Mikrowellensysteme für die in-situ-Feuchtemessung zur Bauwerksüberwachung (English shop)

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Owing to the age structure of buildings and structures in Germany, financial resources of macroeconomically relevant magnitude will have to be raised for their maintenance in the future. In order to deploy these resources as efficiently as possible, continuous monitoring of the condition of a structure is necessary. A key parameter for assessing the condition of a structure is moisture. This thesis therefore investigates non-destructive measurement systems for the in-situ determination of moisture.

First, potential measurement methods are presented. Due to their low cross-sensitivity to the ionic conductivity inherent in concrete, methods in the frequency range between 2.0 GHz and 3.5 GHz are used. Furthermore, the measurement environment is described and the resulting requirements for measurement systems are derived. The measurement systems consist of sensors and evaluation units.

Starting from the state of the art, several sensors are investigated under identical boundary conditions and systematically compared with one another. The main comparison parameters are moisture sensitivity and temperature cross-sensitivity, integrability into measurement systems, and reproducible manufacturability. Resonators that can be easily produced lithographically show a particularly high moisture sensitivity, can be readily integrated into measurement systems and are therefore used. In order to enable density-independent moisture measurements, moisture-sensitive substitute materials made of porous ceramics are employed. The properties of different ceramics are investigated. It is shown that hysteresis-free moisture determination is fundamentally possible.

The realisation of two different evaluation units and their integration with suitable sensors to form measurement systems is described. The first system investigated is a compact vectorial network analyser with an integrated self-calibration network. The second evaluation unit is based on an active sensor whose oscillation frequency is a function of moisture. For determining this frequency, a frequency counter and a novel frequency comparator are presented. Due to the advantages of the comparator, it is combined with the active sensor to form a self-oscillating measurement system. Compared with the network analyser, this system shows clear advantages in terms of size, circuit complexity, cost, manufacturability, energy requirements and measurement accuracy, and is therefore used exclusively for the further investigations.

The suitability of the self-oscillating system is verified under realistic conditions. For this purpose, the cross-sensitivity of the system to the ionic conductivity inherent in concrete is first investigated. In a further step, a two-stage moisture calibration is carried out. This calibration is applied in the subsequent investigations under realistic conditions. Here, several self-oscillating measurement systems are cast into both hardened cement paste and concretes of different compositions. The drying during the curing period and after formwork removal as well as the onset of moistening are monitored. The results essentially correspond to the expectations derived from the fundamental considerations and lie within the system accuracy determined.

The thesis concludes with a summary and an outlook.