Chloride free concrete

In non-carbonated and chloride-free concrete, the concentration of hydroxyl ions (OH ) varies from 0.1 M to 0.9 M, due to the presence of both NaOH and KOH (the latter is predominant, especially in Portland cement). Other ions, e. g. Ca and S04 , are present only in very low concentrations. Addition of blast furnace slag or fly ash to Portland cement results in a moderate reduction of ionic concentration, and thus in pH. From hydroxyl ion concentrations in Table 2.1, values of pH of 13.4-13.9 can be calculated for Portland cement, and pH values of 13.0-13.5 for blended cements. Addition of condensed silica fume in higher percentages may lead to a decrease in the pH to values to below 13 [4, 10]. 

Applying the concept of transport numbers (Eq. (12)) to concrete it can be shown that for chloride-free concrete, assuming that the pore solution contains 0.5 mole/1 of NaOH, the transport numbers for OH and Na are 0.8 and 0.2, respectively. For concrete contaminated by chloride salts, assuming that the pore solution contains 0.5 mole/1 of NaOH and 0.5 mole/1 of NaCl, the transport number of OH, Na, and CT are 0.52, 0.20 and 0.28, respectively. In a general sense, these estimated ion transport numbers have been confirmed by experimental works [31, 32). 

For passive reinforcement in non-carbonated and chloride-free concrete, current can flow only if there is a great enough increase in the potential of the anodic area to exceed the threshold for oxygen evolution (Figure 9.3). It was shown in Chapter 7 that at potentials below +600 mV SCE no iron dissolution or any other anodic process takes place, and thus it is impossible for the current to leave the metal. 

Figure 9.3 Schematic representation of electrochemical conditions in the cathodic and anodic zones of reinforcement in non-carbonated and chloride-free concrete that is subject to stray current...

DC stray currents may have more serious consequences in chloride-contaminated concrete. On passive reinforcement in concrete containing chloride in a quantity below the critical content and thus in itself insufficient to initiate localized corrosion, the driving voltage AE required for current to flow through the reinforcement is lower than in chloride-free concrete and decreases as the chloride content increases (Figure 9.7). This is a consequence of less perfect passivity, and in particular a lower pitting potential. 

We have seen that stray current can hardly induce corrosion on passive steel in non-carbonated and chloride-free concrete. However, the potential adverse effects of stray current on concrete structures may become increasingly important with the increased use of underground concrete construction. Stray-current effects are rarely recognised as such. The importance increases further due to the increase of the required service lives (i. e. 100 y or more). 

If steel is passive and is embedded in non-carbonated and chloride-free concrete, the first stage does not take place and, consequently, also the others do not. Conversely, it can take place if concrete is carbonated or chloride contaminated. Similarly, it can occur if part of the steel surface is not embedded in concrete and is in contact with soil, aqueous solutions with neutral or acid pH or simply with moist atmosphere. 

Non-carbonated and chloride-free concrete. In concrete that is not carbonated and does not contain chlorides, and in the absence of external cathodic polarization, hydrogen evolution, and thus consequent embrittlement, cannot take place. In this type of concrete, characterized by a pH above 12, hydrogen evolution can only occur at potentials below about —900 mV SCE. Passive steel under free corrosion conditions has much less negative potentials (Chapter 7) in the case of atmospherically exposed structures, the potential is between 0 and —200 mV (zone A of Figure 10.9). 

In non-carbonated and chloride-free concrete, the passivity of low-alloyed steels is not influenced appreciably by their composition, stracture or surface conditions. Therefore, the usual thermal or mechanical treatments or the roughness of the surface of the rebars have negligible influence on their corrosion behaviour. 

Even the presence of magnetite scale that often covers the surface of the bars, which can cause dangerous localized attack on steel in contact with neutral solutions (such as fresh water or seawater), is not dangerous in concrete. In fact, non-carbonated and chloride-free concrete passivates all the surface of the steel. If adherent oxide films are present, they do not create problems. If the oxide layer contains chlorides, because for example it is formed in a marine environment, it must be removed completely because it can hinder passivation. 

Figure 15.4 Macrocell current density exchanged between a corroding bar of carbon steel in 3% chloride-contaminated concrete and a (parallel) passive bar of carbon steel in chloride-free concrete, 316L stainless steel in...

This approach is also popular in laboratory corrosion studies and has been developed as an ASTM procedure known as ASTM G109 (ASTM, 2005a) concrete prisms are made with a single top rebar in chloride containing concrete and two bottom rebars in chloride-free concrete. The current flow between the top and bottom is monitored as shown in Figure 4.22. 

Summing up these results it can be concluded that stainless steels improve the durability of reinforced concrete constructions considerably. Depending on the expected severity of the environment different steel grades should be selected for carbonated, chloride-free concrete ferritic stainless steels with low chromium content (e.g. DIN... 

Patch rqiair A repair procedure in which small areas of chloride contaminated concrete are replaced by fresh chloride-free concrete. 

All these factors promote differences in the environment (oxygen, moisture and chloride content) along a given piece of reinforcement. Furthermore, most structures contain reinforcement at different depths, and, because of the procedures used to fix the steel, the steel is electrically connected. Thus, when chloride penetrates the concrete, some of the steel is in contact with chloride-contaminated concrete while other steel is in chloride-free concrete. This creates a macroscopic corrosion cell that can possess a large driving voltage and a large cathode to small anode ratio which accelerates the rate of corrosion. 

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