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Control and evaluation of the corrosion protection of post-tensionned tendons (AGB1998/105)

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Background & Policy context

Ducts for bonded post-tensioning tendons have traditionally been made from steel strips with a special corrugation. Despite a generally good durability with increasing age of the structures damages due to corrosion of the high-strength steel occur. Unfortunately, no established method exists to locate the areas of damage in an early state. Corrugated plastic ducts have been used for ground anchors in the bond length. Between 1968 and 1974 corrugated black poyethylene ducts have been installed in Switzerland and no deterioration has been observed. Only recently thick walled plastic ducts become increasingly popular for post - tensionning tendons. They present attractive features as increased fretting fatigue resistance and the feasability for electrical monitoring of the tendons.
Measuring the electrical resistance between the prestressing and the reinforcing steel can indicate the absence of defects and thus corrosion protection.

The influence of the measuring frequency and thus the capacitance of the plastic duct on the results is studied on model systems with different defect size. The laboratory results are checked on reals structures with fully isolated post-tensioned tendons.


Define better the limiting values of the electrical resistance (better impedance) of a corrugated plastic duct in post-tensioned tendons.


Laboratory experiments and numerical modelling.


Parent Programmes
Institution Type
Public institution
Institution Name
Swiss Government: State Secretariat for Education and Research
Type of funding
Public (national/regional/local)


  • Measurements can be performed only with impedance instruments (LCR meter), simple electrical resistance measurements are not suitable.
  • limiting value for the leak-tightness of the duct and thus for the electrical isolation of the tendons with durable corrosion protection depends on the diameter of the duct and on the length of the tenton. A specific value of 500 kilo Ohm for a duct with 59 mm has been found.
  • The loss factor for a perfectly intact duct was found to be D = 0.023. A value less than 0.1 can be considered sufficient when taking into account very small leakages.

The ohmic resistance measured between the prestressing steel in the tendon and the normal rebar network increases asymptotically with time due to the hydration and drying out of both grout and concrete. A decrease of the ohmic resistance allows to detect in a very early stage the ingress of water at a defect in the duct - thus the non-destructive monitoring of the corrosion protection of the tendons is possible for the first time.


Lead Organisation
EU Contribution
Partner Organisations
EU Contribution


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