Durability of concrete and cement composites

Test method with electrolyte solution representative of actual service DIBt test

Various failures of prestressed concrete structures caused by hydrogen-induced stress corrosion cracking at the beginning of the 1980s led to the development of a constant load test based on laboratory as well as on-site investigations.20,60 In  this test, the prestressing steel specimens as delivered were stressed in a stressing frame under constant strain to 0.8 Rm, at 50ëC. Based on the results from chemical analyses of on-site samples of water taken from prestressing ducts, the electrolyte solution for the test contained 0.014 mol/l chloride, 0.052 mol/l sulfate and 0.017 mol/l thiocyanate (pH ˆ 7.0). The SCNÿ ions promote hydrogen uptake and therefore made conditions more severe than under normal conditions. As noted previously, thiocyanate ions have been found in some cases where damage had occurred,6 and were shown to have originated from concrete admixtures (in which SCN± is no longer permitted).

During a test period of 2000 h, a measurable amount of hydrogen was absorbed by the specimens and led to embrittlement. Specimens made of susceptible materials failed before 2000 h was reached. Compared to tests in concentrated thiocyanate or nitrate solutions, this test has the advantage that the decisive criterion is the occurrence or non-occurrence of a fracture within a certain time period, rather than within a relatively short failure time. Further advantages arise from the fact that the test solution closely simulates corrosion conditions around the prestressing steels within the ducts under construction conditions, and that the results obtained with this test for different prestressing steels are reproducible and correspond to practical experience.

In comparison with the FIP test, slightly higher initial and then nearly constant low hydrogen activity at the prestressing steel surface is required to ensure reproducible results. This has been proved by permeation measure- ments.20,60,61 The higher the pH of the solution, the lower the general corrosion rate. The more favourable conditions for surface layer formation result in a longer testing time. However, with this test the results can be better extrapolated to the behaviour of prestressing steels in practice. Experiments using what is now known as the DIBt test have been carried out successfully for a wide range of steels.

This method has been described by Grimme et al.20 It was found (Fig. 6.15) that all approved prestressing steels on the market passed the time limit of 2000 h without fracture, while steels known to be susceptible to SCC, from cases of reported damage or known not to be suitable for construction practice (St 1080/ 1320, bainitic microstructure3), failed within 2000 h due to hydrogen-induced SCC. A criterion based on time to failure could thus be defined that appeared valid for all types of prestressing steels tested.

Applying the DIBt test to prestressing steels that had failed due to SCC after long-term use in actual structures, showed that this test would have classified them as susceptible and hence not approved, supporting the suitability of this method. Results23,64 showed that annealed prestressing steels of the `old type’ (i.e. without Cr-alloying and manufactured prior to 1965) and also comparable steels from the former GDR do not pass the test and should be classified as `not approvable’ according to today’s standards.

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