time to corrosion initiation

Steel embedded in concrete is effectively prevented from corroding in most, but not all, circumstances by the formation of a protective passive layer or film on the steel surface. This protective layer is maintained by the highly alkaline concrete environment.

However, if the concrete becomes contaminated with chemical species that disrupt the passive layer or reduce the concrete’s alkalinity then, providing there are sufficient concentrations of moisture and oxygen present, corrosion of the reinforcement can occur in areas where the passive layer no longer protects the steel.

Loss of steel passivity within concrete structures largely occurs through the action of two processes namely, carbonation and chloride contamination. Corrosion of the steel reinforcement can occur through either process individually or through a combination of both.

  • Carbonation of concrete is brought about by the reaction of the alkaline phases of the cement matrix with acidic carbonaceous gases, particularly carbon dioxide, present in the atmosphere. This reaction leads to a reduction in alkalinity of the concrete and ultimately results in the depassivation of the steel reinforcement through general dissolution of the protective passive layer.
  • The contamination of concrete through the ingress of chloride ions, from a variety of sources such as de-icing salts, marine environments and contaminated water results in a more localised breakdown of the passive layer leading to depassivation of the steel through pitting corrosion.
    It is possible for chlorides to be cast-in to concrete at the time of construction through contaminated ingredients or admixture.

However, for either corrosion processes to take place and be sustained sufficient concentrations of moisture and oxygen are required at the areas where the passive film no longer protects the embedded steel reinforcement.

The corrosion process for steel reinforced concrete can be simplified into a two-stage process namely, the ‘initiation phase’ and the ‘propagation phase’ as shown in the figure below.

  • Initiation phase is the time taken for conditions to become conducive to corrosion. i.e. loss of passivation leading to the onset onset of corrosion, the latent or hidden damage phase.
  • Propagation phase is the period in which the accelerated corrosion of the steel reinforcement ultimately leads to rust staining, cracking and spalling of the cover concrete.  i.e. the visible effect of corrosion

 

 Figure 1: Simplified Corrosion Model (after Tuutti)

Once the carbonation or chloride data for the structure has been assessed using the Carbonation or Chloride tool developed by John Broomfield et al, it should be possible to establish where the structure, or key elements of the structure, lie in relation to this graph.

If it is found to be within the 'corrosion initiation phase' it is desirable to be proactive with the maintenance programme as it is highly likely that significant latent or hidden damage in relation to corrosion has not yet occured.

If, however, an assessment suggests that the structure effectively lies within the 'corrosion propagation phase' then a more reactive maintenace programme would be necessary.

The concept for obtaining an intervention strategy(s) based on the analysis of the carbonation or chloride data using the Carbonation or Chloride link above is further described under  Repair Strategy