The reinforced concrete structure will go through three separate phases, shown in Figure 2-3, in the development of corrosion (Heckroodt, 2002). Phase 1 is the initiation period, by where there is little to no damage visible, but this is the stage where the ingress of harmful substances takes place, making the steel thermodynamically unstable, vulnerable to corrosion. The occurrence of this initiation stage is based on the quality of the concrete. The less penetration allowed, the less chance of this stage occurring. The second phase involves the propagation phase, from where the now activated corrosion expands, cracking the cover concrete and exposing more of the steel to harmful substances. The final phase is the acceleration period, where the corrosion rate increases rapidly, from the easy access of harmful substances (Alexander, Beushausen, & Otieno, 2012). One of the most well-known conceptual models for this phase system is the Tuutti (1982) model which consists of the first two phases discussed (Cabrera, 1996).
Once all three phases are in play, the steel can expand to nearly ten times its original size which could cause major cracking and spalling of the concrete cover. Pitting could also occur, causing a reduction in the cross sectional area of the steel (Heckroodt, 2002).
The Corrosion Process
Representing the three phase model on the corrosion process