Analysis of the Condition of Tendons in Prestressed Concrete Structures
Authors:   Hillemeier & Knapp GmbH, Hillemeier Bernd ,   Hillemeier & Knapp GmbH, Luther Thomas ,   Hillemeier & Knapp GmbH, Simon Knapp 
Published: January 2023

Prestressing Steel Fracture Detection

Bernd Hillemeier, Simon Knapp, Thomas Luther

Abstract

The earliest possible non-destructive location of a prestressing steel fracture is a challenge of our time. The endangerment of prestressing steels through stress cor-rosion cracking and hydrogen embrittlement is not visible from the outside.
The estimated service life of existing prestressed concrete structures must take into account the construction plans as well as the construction year. The design criterion "crack before break" did not always have priority.
Magnetic fields open up a view of the interior of the supporting structure. They show the condition of the prestressed reinforcement in a picture document. The method is practicable and has already developed so far that the DGZfP published the position paper „Magnetische Verfahren zur Spannstahlbruchortung" ("Magnetic methods for prestressing steel fracture location") in 2017.
The know-how of prestressing steel fracture detection is to imprint magnetic fields into the prestressed reinforcement. This happens with electromagnets or permanent magnets. The smaller and more manageable the devices, the less their depth effect. The specific advantage of those devices lies in measuring prestressed concrete gird-ers with direct bond and low concrete cover. How do you guide the magnets, how do you interpret the data? Transverse tendons in carriageway slabs are tested at pe-destrian speed to a width of 3.5 m with a 3 t heavy self-propelled electromagnet, preferably at night due to traffic conditions. Despite the fact that our many years of practical experience from Germany, Switzerland, GreatBritain and the USA lead to the constant further development of the process steps, more and more specific measuring tasks are carried out more practically and economically.
shows that the probability of failure without early announcement is low. The answer to the question of whether there are cracks in tendons and, if so, in what number and how they are spatially distributed, significantly increases the reliability of the in-formation concerning safety. The non-destructive testing method based on rema-nence magnetism localizes fractures in prestressing steels in a direct bond or in ducts.
The presentation will show examples from the areas of prestressed concrete girders of bridge structures, carriageway slabs of road and motorway bridges with a tested area of around 4000 m² per day, floor slabs of parking garages, prestressed con-crete girders in swimming, sports and industrial halls, VT folds of hall structures and Tank container in prestressed concrete construction.
Nevertheless, if the complexity of a structure with regard to the arrangement and concentration of the mild and prestressed concrete reinforcement is great, the labor-atory test with the simulated reinforcement situation is recommended. In the case of the shell roof of the town hall in Biel (Switzerland), in addition to the laboratory repro-duced structure in Berlin, the client manufactured another test element for experi-mental verification. The presentation also reports on this.