Page 375 - Failure Analysis Case Studies II

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Fig. 4. SEM photograph of the fracture surface of a specimen with Corr. d. = 0. x 13

Fig. 5. SEM photograph of a specimen with Corr. d. = 0, showing the tough fibrous zone. x 2000

with the smaller and shallower dimples (those with a low fracture energy), is more rapid in the
brittle regions of the material. These brittle areas were formed due to the effects of stress corrosion
which is now increasingly more attributable to hydrogen embrittlement. Examples of significant
shallow dimples are shown in Fig. 8, whereas Fig. 9 shows a cleavage fracture surface. Such
fractographic analyses clearly show that in the investigated viaduct there exist very brittle wires.
Stress corrosion cracking is connected with the free chlorides, the proportion of which in the
grouting mixture in the direct vicinity of the cables is up to 1.01 %. This is a very high concentration
so that the grouting mixture, with a pH which was found to be within the limits of 11.3 to 11.6, is
unable to provide the necessary passivation of the steel [l, 21. The local depassivation of the
prestressing wire made possible the start of the corrosion process. On the basis of the results of the