The impressed current anodes are usually surrounded by a carbon backfill. In addition to reducing the strength of the ground, the carbon backfill increases the flow and life time of the anode. According to IPS-M-TP-750/2 standard, there are three types of carbon backfill:

1. Coal coke breeze
2. Calcined petroleum coke breeze
3. Special calcined petroleum coke breeze

The following table shows the chemical properties of these three types of backfill:

Types 1 and 2 coke breeze are to be used for shallow (horizontal or vertical) groundbeds and type 3 coke breeze for deep well groundbeds.

The particle sizes of backfill materials shall conform to the following size distribution:

Resistivity

For Types 1 and 2 backfill material the resistivity shall be less than 100 ohm-cm when dry and lightly tamped and prior to the addition of lime. For Type 3 backfill material the resistivity of the tamped dry backfill shall not exceed 0.1 ohmcm.