Technology

The technological method: Magnetic stray field measurement

Magnetic stray field measurement, also known as remanent magnetism method and prestressing steel crack detection, is a non-destructive testing method used to determine fractures, cracks or pronounced corrosion areas in concrete and prestressing steels in concrete structures.

The method allows a fast, efficient and reliable investigation. With prestressing steels, it can be carried out both in immediate bonding (precast prestressed concrete) and in subsequent bonding (in pressed clad pipes) without the construction having to be opened locally in a destructive manner. Our testing and analysis systems independently detect the position of the reinforcing and prestressing steel during the measuring process so that their prior detection is not necessary.

As a result, a current condition analysis of the tested tendons is available. Depending on the respective influencing parameters, it is possible to detect cracks or fractures in tendons or tendon wires even with a cross section reduction of about 50% of the total cross section.

In normal cases, corrosion and damage to structural components only occur locally. The exact detection of the damage to tendons makes it easier to decide whether repair work is necessary and to plan precise intervention in the structure.

Numerous scientific investigations, such as five dissertations at the Technical University of Berlin (TU Berlin), and many years of experience in practical measurement have been incorporated into the development of this procedure. 

The technological method used in our testing and analysis systems was already awarded the Innovation Prize of the Länder Berlin-Brandenburg in 2006. Since then, it has been continuously further developed and has been used successfully both nationally and internationally. There is still no comparable method known in the world that can provide such a high degree of accuracy and significance with such a fast response on the condition of reinforcement steels.

Previous successful fields of application are:

• Roadway slabs of bridge structures (road and highway bridges),
  
• Prestressed concrete girders of bridge structures,
  
• Floor slabs of parking lots (parking decks),
  
• Prestressed concrete girder for hall structures (swimming, sports and industrial halls) and storey ceiling,
  
• Prefabricated trapezoidal corrugated girder for hall roofs,
  
• Tank container in prestressed concrete structure.

The measuring principle

The magnetic stray field measurement uses the ferromagnetic properties of the steel, which is characterized by an undefined impressed magnetic field. In order to obtain a defined magnetic field necessary for the examination, the steel to be examined is magnetized from the component surface. Magnetic stray fields occur at the crack points of individual tension wires and are comparable to stray fields at broken bar magnets (dipole formation at the crack point).

Principle of magnetic stray field measurement for detecting the crack points on steel wires

The strength of the signal received allows a statement to be made about the weakening of the steel cross section. The measurement result is influenced, among other things, by

• the concrete covering (up to 25 cm),
• the arrangement and degree of reinforced concrete reinforcement,
• steel built-in components as well as
• metal-clad sealing membranes.

Source:

Chol-I Pak, "Optimization of the remanent magnetism method for steel crack detection in buildings - Theoretical further development", Dissertation, TU Berlin

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