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C AT H O D I C P R O T E C T I O N FIguRE 1 ing low-voltage power distribution systems. In the power transmission system, lightning arresters are used. In the power distribution system, one phase of power has been grounded (Figure 2). In both (a) A typical clamp before and (b) after the occurrence of stray AC in a power cases, the grounding system and concrete concrete pole. pole are two electrical parallel paths that pass stray current, and the amount of FIguRE 2 electric current in each path depends on its resistance. The lightning arrester is a noncontinuous grounding system and it will close in specifed high over-voltage such as lightning. In the special conditions explained previously, some current can leak to the concrete pole. In the area that was investigated, there were other problems—the grounding resistance was high (>50 Ω) and most of the grounding cables were cut and stolen. Under such conditions, stray current through concrete poles can be increased during rainfall. Therefore, the grounding systems were repaired and their resistance reached 3 Ω. Concrete Poles (a) Grounding in a high-voltage power transmission system and (b) grounding in a power distribution system. In some concrete poles, the concrete cracked after the rebar had corroded; but FIguRE 3 in others, rebar corrosion occurred after the concrete cover had cracked. The cracks were vertical and parallel, and weren't similar to cracks caused by an alkali silica reaction (Figure 3). Based on field observations, stray AC occurred during rain, when the concrete pores absorbed water and reduced the concrete's electrical resistance. That caused rapid warming of the concrete pole, which released water vapor from its surface. Meanwhile, a slow "jik-jik" sound was heard, which was related to initiation and propagation of cracks. Thermal stress and shrinkage stress from rapidly drying concrete pores caused micro- and macrocracks in the concrete pole.2 These cracks increased the permeability and reduced the Cracks in concrete power pole caused by stray AC. NACE International, Vol. 52, No. 5 May 2013 MATERIALS PERFORMANCE 37