Corrosion mechanism concrete cover

Besides concrete quality, a minimum value of the concrete cover also has to be specified. Eurocode 2 [3] fixes minimum values ranging from 10 mm for a dry environment up to 55 mm for prestressing steel in chloride-bearing environments, as shown in Table 11.5. It should be kept in mind that these values are minimum values that should be increased to obtain nominal values by 10 mm, to also take into consideration construction variability. Besides the protection of steel to corrosion, further requirements of minimum cover depth are fixed to ensure adequate transmission of mechanical forces and fire resistance. 

There are many studies covering the mechanism of corrosion in concrete and assessment techniques (1-6). Specifications and recommended practices on how to select and apply repair methods (NACE SP1290 and 0390, BSEN 12696, BSEN 1504 and ACI222R-01) are given in the literature. 

Though not considered a major problem in North America, carbonation of concrete is a major cause of steel corrosion in the world. Calcium hydroxide in the concrete can react with carbon dioxide or carbon monoxide to produce calcium carbonate. The calcium carbonate is not detrimental to the mechanical properties, but in the process the pH of the concrete drops below 10. At that point the corrosion rate of embedded steel can significanfly increase. This phenomenon is mostly found in concretes with low cement contents, high permeabiUly, and low concrete cover over the rebars. 

Concrete itself can be destroyed by physical, mechanical, chemical or biological actions. Concrete deterioration can be the first step in corrosion of the reinforcement (e.g. when freeze-thaw processes induce cracking and spalling of the concrete cover). The chemical and physical attacks of concrete are described in detail in (Biczoc, 1986 Niirnberger, 1995). 

Requirements specified in this way are deemed-to-satisfy rules. Such rules cannot be used to quantify the performance of the structure in general, specific effects of additional measures (for instance increasing the cover to the steel), or the consequences of sub-standard practice (for example using a higher w/c). In this respect it is important to note that EN 206 also allows the use of alternative performance-related design methods with respect to durability that consider in a quantitative way each relevant deterioration mechanism, the service life of the element or structure, and the criteria that define the end of the service life. Such methods should draw a picture of the characteristics that the concrete must possess to protect the reinforcement for the service life requested from a predictive model of the corrosion attack. These refined methods (as opposed to standard methods) may be based on long-term experience with local practices in local environments, on data from an established performance test method for the relevant mechanism, or on the use of proven predictive models. 

The attack by external agents is more characteristic of steel fibres, where penetration of chlorides can depassivate the steel and lead to its corrosion [8-11]. Although this type of influence might be expected to be particularly harsh in steel fibre reinforced concrete because of the small cover over the fibres, experience has indicated otherwise [8], with such systems performing even better than those of conventional reinforcement. Several mechanisms have been proposed to account for this difference [10,11] and they are addressed in Chapter 7. 

The discussion to this point has dealt only with the mechanical properties of SFRC. However, the durability of SFRC is at least as important, and there have been a number of studies in this area. Atfirst glance, itwould appear that steel fibres would be susceptible to severe corrosion, particularly near the surface of the concrete, where the cover is quite small. Since the diameter of the fibre is effectively reduced by corrosion, any substantial corrosion of the relatively thin fibre would lead to a considerable decrease in both the strength and the toughness of the SFRC, as has been confirmed experimentally by Kosa and Naaman [127]. They also found that the mode of failure could change from fibre pull-out to fibre fracture, which... 

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