Prepared Backfill and Packaging
Most magnesium and zinc anodes used in soils requires the use of prepared backfill around the anode for the following reasons.
- Increases the effective surface area which lowers the anode to earth contact resistance.
- The bentonite clay absorbs and retains moisture.
- The gypsum provides a uniform, low resistance environment.
- The sodium sulphate (a depolarizing agent) minimizes pitting attack and oxide film formation.
Today the standard backfill for magnesium and zinc consists of…
- 75% Hydrated Gypsum (CaSO4 2H20)
- 20% Bentonite Clay
- 5% Sodium Sulphate
When properly combined these elements will provide a uniform resistance of 50 ohm centimeter backfill when measured by the ASTM G-58 Soil Box Test Method and corrected for temperature variations. Most reputable anode fabricators will test and document the resistance values for each batch of backfill. As a historical side note, zinc was formorly thought to perform better in a backfill of 50% gypsum and 50% bentonite. Over the years, it has been determined that zinc may polarize and provide inaccurate readings if sodium sulphate is not used in the backfill.
To keep the backfill uniformly around the anode, the bars with the lead wire attached are placed in cloth bags or cardboard boxes with the prepared backfill added. Cloth bagged anodes are usually encased within a triple layered paper bag and become resistant to short periods of inclement weather and handling damage. Prior to backfilling, the paper bag is removed and discarded, permitting the cloth bag containing the backfill to readlily absord moisture, allowing for more rapid acquisistion of fluid measurements after installation. The bags usually shift in the backfill, either during transportation or manual handling of the units. In high resistance soils the exposed anode surfaces may result in a reduction of current output, caused by a higher ground bed resistance. This condition seems to be a concern when soil resistance exceeds 50,000 ohm centimeters and when more than 25% of the anode bar is exposed.