Critical chloride content

Initiation of pitting corrosion takes place when the chloride content at the surface of the reinforcement reaches a threshold value (or critical chloride content). A certain time is required from the breakdown of the passive film and the formation of the first pit, according to the mechanism of corrosion described above. From a practical point of view, the initiation time can be considered as the time when the reinforcement, in concrete that contains substantial moisture and oxygen, is characterized by an averaged sustained corrosion rate higher than 2 mA/m [8], The chloride threshold of a specific structure can be defined as the chloride content required to reach this condition of corrosion. 

For a given potential of the steel, the highest content of chlorides compatible with conditions of passivity is the critical chloride content (or chloride threshold) at that potential. As already discussed in Chapter 6, for structures exposed to the atmosphere (whose reinforcement operates at a potential around 0 V SCE) the critical level is usually considered to be in the range of 0.4% to 1 % of the cement content. For structures immersed in water (whose reinforcement operates instead at a much lower potential, around —400 to —500 mV SCE) or for reinforcement that is cathodically polarized for any reason, the chloride threshold is much higher. 

Table 11.4 Initiation time of corrosion as a function of thickness of concrete cover and wjc ratio, calculated on the basis of a critical chloride content of 1 % by mass of cement, a surface content of 4% and constant diffusion coefficients for chlorides (Dapp) determined on two concretes (with 420 kg/m of Portland cement and wjc of 0.4 or 300 kg/m and w/c 0.54) submerged in the North Sea for 16 y [8]...

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]. 

The design value of the critical chloride content can be found by... 

Table 11.8 Critical chloride content for different environments and water-binder ratios for concrete made of ordinary Portland cement (% by mass of binder), from DuraCrete [21]...

Fields of applicability. Figure 15.3 depicts the fields of applicability of pickled stainless steels in chloride-contaminated concrete exposed to temperatures of 20 °C or 40 °C. Fields have been plotted by analysing the critical chloride values obtained by different authors from exposure tests in concrete or from electrochemical tests in solution and mortar and taking into consideration the worst conditions [11-28]. Nevertheless, it should be pointed out that values are indicative only, since the critical chloride content depends on the potential of the steel, and thus it can vary when oxygen access to the reinforcement is restricted as well as when stray current or macrocells are present. For instance, the domains of applicability are enlarged when the free corrosion potential is reduced, such as in saturated concrete. Furthermore, the values of the critical chloride Hmit for stainless steel with surface finishing other than that obtained by pickling can be lower. 

In carbonated concrete, or in the case where the concrete is extensively cracked, the critical chloride contents are remarkably lower. Situations where carbonated concrete and high chloride levels are simultaneously present are rare, but can be found, for instance, inside road tunnels [29]. The more highly alloyed stainless steels should be preferred in these more aggressive conditions. It is well known that for austenitic and duplex stainless steel, an increase in the content of chro-... 

The critical chloride content decreases as temperature increases for instance Eigure 15.3 shows the expected variations between 20 °C and 40 °C. Thus bars made of steels containing molybdenum should be preferred in hot cHmates. 

Interpretation. The presence of an above-critical amount of chloride ions at the rebars leads to depassivation and in the presence of oxygen and water to corrosion attack. From chloride profiles information on the transport of chlorides into the concrete (Chapter 6) can be obtained. In combination with results from potential mapping, the critical chloride content for the specific structure can be obtained. On chloride-contaminated structures an empirical correlation between chloride content and half-cell potential could be established, thus the chloride distribution can be roughly obtained from the potential map. 

Cathodic prevention (CPre) is applied to new structures that will presumably be contaminated by chlorides the passive reinforcement is cathodicaUy polarized with the aim of increasing the critical chloride content necessary to initiate pitting attack in such a way that it will not be reached within the service life of the structure. 

Rapid-Chloride-Migration test. The chlorides in sea water do not attack the mortar itself. However, exceeding the critical chloride content in the mortar near the steel surface might lead to a high corrosion risk. In that case, a localized corrosion occurs, which might spread quickly to the center of the steel tower. 

Figure 8-9. Influence of environment and concrete quality on the critical chloride content (according to CEB Bulletin 182).

Figure 8-13. Critical chloride content for the initiation of corrosion on different stainless steel grades exposed to solutions simulating carbonated and alkaline, chloride containing pore solutions of concrete. Potentiostatic tests at +0.2 V SCE (after Bertolini et al., 1996a).

Initially, reinforcement steel in concrete is protected by the high alkalinity of the concrete and no corrosion occurs (Val Melchers 1997). However, in case of chloride-induced corrosion, the diffusion of chlorides through the protective concrete cover results in the depassivation of the concrete. As soon as a critical chloride concentration is reached, the corrosion process starts. Models for the depassivation of concrete can be found in Stewart Rosowsky (1998) and Vu Stewart (2000). Generally, the length of the initiation period depends on the concrete cover, the diffusion coefficient, the critical chloride content, the initial chloride concentration in the concrete and the chloride concentration at the surface of the concrete element. 

The determination of a critical chloride level, below which serious rebar corrosion damage does not occur, for design, maintenance planning, and life prediction purposes is appealing. Not surprisingly, then, several studies have been directed at defining such a parameter. Unfortunately, the concept of a critical chloride content as a universal parameter is unrealistic. Rather, a critical chloride level should be defined only in combination with a host of other parameters. After all, a threshold chloride level for corrosion damage will be influenced by variables such as... 

For prestressed and reinforced concrete that will be exposed to chlorides in service, it is advisable to maintain the lowest possible chloride levels in the mix to maximize the service life of the concrete before the critical chloride content is reached and a high risk of corrosion develops. Consequently, chlorides should not be intentionally added to the mix ingredients even if the chloride content in the materials is less than the stated limits. In many exposure conditions, such as highway and parking structures, marine environments and industrial plants where chlorides are present, additional protection against corrosion of embedded steel is necessary. 

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