Inspection of Prestressed Concrete Structural Members with Magnetic Methods

Inspection of Prestressed Concrete Structural Members with Magnetic MethodsThe methods of magnetic stray field measurement and of magnetic remanence measurment are well suited for nondestructive detection of ruptures in prestressing wires of concrete structural members. Using a yoke magnet with integrated magnetic field sensors, the members are scanned several times with varying magnetization field strength. The signal analysis is based on the comparison of measurements after different premagnetizations as well as on the determination of the correlation with idealized rebar and crack signals. The limits of detection are discussed on a case-by-case basis. The application in inspection practice is illustrated with three examples

Radar-Magnet-Betontest - Eine neue Methode zur Bestimmung der Feuchte und des Chloridgehaltes von Brückenfahrbahnplatten aus Beton

For the assessment of the condition of concrete bridge decks it is important to know the moisture and the salinity of the concrete. In this paper, a novel technique for nondestructive determination of moisture and salinity of concrete through the pavement is presented. Using Ground Penetrating Radar, the transient time of the electromagnetic wave to the upper layer reinforcement bars (rebars) is measured. It depends on the dielectric properties of the intermediate materials. The depth of the rebars is determined by a measurement of the static magnetic field of the premagnetized rebars. This magnetic depth determination technique is independent of the dielectric properties of the materials covering the rebars. Comparison of the Radar delay and the magnetically extracted depth yields the averaged real part of the dielectric permittivity of the intermediate material. The imaginary part is estimated from the reflected Radar amplitude. Then, moisture and salt content of the concrete are derived from calibration curves. The method was verified in the lab using concrete test blocks with and without asphalt cover A Radar-magnetic measurement vehicle was developed and tested on two highway bridge decks. The dielectric permittivity of the concrete close to the surface was mapped on a large area, The technique is well suited for quick and nondestructive scans over large areas in order to identify potentially damaged regions. These conspicuous locations should be analyzed in more detail afterwards