Time domain reflectometry (TDR)

For many years, time domain reflectometry (TDR) is applied as an established method for the determination of moisture and salt content in the subsurface soil. Therefore, it is very promising to modify and apply this technique for the investigation of building materials.

BAM has developed a probe consisting of two parallel rods having each a diameter of 11.5 mm with a distance of about 50 mm. Thus, this probe is much larger than these applied to soil investigations. Cores have to be drilled into the structure under investigation for positioning of the TDR probe. The same cores could be used as for the microwave borehole method. A strong contact of the probe to the embedding material is not necessarily important. If the probe is positioned inside the structure, it is connected with a high frequency coaxial cable to the TDR control unit. Depending on these systems, step impulses with a rise time from 20 to 200 ps are generated. These impulses are reflected at changes of the impedance along the wire, e. g. at the connection between the coaxial cable and the probe or at the end of the probe. In first approximation, the run time of the step impulse along the probe depends on the relative permittivity of the embedding material. From the damping of the signal which is reflected at the end of the probe, the electric conductivity of the material can be determined.

Investigations of moist sand lime bricks showed that with decreasing moisture content the reflection from the end of the probe appears earlier. Measurements performed with the probe inside NaCl solutions with different concentrations of NaCl demonstrated that with increasing electrical conductivity, the intensity of the reflected impulse decreases. Thus, with this method the moisture content as well as the content of solved salt ions can be determined locally. The costs for one of these probes is relatively low. Therefore, it is possible to install these at selected positions for longer times and to perform repeated measurements with the TDR equipment regularly.

A further area of application of TDR is the localisation and characterisation of corrosion of single wires inside tendon ducts. Parallel to the wires a sensor wire has to be built in to achieve a two rod system. Changes of the diameter of the tendon wire implies changes of impedance and thus a characteristic reflection of the microwave impulses, which possibly could be related to corrosion.

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