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2015WinterShotcreteEMag

Technical Tip The anodes corrode in place of the reinforcing steel. To achieve this, there are some stipulations that need to be addressed to ensure adequate protection. First, the anodes need to be spaced properly. To properly space the anodes, some information needs to be acquired. For instance, what is the chloride content of the existing struc- ture (in concrete repair cases) or what is the expected chloride content (for new structures)? To properly determine the chloride content of the structure, the surrounding content should be tested in accordance with ASTM C1218/ C1218M. Once the chloride content is deter- mined, the steel reinforcement needs to be identified. The reinforcing bar size, density, and distribution are also considered in the spacing Fig. 1: Steel density of the anodes. Examples of how this information is used to determine the anode spacing is shown in Fig. 1 and 2. This type of spacing criteria is used when the concrete/shotcrete has a volumetric resistivity of less than 15,000 ohm-cm. When the concrete/ shotcrete has a volumetric resistivity higher than 15,000 ohm-cm, a correction factor is typically applied. Shotcrete often includes silica fume in both wet and dry mixtures. Silica fume will naturally increase the matrix resistivity, so testing the resistivity is highly recommended. The testing equipment is relatively inexpensive and easy to operate. Once the chloride content (or expected chloride content for new structures) is determined, the reinforcing bar density is Fig. 2: Anode spacing identified and the resistivity is tested; the anode spacing factor is then set. For the anodes to work properly, they should be in full contact with the reinforcing steel that they are going to protect and also need to be fully encapsulated in the surrounding concrete or shotcrete. Normally, this is not an issue. However, in shotcrete, full encapsulation takes a little bit more attention. Shotcrete should be placed by ACI certified and experienced nozzlemen. Special attention should be used to ensure that adequate encapsulation of the anodes is performed. If complete coverage is not obtained, there is a risk of more concen- trated chloride migration to more focused areas of the anode, which would lead to premature repair of the area. Anode spacing in shotcrete deserves a clear explanation. Cathodic protection depends on chloride migration, or movement, through the concrete/shotcrete matrix. With the higher volumetric resistivity of most shotcrete mixtures due to high cement factors, addition of silica fume, and lower water-cement ratios (w/c), the Fig. 3: Reinforcing bar sentinel tie-in Shotcrete • Winter 2015 67


2015WinterShotcreteEMag
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