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.