PSI - Issue 10

A. Drakakaki et al. / Procedia Structural Integrity 10 (2018) 59–65 A. Drakakaki et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Mass loss rate constitutes a commonly accepted damage index of a metallic material, when corrosion has occurred. However, in order to achieve a more complete correlation, further investigation of the damage effects is demanded. Precisely, the geometrical characteristics of pits and the influence of the corrosive agent on the geometry of ribs, which are associated with the development of the bond between steel and concrete, should be studied Given the different internal structure of the two steel categories (single phase and dual phase), besides their cor rosion resistance it was necessary to evaluate their mechanical performance as well, before and after corrosion process. Specifically, Table 5 presents the drop of the mechanical properties of the two steel bar categories, after exposure for 300 hours to 0.5 mA/cm 2 constant current density. To estimate the performance of the two steel classes 6 tensile tests were performed for each case, 3 for non-corroded samples and 3 for corroded specimens. Table 5. Drop of the mechanical properties of S400 and S500s steel bars, after 300 hours of corrosion under 0.5mA/cm 2 (constant current), according to the results of the tensile tests executed. S400 S500s Mechanical Properties Values Mechanical properties Values Mass Loss (%) 4.23 Mass Loss (%) 3.18 Drop of Yield Stress (%) 2.11 Drop of Yield Stress (%) Constant Drop of Tensile Strength (%) 3.66 Drop of Tensile Strength (%) 1.7 Drop of Agt (%) 18.31 Drop of Agt (%) 17.95 Drop of Energy Density (%) 21.97 Drop of Energy Density (%) 18.42 Table 6 presents the drop of the performance of the corroded specimens of the S400 and S500s steel bar categories, after the performance of the Low Cycle Fatigue tests. Table 6. Drop of the performance of S400 and S500s steel bar categories that were corroded under constant current, after Low Cycle Fatigue tests. S400 S500s Mass Loss (%) Drop of Numbers of Cycles to Failure (%) Drop of Dissipate Energy (%) Mass Loss (%) Drop of Numbers of Cycles to Failure (%) Drop of Dissipate Energy (%) According to the results presented in Tables 5 and 6 it can be concluded that behavior of the two steel categories (S400 and S500s) against corrosion, as far as mass loss and mechanical performance is concerned, changes over time. Specifically, in Table 5, it is clear that for the same exposure duration to corrosion, under 0.5 mA/cm 2 current density, S400 records higher mass loss against S500s. This conclusion is in agreement with the experimental results presented in the existing literature, referring to the method of the salt spray fog chamber, for mass loss percentages lower than 7%. For the same mass loss values, from Table 5 and 6, it is obvious that S400 demonstrates higher drop, concerming its mechanical performance. This remark though, is not valid in cases when mass loss values are higher than 7%. Table 6 presents the drop of the performance of both steel categories, after the execution of Low Cycle Fatigue Tests. The free length of the specimens tested was 6Φ and control led strain equal to ±2.5% was defined. The drop recorded for S500s steel class, was twice as high as the drop of S400 steel grade. However, the results referring to the mass loss percentages of the short specimens, used for the purposes of Low Cycle Fatigue tests, cannot be directly correlated with the previous corresponding findings (concerning mass loss rates), referring to long specimens. According to Angst (2017), this difficulty lies in the fact that the corrosion damage (mass loss) recorded on the short or long exposed lengths of the steel bars, is considerably different (Fig.5). The reason for this variation is that for equal exposure to the corrosive agent, the concentration of chloride ions recorded in short samples is higher than in long bars. Consequently, the corrosion damage in short-length specimens is more intense (higher mass loss percentages and harsher pits). At the same time, the probabilities of existence of surface sulphides MnS, FeS (surface or subcutaneous) in steel, are directly related to their chemical composition. Indeed, quite often, existence of surface sulphides is responsible for the development of corrosion paths (Fig.6). 7.94 8.7 9.18 6.27 18.42 19.96