Thickness of the 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. 

An increase in the thickness of the concrete cover brings about different beneficial effects and in extreme cases some adverse effects. First of all, increasing the cover increases the barrier to the various aggressive species moving towards the reinforcement and increases the time for corrosion initiation, even though different transport laws apply depending on the characteristics of the concrete and the cause of corrosion (carbonation or chlorides). 

It may be remembered that the carbonation depth in time assumes values equal to or (for long periods) below those expressed by the law s — k Therefore if 

Action Exposure class Minimum cover thickness (mm) to  

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It is evident that RubCon is a corrosive-stable structural material because its chemical resistance coefficient during 10 years of operation in aggressive environments exceeds 0.5. The obtained data allow determination of the thickness of the concrete cover of a reinforced RubCon structure. 

The time for initiation of carbonation-induced corrosion is the time required for the carbonation front to reach a depth equal to the thickness of the concrete cover. It depends on all the factors mentioned above (that influence the carbonation rate) and on the thickness of the concrete cover. If the evolution of carbonation in the course of time and the thickness of the concrete cover are known, the initiation time can be evaluated. It should, however, be taken into consideration that the carbonation front may not be uniform across the concrete surface. 

Furthermore, Eq. (1) is also used for the prediction of long-term performance of structures exposed to chloride environments, e. g. during the design stage or the evaluation of the residual life of existing structures. In principle, if D pp and Q are known and can be assumed to be constant in time, it is possible to evaluate the evolution with time of the chloride profile in the concrete and then to estimate the time t at which a particular chloride threshold will be reached and corrosion will initiate. For example. Figure 6.6, plots chloride profiles calculated for a surface chloride content of 5 % by mass of cement and a time of 10 y as a function of the apparent difiusion coefficient. Figure 6.7 shows the initiation times that can be estimated from those profiles as a function of the thickness of the concrete cover when a chloride threshold of 1 % by mass of cement is assumed. 

Some compositional features also have a strong influence on the mechanical strength of the concrete, in particular the wjc ratio. However, in particular in chloride-contaminated environments, the cement type is even more important. In previous chapters, the microstructure of the cement paste and the beneficial role of blast furnace slag and pozzolana such as fly ash have been outlined. The other most important factor is of course the thickness of the concrete cover, which will be discussed in Section 11.4. 

In most structures exposed to the atmosphere, the informative recommendations on cement content and iv/c together with the minimum thickness of the concrete cover required by Eurocode 2, wiU provide a service Hfe of at least 50 y. Therefore, by simply following these standards it would be possible to eHminate the vast majority of forms of deterioration, including corrosion, which are found today and that are connected to incorrect design, material composition or construction practice. 

Figure 11.2 Reduction of the initiation time of corrosion due to iocai reductions in the thickness of the concrete cover [1]...

As the environmental aggressiveness increases, it is theoretically possible to maintain a constant level of durability by increasing the thickness of the concrete cover. In reality, however, the cover thickness cannot exceed certain limits, for mechanical and practical reasons. In particular a very high cover may have less favourable barrier properties than expected. In extreme cases, a thick unreinforced layer of concrete cover may form (micro)cracks due to tensile forces exerted by drying shrinkage of the outer layer, while the wetter core does not shrink. In practice, having cover depths above 70 to 90 mm is not considered realistic. 

As described in previous sections, the standard method for treating durability in EN 206 and Eurocode 2 is based on the definition of the exposure class and determining for each class, the maximum w/c ratio, the minimum cement content and the minimum thickness of the concrete cover. EN 206 provides recommended (informative, that is non-normative) values for concrete composition in terms of maximum w/c ratio, minimum strength class and minimum cement content, assuming an intended working life of 50 y, the use of Portland cement (CEM I) and maximum aggregate size between 20 and 32 mm. In national documents these values and additional requirements can be further specified as normative values, as has been done for example in NEN 8005 (nl) for The Netherlands [12]. 

