What is stress corrosion of prestressing steel

Verification of the risk of stress corrosion cracking (SCC) from Hennigsdorfer prestressing steel for the production period to 1993

Table of contents

      1. Project data
      2. Report in the yearbook 2010

Project data

Report in the yearbook 2010

Tests on Older Prestressing Bars

Over 70 years ago, the first prestressed concrete bridge in Germany was built in Aue. In the mean time the prestressed concrete has developed to a robust and highly efficient construction method. Yet there were also repeatedly cases of damage due to stress corrosion cracking (SCC). The main damage reasons were (1) the use of prestressing steel that was especially sensitive for this particular type of corrosion and (2) shortcomings in construction, often because moisture could freely access the prestressing steel.

Stress corrosion cracking takes place in three phases: preliminary corrosion processes and crack initiation, crack propagation and failure. The process can take a few days, but also many years. SSC results typically barely corrosion products. Particularly critical is that the steel with SCC can fail suddenly and extremely brittle. For prestressed concrete structures, this means that an imminent failure would not announce by previous cracking.

For two of the previously used quenched and tempered steel types, the susceptibility to this type of corrosion has long been known. Therefore, this steel has not been manufactured since many years. Whether the prestressing bars from the steel mill Hennigsdorf (former GDR) is at risk also is the subject of ongoing investigations. In this project, material tests are carried out on prestressed steel from different manufacturers, but predominantly from Hennigsdorfer production from different production years in order to gain knowledge about the entire production period to the early 1990s.

The samples are usually won in bridge break offs from various bridge types. First, single large specimens are separated from the structure. Afterwards the parameters of the concrete are determined, such as strength or carbonation depth. Thereafter, the ducts are exposed and evaluated visually. The injection mortar is also first examined visually, and then follows the analysis of chemical composition. Finally, some tension rods are extracted. These will be investigated with regard to existing cracks before the tensile strength and the stress-strain curves are determined. So-called FIP tests on the MFPA Leipzig - that are longtime tension tests in a highly aggressive medium - will also provide parameters on the susceptibility of prestressing steels.