Alkaline and Chloride-free Concrete

Consequences of DC stray current in reinforced concrete change according to the properties of the concrete (alkahne, carbonated or contaminated by chlorides), to the duration of the current circulation and to the current density. It is therefore necessary to distinguish concrete structures not contaminated by chlorides and not carbonated from those contaminated by chlorides in quantities insufficient to initiate corrosion and, finally, from those that already have corroding rebars because of chlorides or carbonation. 

Furthermore, even if such a condition is reached and current circulates through the reinforcement, this does not automatically lead to corrosive attack, since the anodic process that takes place at potentials higher than about +600 mV SCE is oxygen evolution, instead of iron dissolution. Nevertheless, attack may occur if the current flows for sufBciently long periods of time [4,5]. Initiation of corrosion can be ascribed to the depletion of the alkahnity in the vicinity of the anodic area promoted by the anodic reaction of oxygen evolution (2H2O — O2 + 4H + 4e.  

Consequently, for corrosion to initiate, two preconditions are necessary a) the electric field must be strong enough to create conditions for the circulation of current through the reinforcement, and b) it must persist for a time long enough to lead to acidification and the destruction of passivity. 

First precondition. Passive reinforcement in non-carbonated and chloride-free concrete offers a high intrinsic resistance to stray current, since the driving force required to produce the circulation of an appreciable current density in the anodic areas (i. e. 2 niA/m ) is at least 600 mV [2]. 

Therefore the driving voltage required to force the circulation of current through the reinforcement is  

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