For carbonation-induced corrosion, the service life (ti) is expressed as the sum of the initiation (h) and propagation (tp) periods up to the threshold at which deterioration becomes unacceptable = h + tp (Figure 4.1). The initiation time (h) may be calculated as a function of the properties of concrete, in particular the coefficient K of carbonation, the environment and the thickness of the concrete cover x), for example with models by Tuutti, Bakker, or Parrott (Chapter 5). The propagation time (tp) can be estimated if the corrosion rate is known, once the maximum acceptable penetration of corrosion has been fixed. A maximum penetration for corrosion attack that is often accepted in reinforced (but not prestressed) concrete elements is 100 tm. 

The initiation time ( ) may be calculated as a function of the chloride transport properties of concrete (usually the apparent diffusion coefficient), the surface chloride content dictated by the environment, the thickness of the concrete cover and the critical chloride content determining the onset of corrosion. The arrival of the critical chloride content at the steel at depth x at time t is calculated using Fick s second law of diffusion (Chapter 6). 

Using this type of calculation, it is possible to find values for D pp (assumed constant) which can be used to obtain a particular service life as a function of the thickness of the concrete cover and the critical chloride content C, assuming a fixed chloride surface content C, as seen in Table 11.6 [16]. 

In principle, stainless-steel reinforcement can be a viable solution for preventing corrosion in a large number of applications. The chloride threshold is much higher than the chloride content that is normally found in the vicinity of the steel even in structures exposed to marine environment or de-icing salts. There is no objection to using stainless steel only where its improved protection is necessary, combined with normal steel at other areas. Hence, stainless-steel bars can be used in the more vulnerable parts of structures exposed to chloride environments, such as joints of bridges or the splash zone of marine structures. Similarly, they can be used when the thickness of the concrete cover has to be reduced, such as in slender elements. 

Conventional repair. The most utilised method consists in the removal of carbonated concrete and its replacement with alkaline mortar or concrete. This method is convenient when the corrosion attack is limited to zones of small extent (for example when the thickness of the concrete cover is reduced locally). In that case it is usually called patch repair. Conversely, it may be rather expensive when repair is required on large surfaces. In fact, concrete must be removed in all the zones where carbonation and subsequent corrosion of steel are expected to damage the structure within the design life of the repair. Even structurally sound concrete must be removed where the corrosion rate of the embedded steel is expected to... 

The anodic process can be stopped by applying a coating to the reinforcement that acts as a physical barrier between the steel and the repair mortar. For this purpose only organic coatings, preferably epoxy based, should be used. Protection is entirely based on the barrier between the reinforcement and the mortar, and passivation of steel cannot be achieved because contact with alkaline repair material is prevented. This method should be used to protect depassivated areas of the reinforcement only as a last resort, i. e. when other techniques are not applicable and only for small specific applications [1,4]. It may be used, for instance, when the thickness of the concrete cover is very low and it is impossible to increase it to the proper level, so that the repair material cannot provide durable protection to the embedded steel. 

While the thickness of the concrete cover and the chloride profile can be measured, the chloride threshold can only be estimated. This is a critical point for successful repair of chloride-contaminated structures. Because of the many influencing factors (Section 6.2.1) the evaluation of the chloride threshold is often rather difficult, especially if the actual concrete composition of concrete is not known. From a safe point of view a value of 0.4 % may be usually considered, but even lower values may be possible for certain concretes and exposure conditions (Chapter 6). 

Cover thickness. The thickness of the cover produced by the repair material should be designed, as in the case of new stractures, to be sufficient to protect the reinforcement for the required time. Therefore, it depends on the resistance of the repair material to carbonation and chloride contamination (Section 19.4.3), on the aggressiveness of the environment and on the design life of the repair. Often for geometrical and aesthetic reasons the original thickness of the concrete cover is reconstmcted however, if this is not sufficient to obtain the required durability, it should be increased or additional protection should be used (Section 19.5). 

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