ABSTRACT
Bridge closures (Roosevelt Bridge USA 2020) and bridge collapses (Morandi Bridge Italy 2019, Lake View Drive Bridge USA 2005) increase awareness of the problem of prestressed concrete structures. In what condition is the prestressing steel? Locating fractures of prestressed steel in prestressed concrete structures as early as possible and non-destructively is an increasingly important task due to the natural aging of prestressing steel. Non-destructive investigation methods are available which, in the case of existing structures, open up a view of the interior of the supporting structure with the help of magnetic fields. They show the condition of the prestressed reinforcement in the picture document. The magnetic leakage flux method (MFL) makes it possible to estimate whether corrosion is developing. Depending on the concrete cover of up to 30 cm and the number and density of the reinforced concrete reinforcement, electromagnets, permanent magnets and experience with measurements in both active and remanent magnetic fields are available. The article shows application cases for the various types of magnetization and measurement, from small magnets for tension wires close to the surface to a self-propelled 3.50 m wide and 30 KN heavy magnet for driving on lanes on bridges to measure the tension wire situation in metallic ducts. In Germany, the method is so well developed that the DGZfP published the position paper “Magnetic methods for prestressing steel fracture location” in 2017 (DGZfP 2017).
