Abstract
A wide variation of chloride thresholds can be found in the literature. Possible causes for this variation, which were mentioned are: method of threshold determination, cement chemistry, and concrete microstructure. Regardless of the reasons for these variations, a probabilistic method can be used to ensure the durability of reinforced concrete structures for a specific period. A probabilistic method gives a threshold for design for given required confidence. A former research analyzed the micro-structure of concrete around steel rebar, by means of BSE automated image analysis, and the chloride threshold. The research found a statistical significant correlation between the maximal distance of steel from the closest concrete solid on the rebar perimeter and the chloride threshold. Theory of statistics of extreme values state, that the distribution of maxima data is bonded to be general extreme value distribution (GEVD). Re-analysis of the data from the abovementioned research found that as expected from the theory of statistics, the maximum steel-concrete distance distributed according to GEVD. Therefore, since the chloride threshold depends on the steel-concrete distance, its distribution is bonded to the GEVD. The analysis in this paper show that the received chloride threshold is GEVD as the theory predicted. From the theoretical point of view, GEVD may be the distribution of many other corrosion processes. The recognition of GEVD as the correct distribution for describing corrosion initiation in reinforced concrete (RC) structures, can enable more accurate planning for corrosion protection.
Highlights
When the chloride content near the rebar surface exceeds a certain limit, referred to as the chloride corrosion concentration limit (CCCL, denoted as "threshold" in the followings), depassivation of the steel occurs and the steel is susceptible to corrosion; the corrosion hazard to the embedded reinforcement increases drastically above this level [1].The chloride threshold for depassivation of steel embedded in concrete is an important parameter for the calculation of the time expected to pass before the initiation of corrosion of reinforced concrete
Ignoring the distribution of the steel-concrete distance, the distribution of the maximum steel-concrete distance in an image, or the maximum steel concrete distance that can be found around a rebar, have a generalized extreme value distribution (Figure 5 and Figure 6 respectively)
A variable that is linearly related to another variable that is distributed according to general extreme value distribution (GEVD), has a GEVD as well [19]
Summary
When the chloride content near the rebar surface exceeds a certain limit, referred to as the chloride corrosion concentration limit (CCCL, denoted as "threshold" in the followings), depassivation of the steel occurs and the steel is susceptible to corrosion; the corrosion hazard to the embedded reinforcement increases drastically above this level [1].The chloride threshold for depassivation of steel embedded in concrete is an important parameter for the calculation of the time expected to pass before the initiation of corrosion of reinforced concrete. When only the free chloride was measured, the range of the threshold is from 0.03% of cement [7] to 4% of cement [6]. Where Cl-/OH-is used for presenting the chloride threshold a range of 0.12 to 20 can be found in the literature [6]. These variations were attributed to several causes, including (a) the method of threshold determination [4], (b) cement chemistry [8], and (c) concrete microstructure [3, 9,10]
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