Issue 50

Ch. Apostolopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 548-559; DOI: 10.3221/IGF-ESIS.50.46 558 From a structural point of view, dissolved MnS sites and porosity represent high stress raisers, which upon loading can lead to the development of microcracks. These phenomena within the martensitic zone, near the location of surface pitting, result in local disruption of the material and a possible conjunction of extended imperfections with the surface pits. In these locations, the local disruption of the material is a crucial factor of its mechanical behavior since during the axial load an intense stress concentration is developed and the subsequent crack propagation is inevitable and rapid [8]. Rapid depletion of the ductility or even failure may occur in high strength and ductility dual phase steel bars, due to the combination of interior and exterior damage phenomena under strong stresses. Consequently, an additional purpose of this study is to highlight the significance of the corrosion factor on the mechanical performance of the dual phase steel bars and to estimate the degradation of its mechanical properties, which is the result of the cooperation of the internal and the external damage, under those hazardous circumstances [25]. The results of the mechanical tests confirmed that corrosion, due to chloride ions, is one of the main degradation factors of steel reinforcement. C ONCLUSIONS he conclusions of the present study can be summarized as follows: • Steel bar category B450c recorded slightly higher mass loss percentages than B400c steel class, a fact that demon- strates that the first category is more vulnerable to corrosion. • In both steel categories, a slight mechanical degradation was recorded, in terms of yield and maximum strength, while ductility and dissipated energy were dramatically diminished. • Due to buckling and buckling reversal, materials demonstrate limited ductility especially at 4%. • When cyclically stresses occur, sulphides, FeS and MnS sites will host crack nucleation leading to sub-surface crack propagation, which interact with external pits. The case becomes very complex in terms of analysis, especially under fully reverse loading and high plastic strain levels leading to buckling • Among two specimens of the same steel class, the one with smaller nominal diameter records higher performance against LCF tests. • The areas with MnS compounds present a significant development under the presence of (Cl − ) ions, which combined with the occurrence of other impurities, as well as pits at the surface of steel, may inevitably induce mechanical stress con- centration during the tensile loading • Therefore, the mechanical degradation of precorroded steels that were examined can result from the synergy of mass loss effect, external pitting, and a variety of inevitable side effects from regions with MnS compounds within the marten- sitic zone. • Examining the performance of both steel categories under LCF tests, it seems that reference B450c steel has grater life cycle and higher energy stocks than the reference B400c steel. However, when both categories record 5%-10% mass loss, under corrosion conditions, life cycles of both steel categories are approximately equal. A CKNOWLEDGEMENTS he present work has been implemented within the framework of the European Research project Rusteel (Effect of corrosion on low-cycle (seismic) fatigue -Behaviour of high strength steel reinforcing bars) (2008-2012), Project funded by RFCS - Contract No. RFSR-CT-2009-00023 R EFERENCES [1] Apostolopoulos, C.A., Papadakis, V.G., (2008). Consequences of steel corrosion on the ductility properties on rein- forcement bar, Construction and Building Materials, 22, pp. 2316-2324 [2] Almusallam, A., (2001). Effect of degree of corrosion on the properties of reinforcing steel bars, Construction and Building Materials, 15(8), pp. 361-368 [3] Apostolopoulos, Ch., (2007). Mechanical behavior of corroded reinforcing steel bars S500s tempcore under low cycle fatigue, Construction and Building Materials, 21, pp.1447-1456 [4] Papadakis, V.G., (1999). Supplementary cementing materials in concrete-activity, durability and planning, Danish Technological Institute Concrete Center T T