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Development and Application of a Portable NMR Moisture Measuring System in One-Sided Access Technique

: Wolter, B.

Kupfer, K. ; Hochschule für Architektur und Bauwesen Weimar -HAB-, Materialforschungs- und -prüfanstalt -MFPA-:
Fourth International Conference on Electromagnetic Wave Interaction with Water and Moist Substances 2001 : Papers, Posters and Exhibits
Weimar, 2001
International Conference on Electromagnetic Wave Interaction with Water and Moist Substances <4, 2001, Weimar>
Fraunhofer IZFP ()
moisture absorption; moisture; one-sided access NMR (OSA NMR); density

Hydrogen Nuclear Magnetic Resonance (¹H-NMR) is known to be an versatile method for accurately determining the content and the binding degree of moisture in solid materials. In industrial applications it is used as a laboratory method for moisture determination in foodstuffs, powders and other bulk materials. As an hindrance of conventional instrumentation, it is not suited for building inspection. During measuring the sample has to placed into the probe interior. Furthermore, conventional NMR equipment is not portable. Even though, the physical principle of the method is nondestructive, in case of large or stationary objects (e.g. building wall), it is necessary to take a sample. By this, appearance or even functionality of the specimen will be damaged.
With a view to use NMR for oil-well logging, respectively for moisture measurements in near-surface soils, already in the early eighties instrumentation have been developed for measuring NMR signals in objects external to the probe. For the first time, this one-sided access (OSA) technology opened up the possibility to use NMR in a completely nondestructive manner. But up to now, the equipment of this technology have been oversized by far, to use it for on-site application on building components.
The testing system "NMR-INSPECT" is the worldwide first completely portable, battery-powered moisture measuring system based on one-sided access nuclear magnetic resonance (OSA-NMR). At present, is the only completely nondestructive method for depth-profiling of the moisture content in one-sided application to the component.
Compared to earlier OSA-NMR technology, the size of the electronic device of this equipment was reduced by more than the factor twenty. This was achieved by using miniaturized highly efficient electronic components. The sensor design was optimized by application of numerical methods for maximizing the ratio between efficiency and weight and by utilizing new magnet materials with highest energy-density. In contrast to prior equipment, "NMR-INSPECT" allows to vary the measuring depth without changing the distance between probe and specimen surface (constant lift-off).
The measuring system determines a depth-profile of the content and the mobility of hydrogen in the material. The hydrogen content is proportional to the moisture content. Based on the molecular mobility of the hydrogen, the binding degree of water can be characterized. By this means, chemically/ physically bonded moisture as well as free moisture can be determined individually. The accuracy of moisture content measurements in porous solids is between 0.3 and 1 mass-%. Monitoring a depth-profile up to 25 mm with a depth-increment of 1 mm will take approximately 30 minutes.
Depth-resolved measuring of the moisture distribution allows to determine the moisture penetration depth. By comparison the moisture profile before and after a defined one-sided wetting, the water permeability of a building material can be determined and by this means, its tightness against pollutant penetration can be evaluated. Since the method is contact-free, it is also applicable in case of components with pronounced surface roughness.
During hardening of cement stone, the binding state of water in concrete is changing continuously, originating a decided effect on the NMR signals. Investigations on different mortar mixtures have shown a clear correlation between NMR measuring signals and the (bending) compressive strength, which was determined with a destructive reference method.