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Accelerated carbonation testing of mortar with supplementary cementing materials - Limitation of the acceleration due to drying

J.H.M. Visser
TNO Technical Sciences, Structural Reliability, Delft, the Netherlands

In the design stage of a concrete structure, decisions have to be made on how to fulfil the required service life and consequently, what concrete composition to use. Concrete compositions can be chosen on account of known performances but this will limit the choice of compositions and materials to those that have already been in use. Other methods may give a wider choice in concrete compositions, but mostly require proof which is generally obtained by means of testing. When limited time is available for testing, accelerated tests often are performed. If there is no good insight in the underlying principles of the effect of the acceleration, some serious mistakes in the service life designs will be made.

In this paper, an example of accelerated testing is shown for carbonation. Accelerated carbonation tests at 2% CO2 and natural carbonation tests at ambient CO2-level have been executed. Based on the results, the resistance against carbonation has been calculated. Since this resistance is a material property, it should be similar in both tests. For two of the tested concrete compositions this proved to be the case, a third type of concrete made with fine cement it did, however, not. It was speculated that in the accelerated test, a different mechanism was becoming dominant for this concrete. Instead of the transport of CO2, now drying out was thought to be dominant. The drying out is a necessary step in the carbonation process as during the carbonation a relatively large amount of water is generated that, when saturating the pore space, prohibits CO2 to be transported to the carbonation front. A new simple model was derived for this case. The modelling gave a similar resistance against carbonation for the fine OPC as determined in the natural carbonation case where transport of CO2 was the dominant step in the carbonation process. If this change in dominant step had not been made, a far too high resistance in carbonation would have been calculated, seriously overestimating the service life of this fine OPC concrete in structural applications.

Key words: Accelerated testing, modelling, carbonation, moisture, pore structure