Predictive models

Predictive models

Predictive models aim at assessing the chloride and/or CO2 diffusion into concrete. Basically, they predict the time it will take for chlorides to reach a critical value (Critical Chloride Threshold Level, CCTL) on the rebar surface. The generally accepted CCTL value is 0.4% of the cement content in the concrete.

Such models provide meaningful results only if the physical mechanisms are those modelled and if the input data are correct. The latter may be a challenge, in view of  i) the wide variation of diffusion coefficients between different types of concrete (up to 25 times) ii) the degree of exposure iii) the temperature variations and iv) the role of cracks.

In addition, poor workmanship may occur and induce unknown factors.

As a result, and in order to simplify choices, simple rules have been implemented in the building codes.

For carbon steel the European building code (Eurocodes ) takes into account:

  • The design working life of the structure (typically 50 years for a building and 100 years for a bridge)
  • The environment in which it is located

And from these two parameters a minimum concrete cover of the rebar is specified. [12] [13] [14]
In case of corrosion resistant stainless steel rebar, no extra concrete cover should be necessary* and an important saving in concrete and weight could be achieved.

 * This value is left up to each country

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