Issue 50

Ch. Apostolopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 548-559; DOI: 10.3221/IGF-ESIS.50.46 554 Additionally, due to buckling and buckling reversal, the material's ductility especially at 4% is particularly limited. Explicitly, the free length of the specimen strongly influences the results of the experimental tests, since buckling phenomena due to compression axial loads can lead to premature unexpected failures of the rebars (Fig.5); in order to avoid this problem, actual standards for reinforced concrete constructions (EN 1998-1:2005, D.M. 14/01/2008) prescribe the adoption of opportune limits for the free length of reinforcements between stirrups, that shall be lower than 6 or 8 times the diameter for buildings respectively designed in high, medium or low ductility class. For comparison reasons, a few more LCF tests were performed on reference B450c steel bar specimens, of 12mm nominal diameter. The corresponding results are presented in Fig.6. Figure 5 : Buckling effect on a tested rebar. 0 50 100 150 200 250 300 350 400 450 500 0 20 40 60 80 0 100 200 300 400 500 600 700 800 0 2 4 6 8 10 12 Ncycles Energy (MPa) Mass Loss (%) Energy stock-mass loss-6D Ncycles-mass loss-6D Energy stock-mass loss-8D Ncycles-mass loss-8D Figure 6 : A graphical depiction of energy stocks and number of cycles of the B450c (Φ12) specimens tested in LCF, in reference to their corresponding mass loss, before and after their exposure to corrosive conditions Comparing the results of Fig.4, with the corresponding results of Fig.6, where the same steel category- but with different nominal diameter- was used, it seems that specimens with 12mm diameter record lower mass loss percentages. This is owed to the fact that corrosion process, that is conducted in the salt spray chamber, is a method that mainly affects the external surface of the material. In the case of 16mm nominal diameter, the spread of the exposed surface is greater than in the case of 12mm, and it is a fact that results in a greater surface attack. This remark, in combination with the existing knowledge, related to the differential aeration corrosion phenomenon, can sufficiently explain that the rate of the electrons flow- and therefore the corrosion rate- depend on the volume of the exposed material as well as the volume of the protected (not exposed to the aggressive conditions) part of the steel reinforcement.