In 1982, Tuutti proposed that deterioration caused by reinforcement corrosion can be represented by two broad stages: corrosion initiation and corrosion propagation. This simple model is still useful, but the end of service life must be tied to a stated limit state. Depassivation, visible cracking, spalling, and loss of structural capacity do not occur at the same time.
Corrosion initiation and propagation
The initiation period, Ti, starts when the concrete is placed and ends when the passive film protecting the reinforcing steel is lost. Chlorides reaching the steel or carbonation lowering the alkalinity around the steel are two common causes of depassivation.
The propagation period, Tp, begins after depassivation. Corrosion products then accumulate around the bar. Because these products occupy more volume than the original steel, tensile stresses develop in the concrete cover. With sufficient corrosion, cracking, delamination, and spalling may follow.
Service life depends on the selected limit state
For a selected corrosion-related limit state, the service life may be written in a simplified form as:
Tservice = Ti + Tp
where Tservice is the time to the selected limit state, Ti is the initiation period, and Tp is the propagation period from depassivation to that limit state. If depassivation itself is selected as the limit state, Tp is zero.
| Stage or limit state | What is happening | Possible evidence | Meaning for service-life assessment |
|---|---|---|---|
| Initiation | Chlorides or the carbonation front move through the concrete cover. | Usually no visible corrosion damage. | Ends when steel depassivates at the location being assessed. |
| Early propagation | Corrosion starts and corrosion products accumulate. | Electrochemical activity may be detected before visible cracking. | Damage rate depends on moisture, oxygen, temperature, resistivity, cover, and corrosion rate. |
| Cover cracking | Expansion around the bar exceeds the tensile capacity of the cover. | Longitudinal cracks, often following reinforcement. | A common serviceability limit state, but not the only possible definition. |
| Delamination or spalling | Cracks join and the cover separates or falls away. | Hollow-sounding concrete, rust staining, delamination, or exposed steel. | May create falling-concrete hazards and accelerate deterioration. |
| Strength-related limit state | Bar section, bond, anchorage, or concrete section has deteriorated significantly. | Measured section loss, reduced bond, deformation, or other structural distress. | Requires an engineering assessment rather than a visual estimate alone. |
Why the propagation period is not a fixed number
The propagation period should not be treated as a universal number of years. It changes with the selected limit state and with the exposure, moisture, oxygen availability, concrete resistivity, cover depth, bar size, cracking, and corrosion rate. A short assumed period used in one prediction model is an input to that model, not a rule for every structure.
For this reason, a service-life report should state the deterioration mechanism, the limit state being predicted, the input assumptions, and the uncertainty. Field observations and testing are also needed when an existing structure is being evaluated.
Related corrosion topics
The model describes when corrosion damage develops; it does not replace diagnosis of the cause. See the separate explanations of chloride-induced corrosion, carbonation and corrosion, and corrosion of embedded metals in concrete.