Detailed Explanation About Impressed Current Anodes for Buried Concrete Structures

Detailed Explanation About Impressed Current Anodes for Buried Concrete Structures

Galvanic Anodes
For buried concrete structures, traditional galvanic anodes based on zinc or magnesium may be used. The required minimum net mass of the anodes is determined from the total current requirement, anode alloy capacity, design life and anode utilization factor. The size and number of anodes is also determined from the anode current output, which in turn is based on anode shape, soil resistivity and anode operating potential. For high-resistivity soils (typically >100 Ωm), only magnesium alloys will be able to provided effective current output.

Galvanic anodes may be installed directly or in chemical backfill (typically gypsum, bentonite and sodium sulfate) to optimize their performance. Anodes will normally be connected to the embedded steel in the concrete via an integral anode cable. Normal practice is to connect the anode via a junction box or test box to allow the anode current to be measured.

Impressed Current Anodes
Impressed current anodes for buried steel in concrete typically comprise high silicon cast iron (with chrome in chloride environments), graphite or mixed metal-oxide-coated titanium. Other impressed current anodes can be used including magnetite and scrap iron. Impressed current anodes will be installed either as single anodes or together to form a horizontal or vertical groundbed. The anodes will be embedded in a conductive backfill (typically of calcined petroleum coke) to reduce the resistance to ground (and hence, reduce driving voltage for a desired current output) and to reduce consumption rate on the anode itself.

Outputs for impressed current anode systems can typically range from 1 A to 2 A for single anodes, to typically 5 A to 200 A for horizontal or vertical clustered groundbeds and typically from 10 A to 100 A for deep vertical groundbeds. Selection of the ground bed type will be dependent on the total current requirement, geometry of the structure, access and availability of land for sitting the groundbed. With impressed current anode systems, consideration of the risk of stray current to other structures, including third party structures, should also be taken into account.

Cables for impressed current anodes should be suitable for exposed to buried conditions, be capable of carrying high currents and, at the cable-to-anode interface, be capable of withstanding the aggressive/oxidizing conditions created.