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Tests on the concrete in an existing structure can be used to determine the depth of carbonation at a given age (see Testing/Hardened concrete testing/Carbonation depth). The rate of carbonation is affected by many parameters, but it is generally assumed that the depth of carbonation is proportional to the square root of time. Hence using the measured depth (at a known age) the time for carbonation to reach the level of the steel can be estimated. Further information may be found in Concrete Society Technical Report 54, Diagnosis of deterioration in concrete structures, and in Technical Report 61, Enhancing reinforced concrete durability.
For marine structures and for structures subjected to de-icing salts such as highway bridges, tests can also be used to determine the chloride concentration at various depths from the surface (see Testing/Hardened concrete testing/Chloride ion concentration). Knowing the chloride concentration profile at a given age the time at which the concentration at the level of the reinforcement will reach the critical (or threshold) level can be estimated. Again a simple square root of time relationship may be used, as outlined in Technical Report 54). Alternatively more detailed analysis may be used, which is based on Fick’s second law of diffusion, see Technical Report 61.
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