Carbonation > Evolution

In order to establish the speed of propagation of arbonatation inside a reinforced concrete structure, one must know the phenomena which govern its evolution.

Generally speaking, concrete has a pH between 13÷13,80 which, when covering the reinforcements, keeps them in an alkaline environment, favouring the formation of a thin layer of oxide (a few nano-metres thick), which ensure protection against corrosion. The alkalinity of the concrete diminishes progressively through contact with the carbon dioxide in the atmosphere, so the pH goes down to values of < 9. The reaction of Carbonation may be globally described as:

Ca(OH)2 + CO2  >  CaCO3 + H20

In these new conditions, the oxide film is destroyed and the conditions of passivity of the reinforcements diminish. The destruction of the
film is the pre-condition for corrosion, which will take place in the right circumstances and with the combined presence of water and oxygen. Corrosion of carbonation appears uniformly distributed on the reinforcement throughout the carbonation front. This makes it easy to distinguish from another kind of corrosive phenomenon due to the presence of chlorides, more localized in isolated points.

Carbonation starts on the outside surface of the concrete and then affects the inner regions. Its penetration in time generally follows a parabolic trend with a law of the kind s=Kt½  
where through time (t) the thickness of the carbonatated layer (s), advances as a function of a coefficient K, which is an index of the penetration speed.

The parameter K which defines the penetration speed of carbonation depends on both environmental factors and on factors associated with the features of the concrete.

exposure_carbonationENVIRONMENTAL FACTORS
The speed at which the carbonation spreads diminishes as dampness increases, in fact transportation of carbon dioxide takes place easily through air-filled pores, and is much slower in those filled with water. However, the reaction of carbonation only takes place when water is present, so at relative humidity of less than 50% it takes place at a negligible speed. This is why the most dangerous range of relative humidity for carbonation lies between 50% and 80%. Also, as the content of carbon dioxide grows in the air, the spreading speed of the carbonatated front increases, and at a constant relative humidity, temperature rises make the penetration speed grow.

                  The kind of exposure of the structural element also conditions the progress of carbonation. In fact, the UNI EN 206 standard establishes the levels of the conditions of environmental aggressiveness setting 4 XC levels for carbonation corrosion. When reinforced concrete is exposed to air and dampness, the exposure is classified as follows:

The layer of concrete covering the reinforcements provides them with protection against aggressive agents from the outside environment. The increase of the cement content, with the same w/c ratio, allows a greater quantity of water to get into the mix. However, an increase of cement can lead to cracking because of hydration heat. The water-cement ratio at the time the mix was made is the key factor determining the capillary porousness of the cement mix and hence the penetration strength of the aggressive species. Diminishing the w/c ratio reduces the porosity of the cement mix and hence slows down the penetration of the carbonation. In a similar manner, the cracks may reduce the starting time since they are preferential routes for the entry of carbonation.

In existing structures, thanks to the innovative CARBONTEST® method, one can measure – in the different parts of the work – the penetration (s) of carbonation.
Comparing the measured value with the age of the structure, one can draw up a carbonation propagation graphic, experimentally establishing the coefficient K.
If one then establishes the thickness of the cover (c) one can predict the evolution of the decay of the concrete and estimate the time it will take the carbonation to reach the reinforcements, in order to plan any maintenance operations.