Penetration of Carbonation

The carbonation reaction starts at the external surface and penetrates into the concrete producing a low pH front. The measurement of the depth of carbonation is normally carried out by spraying an alcoholic solution of phenolphthalein on a freshly broken face. The areas where pH is greater than 9 take on a pinkish color typical of phenolphthalein in a basic environment, while the colour of carbonated areas remains unchanged (Section 16.3.1). 

The rate of carbonation decreases in time, as CO2 has to diffuse through the pores of the already carbonated outer layer. The penetration in time of carbonation can be described by  

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Mechanical intrusion is the penetration of the matter to be dated by carbon of a different age from that of the sample itself if not taken into account, mechanical intrusion, too, leads to erroneous ages. The penetration of rootlets from growing plants into buried specimens, the infiltration of windblown organic matter, and the accidental insertion of fibers from brushes or other instruments used to clean a sample are examples of likely modern carbon intrusions into prospective samples, which lead to assigning to a sample later dates than the true ones old carbon intrusions, such as those caused by the penetration of carbonate minerals from groundwater, or of petrol or oil from excavating tools, on the other hand, are conducive to assigning earlier dates than the true ones. 

Hatch, W., 1975. The Implication of Carbonic Anhydrase in the Physiological Mechanism of Penetration of Carbonate Substrate by the Marine Burrowing Sponge Cliona celata. Unpubl. Ph.D. Diss., Boston University, 158 pp. 

Figure 5.2 Example of penetration of carbonation in concrete in relation to the microclimate (average of 27 concretes) [5]...

CO2 concentration. The concentration of carbon dioxide in the atmosphere may vary from 0.03% in rural environments to more than 0.1% in urban environments. Comparatively high concentrations can be reached under specific exposure conditions, such as inside motor vehicle tunnels. As the CO2 content in the air increases, the carbonation rate increases. Accelerated tests carried out in the laboratory to compare the resistance to carbonation in different types of concrete show that, indicatively, one week of exposure to an atmosphere containing 4% CO2 will cause the same penetration of carbonation as a year of exposure to a normal atmosphere [8]. Some researchers suggest that with a high concentration of CO2 the porosity of carbonated concrete is higher than that obtained by exposure to a natural atmosphere, particularly if the concrete has been made with blended cement or has a high cement content However, this is controversial, since it was shown that even 100 % CO2 under increased pressure, produced the same microstructure as natural carbonation [9]. 

Often the parabolic formula is used to describe the penetration of carbonation ... 

In any case, cracks may reduce the corrosion initiation time in that they provide a preferential path for the penetration of carbonation or chlorides (Figure 11.1). Experiments with sectioned steel bars in intentionally cracked concrete beams have shown that the depassivation time decreases as the crack width decreases however, there is no relationship between crack width and corrosion rate actually the corrosion rate decreases with increasing cover of the uncracked concrete (between cracks) due to the influence of the cathodic process [10]. 

Organic coatings are used to block the penetration of carbon dioxide or chloride ions. They form a continuous polymeric film on the surface of the concrete, of a thickness ranging from 100 to 300 pm. The binder can already be present in the liquid paint as a polymer, or the polymer can be formed due to chemical reactions between monomeric components that are mixed just before application. Modem coating systems are usually built up of several layers applied consecutively. They are compatible with the alkalinity of the concrete and are based on various types of polymers (e. g. acrylate, polyurethane, epoxy), pigments and additives, and are made suitable for application by the addition of solvents or diluents. 

The beneficial effects of this type of repair are durable if the external layer offers an effective barrier to penetration of carbon dioxide. The thickness and permeability of this layer must therefore be sufficient to prevent its carbonation during the complete design life of the repair. In fact, the original concrete underlying this layer cannot be counted on to resist carbonation. 

To evaluate the penetration of carbonation, the square-root model can be considered (Section 5.2.1). In most cases it is reasonable to assume that the conditions of exposure of the structure wiU not change in time and thus the carbonation coefficient K can be calculated from the present carbonation depth K = It... 

Surface treatment of concrete. Surface treatment of concrete may be used in association with conventional repair to achieve the required service life of the repair. A specific type of treatment may be used to delay penetration of carbonation or chlorides or to decrease the moisture content (Chapter 14) either in the original concrete or in the repair mortar. The effect of this treatment can be taken into consideration in the evaluation of the residual service fife of the structure, both in the repaired and unrepaired zones. This can lead to a reduction in the extent of the areas to be repaired or in the thickness of the repair material. 

The enantiomeric excess of the interesterification of ibuprofen methyl ester with ethanol in supercritical carbon dioxide at different pressures shows a linear decrease of the enantiomeric excess with increasing pressure. From experiments in organic solvents it is known that penetration into the hydrophobic core of the enzymes can occur. Therefore penetration of carbon dioxide into the enzyme molecule probably takes place with increasing pressure and alters the conformation of the enzyme and hence reduces its affinity to the substrates. 

Carbonization of concrete caused by the penetration of carbon dioxide into concrete leads initially to the formation of calcium carbonate as a result of reaction with calcium hydroxide, and then to calcium hydrogen carbonate, which is washed out by rain water from concrete capillaries. A decrease of the pH of the concrete environment to 8-8.5 is the result and it causes destruction... 

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