Corrosion of steel in concrete can be modelled as a three-stage process. The first, usually called the initiation stage, is the diffusion of CO2 or chlorides to the steel to cause depassivation. The second stage is the ‘activation stage’. This is a less clear cut phenomenon as more of the rebar network starts to corrode, and the rust products are formed. The third stage is deterioration as cracking and spalling occurs. Eventually a situation is reached which is defined as limit state or the end of the functional life and rehabilitation must take place by that point. The process is illustrated in Figure 9.1 using an exponential function and a simple linear fit.