Depassivation reinforcement cover

Carbonation depth sampling can allow the average and standard deviation of the carbonation depth to be calculated. If this compared with the average reinforcement cover then the amount of depassivated steel can be estimated. If the carbonation rate can be determined from historical data and laboratory testing then the progression of depassivation with time can be calculated. 

Reinforcement cover. The time taken for a carbonation front to advance depends on the depth of the concrete cover. If the cover is thick enough, carbon would not reach at depths to depassivate steel. Phenolphthalein solution can be used to check depth of carbonation, by fracturing (not drilling) at a test location. A pink color indicates satisfactory concrete. 

The service life of reinforced-concrete structures can be divided in two distinct phases (Figure 4.1). The first phase is the initiation of corrosion, in which the reinforcement is passive but phenomena that can lead to loss of passivity, e.g. carbonation or chloride penetration in the concrete cover, take place. The second phase is propagation of corrosion that begins when the steel is depassivated and finishes when a limiting state is reached beyond which consequences of corrosion cannot be further tolerated [6, 7]. 

Depassivation of rebars due to carbonation or chloride penetration often does not extend to the whole surface of the reinforcement but, for instance, it is limited to the outer layer of rebars, or to parts where the concrete cover has a lower thickness... 

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. 

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 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. 

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