Protection current distribution in reinforced concrete cathodic protection systems
Current distribution from a surface mounted anode to steel reinforcement in atmospherically exposed concrete is modelled as a function of the condition of the steel, the resistivity of the concrete and anode-steel geometry. The boundary conditions at the steel have a significant effect on current distribution with more uniform distribution arising at low steel corrosion rates. In a typical situation the surface of a steel bar facing the anode may receive 50% more current than the opposite surface. As cathodic protection has proved to be effective in these cases, a basis for many design decisions that influence current distribution is that their effect is small by comparison. When more than one layer of reinforcement is present the current distribution is significantly worse. In this case a surface anode may not be enough and discrete anodes may be necessary to improve current distribution. An increase in the concrete resistivity, cover and the anode to cathode area ratio at a constant anode current density will increase the voltage drop through the concrete inducing an improvement in the environment at the steel that promotes steel passivity.