Time to depassivation

Cover thickness (mm) Time to depassivation (y) Time to depassivation (y)... 

Although the macrocell currents measured with the anode-ladder system can detect the time-to-depassivation, no information regarding the corrosion rate of the... 

We have treated the problem as one dimensional so far, considering the time to depassivation at one particular location. Carbonation depths, chloride profiles and rebar depths are not uniform so the spatial distribution of depassivation or initiation must be included in the calculation unless the ranges are small or the time from depassivation to damage is large. We know that all the concrete cover will not spall off at once so there must be a distribution of depassivation times and of time from depassivation to spalling. We must have realistic estimates of the time from the first spall to end of functional service life. 

This is an S shaped curve that approximates to the two straight lines for To and in Figure 9.1. The curve could therefore be calculated if there are two sets of data. These can be derived from two sets of measurements of cracking, spalling and chloride content separated by several years, or taking one set of measurements at the present time and back calculating data for an earlier time This may be to the time of the first spall or a back calculation of the time to depassivation from the chloride profiles (approximately Tq). We can therefore derive values for A and B. These can be used to project forward the delamination rate and show how costs will escalate if work is deferred or how repair quantities will increase between the survey and the start of patch repair work. 

This reaction leads to a neutralization of the alkaline pore solution of concrete and requires both CO2 and water in order to proceed. As soon as the pH at the reinforcement is near neutrality the steel becomes depas-sivated (ACI222,1985). The time to depassivation of reinforcing steel depends on the... 

This gives fairly accurate predictions, though it tends to overestimate, at least for longer times, the penetration in the case of low-porosity Portland cement concrete. Values of K depend on all the factors discussed above and thus change as a function of concrete properties and environmental conditions. In practice, an accurate prediction of K, and thus the time of depassivation of the steel, is complex, above all because this parameter may change in time or in different parts of a single structure. 

Corrosion potential. Any embedded reference electrode allows the electrochemical potential of the adjacent rebars to be measured. This allows depassivation of the rebars or of any other steel sensor element put at different depths to be detected by a drop in half-cell potential. The corrosion potential will be influenced by concrete humidity and oxygen content (Chapter 7). The depassivation of the steel probe located in the outermost cover concrete will present an early warning and suitable in-depth distribution of a set of steel probes allows the corrosion risk to be evaluated or the time of depassivation of the rebars to be calculated. 

The next step is the determination of the corrosion rate of the depassivated material. In order to keep continuously a part of the immersed sample surface in an active state, the passive film has to be removed by mechanical contacts. It is thus necessary to select a rotation period, troi, which is small compared to treac so that the passive film has no time to regrow in between two successive contact events. It was proposed (Diomidis et al., 2009, 2010) to take the rotation period troi equal to ... 

Figure 8-7. Development of corrosion of steel in concrete with time (adapted from Tuutti, 1982). r, time of depassivation t/, time to reach the final state with corrosion rates a and b, respectively a, b, c increasing corrosion rate (a>b> c).

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. 

It may not be necessary, however, to assume that the early stages of depassivation by annealing are due to buildup of the concentration of H° as AH complexes dissociate. While the equations of Sah et al. have focused attention on the ultimate formation of H2 as the dominant process in the later stages of annealing, it is quite possible that the rate of H2 formation may be relatively higher at early times than would be predicted by the simple n2 dependence in the original Eq. (117). When both H+ and H° are present, formation by H+ + H+ will be slow, because of Coulomb repulsion, and we shall see evidence in Sections 4a and 4b that H+ + H° may... 

If an incubation time is needed to first depassivate the surface, the R function has to be constructed accordingly. A corresponding X(t) curve is also shown in Figure... 

In any case, cracks may reduce the corrosion initiation time in that they provide a preferential path for the penetration of carbonation or chlorides (Figure 11.1). Experiments with sectioned steel bars in intentionally cracked concrete beams have shown that the depassivation time decreases as the crack width decreases however, there is no relationship between crack width and corrosion rate actually the corrosion rate decreases with increasing cover of the uncracked concrete (between cracks) due to the influence of the cathodic process [10]. 

According to the general service life model of Tuutti (Figure 4.1), corrosion inhibitors admixed to the fresh concrete can act in two different ways these inhibitors can extend the initiation time and/or reduce the corrosion rate after depassivation has occurred. From the point of view of the design process and the desired extension of the initiation time, mixed-in inhibitors are more reliable, i. e. it is easier and more secure to add the inhibitors to the mix. 

A number of empirical calculations have been used to derive values of A and n based on such variables as exposure conditions (indoors and outdoors, sheltered, unsheltered), 28 day strength and water cement ratio. A wider range of empirically derived equations is given in Table 3.1. These cover different exposure conditions, curing and concrete properties. The easiest solution for a given structure is to take some measurements of carbonation depth, assume n = 1/2 and calculate A. This can be used to predict the rate of progression of the carbonation front. The time taken to reach the steel can then be estimated and the rate of depassivation calculated. 

The successful determination of copper in beer, a complex system that precludes meaningful measurements under silent conditions, opened up the possibility for analysis in even more inaccessible media such as biological samples. Cavita-tional depassivation provides a remarkable enhancement in measured Faradaic currents whilst the increased mass transport due to acoustic streaming lowered the accumulation times below those required for other hydrodynamic voltammetric techniques such as rotating disk electrodes. 

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. 

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