Tension wire break detection

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The clamping wire break detection , also prestressing steel break detection or magnetic field measurement (stray field measurement ), is a non-destructive test in construction . To locate the cracks, the tendon (see prestressed concrete ) is magnetized and the magnetic stray field of the tendon is measured on the concrete surface.

Tension wire of a bridge plate destroyed by de-icing salts
Measuring system for the location of tension wire breaks in prestressed concrete trusses
Mobile measuring system for the location of tension wire breaks in transverse tendons

Problem Description

In prestressed concrete structures , cast-in prestressing steels form the main structural members. These can crack as a result of external influences or as a result of fatigue corrosion or stress corrosion cracking . Tendons are often affected by bridge structures, as de-icing salts can penetrate the concrete and, if the waterproofing is inadequate, the prestressing steel. This also applies to roof trusses of halls if, due to insufficient concrete quality or damage to the concrete, moisture and possibly chlorides can penetrate to the tendons. Pre-stressed precast concrete elements made from high-alumina cement also posed a hazard to tension wires . The use of alumina cement for prestressed concrete was banned in Germany in 1961. However, most of the components in hall roofs, residential buildings and stables that were manufactured up to that point are still present.

The cracks in the tendons of prestressed concrete components should be determined to determine the load-transferring prestressing steels. From this knowledge, a structural engineer can estimate the residual load-bearing capacity of a component. As tension wire breaks are not visible from the outside, opening the concrete only locally and inspecting the tensioning steel visually or checking the magnetic particles locally is not sufficient.

Procedure

A magnetized tendon is comparable to a bar magnet . The north and south poles form at the breakpoints of individual tension wires. The resulting stray field can be measured on the concrete surface (stray field measurement). The strength of this significant signal provides information about the cross-sectional weakening of the tendon. The most important influencing parameters are:

  • The concrete cover
  • The arrangement of the reinforced concrete reinforcement and other steel fixtures and fittings.

execution

The check of prestressed concrete components for tension wire breaks is carried out in the following steps:

  • Localizing the tendon course (e.g. using georadar )
  • Installation of a suitable guide system for the magnets and the measuring probes according to the tendon profile
  • Magnetization of prestressing steels
  • Measurement of the magnetic field of the remanently magnetized tension wires
  • Signal processing
  • Interpretation of the measurement data

literature

  • Bernd Hillemeier : The detection of tension wire breaks on prestressing steels embedded in concrete . Lectures on concrete day, Deutscher Beton-Verein e. V, Wiesbaden, 1993.
  • Bernd Hillemeier: Assessment of Structural Stability of Prestressed Concrete by Non-Destructive Detection of Steel Fractures . Proceedings Vol. 1 of the International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), German Society for Non-Destructive Testing e. V., pp. 23-29, 1995.
  • Horst Scheel: Structural use of the remanence magnetism method: tension wire break location in tendons of a roadway slab of the motorway bridge near Michelsrombach . Report volume DGZfP building diagnosis 1999.
  • Andrei Walther : Fast NDT Localization of Prestressing Steel Fractures in PT Concrete Bridges . Structural Faults and Repairs, Edinburgh 2008
  • Andrei Walther: Locating breaks in the prestressed reinforcement in prestressed concrete components - quickly, innovatively and in high quality . Symposium on Modern Building Inspection - Technical and Legal Aspects, Federal Highway Research Institute, Bergisch Gladbach 2010; DGZfP report volume BB 123-CD.

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