PSI - Issue 10

Alk. Apostolopoulos et al. / Procedia Structural Integrity 10 (2018) 49–58 Alk. Apostolopoulos and T. Matikas / Structural Integrity Procedia 00 (2018) 000 – 000

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However, in corroded bars the location of crack initiation is significantly affected by the distribution of corrosion pits along the length of the bars. The study of Apostolopoulos and Matikas (2016) says that the gradual volume increase of MnS, FeS compounds and impurities which exist close to the external surface of steel bar, are accounted as areas without mechanical strength during the mechanical stress of rebars. On the other hand, it develops severe internal stress concentrations creating conditions of interaction with other pits in the same position. The detection of areas on steel bar with MnS inclusions inside the martensitic zone and the fact that chloride is prone to be absorbed in and accumulated at the MnS (sulfides) inclusions, constitutes a serious reason of starting the internal corrosion of steel bar (Apostolopoulos et al. (2016)). The above phenomenon of Mns inclusions, also exists in steels without martensitic bark as the inclusions that are examined at this work (B500A and B500B). Because of the fact that chlorides is prone to be absorbed and accumulated at the (FeS, MnS) sulfides inclusions, an increase of volume of sulfides is taking place (FeS, MnS), according to Webb (2001). Sulfide areas play a leading role in the initial stages of internal corrosion of the steel bars. According to the results of tensile and LCF tests and SEM images which are presented in, it can be mentioned that many of the conclusions of Apostolopoulos et al. (2016) study are confirmed. The pitting (local material failure and degradation due to corrosion effect) has significant surface dimen sion and the damage extends in depth. Upon loading these locations act as internal stress concentration leading eventually to the formation of cracks. However, due to their proximity, they force a multiple cracking phenomenon. As such crack coalescence will become critical with the number of loading cycles leading to fast crack growth. The direc tion of the crack appears to tend to expand towards the surface. This rapid expansion of the crack produces brittle ridge-like fracture surface- negating any remaining ductility that had been left in the material. It is also important to note that from the first loading cycles buckling creates an increasing tension on a single side of the rebar. The process is so strong as to produce rapid ductility exhaustion (hardening plateau) and therefore pits within this region will propagate into cracks without any significant ductility signs in their crack path. Even more in cases of corroded steel, which in a way or another, energy reserves are dramatically reduced. It is noteworthy that, approximately, the same conclusions were confirmed to Apostolopoulos et al. (2016) work even if there were a dual phase steel bars of high strength and ductility (C class, B450c and B400c). The assessment of struc ture performance in older buildings which is based only on mathematical models (displacement – pushover method) might lead to unreliable and inappropriate decisions when the degradation of the strength and ductility properties of materials due to corrosion is not taken into consideration beforehand. The above results and observations constitute a useful data for the expected behavior of B500A and B500B rebars which are used in many modern RC structures. The main conclusions of this study are summarized as follows:  The results of mechanical tensile tests confirmed that corrosion is a significant factor of degradation in mechanical properties of steel bars.  The reduced values of U of corroded steel rebars are expected to have a direct negative impact on the service life of the specimens which come after the LCF tests.  The percentage mass loss, the pitting corrosion and the inelastic buckling constitute the main parameters of affecting negatively to the (seismic behavior) low-cycle fatigue life of the steel bars.  The degradation of the mechanical performance of steel on seismic loads can be attributed to mechanism of inelastic buckling influenced by loading history with a combination of the presence of extensive porosity close to the surface of the steel bar.  Generally, the lifetime (N cycles ) decreased with the increase of the level of imposed deformation and free length of the specimen.  Increasing the free length reduces the mechanical (seismic) performance of steel bar. This reduction is more unfavorable in case of corroded specimens where buckling phenomena are encouraged and the corroded bars have highly localized pitting corrosion.  The negative effect of free length on the mechanical (seismic) properties of steel bars is greater than the nega tive effect of the corrosive action, due to buckling phenomena which lead to premature unexpected failures.  Corrosion causes a significant reduction of the ductility properties of the material such as elongation to fracture, energy density, cycles up to failure, levels of dissipated energy and increase to percentage mass loss. 4. Conclusions