A Novel Radar-Magnetic Technique for Dielectric Permittivity Mapping of Bridge Decks in Order to Determine Moisture and Salinity of the Concrete

For the assessment of the condition of concrete bridge decks it is impor- tant to know the moisture and the salinity of the concrete. Usually, the salinity is de- termined by potential measurements and is verified by analyzing samples in the laboratory. However, this method requires the removal of the pavement. We devel- oped a novel technique for nondestructive determination of the complex dielectric permittivity of the concrete which operates nondestructively from a wagon running on the pavement. Using a 1 GHz Ground Penetrating Radar, the transient time of the electromagnetic wave to the upper layer reinforcement bars (rebars) is measured. The depth of the rebars is determined by a magnetic measurement of two compo- nents of the static magnetic field of the premagnetized rebars using feedback- linearized GMR magnetometers. This magnetic depth determination technique is in- dependent of the dielectric properties of the materials covering the rebars. Compari- son 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 of the dielectric permittivity is estimated from the reflected Radar amplitude. The moisture and salt content of the concrete is derived from the measured complex di- electric permittivity using interpolated calibration curves. The method was evaluated using a test block of reinforced concrete with water and chloride contaminated nests and an asphalt cover slab, resembling a typical bridge deck. The system was tested on two highway bridge decks. The dielectric constant of the concrete close to the surface was mapped. The advantages and the limitations of the new technique are discussed.