, anodes can be embedded in individual pockets of low- resistivity material. These pockets should completely encapsulate the anode and com- pletely fill the space between the anode and the concrete substrate.” The recommendation for repair materials to have a resistivity of less than 15,000 ohm-cm allows sufficient protective current to flow from the anode to the steel. Based on our experience, a maximum of 15,000 ohm-cm resistivity approximates to roughly a minimum rating of 1500 coulombs using the Rapid Chloride Perme- ability Test (ASTM C1202). Figure 4 shows a graph based upon data compiled from various sources and could be used as a general guide; Fig. 5: Embedding mortar used to create a conductive path between however, actual resistivity testing of the repair the anode and the concrete substrate material is recommended. As a service to the industry, our company will test submitted repair material samples at no charge. Anode Installation Methods On the surface, the resistivity requirement may cause a problem for many shotcrete mix- ture designs, especially if it contains significant levels of silica fume to densify the mortar and to improve shotcrete build. But as the ACI RAP 8 guide suggests, it is acceptable to place lower-resistivity mortar between the anode and the concrete substrate to create a conductive bridge between the anode and the concrete substrate. When anodes are used with shotcrete, using a conductive bridge of embedding mortar will also eliminate the possibility of shadowing and overspray pockets behind the anode (more Fig. 6: Type 2 anodes placed into holes drilled into the on that later). parent concrete and connected to the steel in the repair Shotcrete • Summer 2014 17


2014SumShotcreteEMag
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