Absolute Moisture Determination in Building Materials with Single-Sided NMR

Abstract

Moisture and moisture related processes are a main damage cause to building materials and
infrastructure. In Germany alone, the annual damage due to moisture exceeds several 10
billion €/a. However, common non-destructive measurement devices cannot reliably
determine the absolute moisture content due to surface effects, cross influences and complex
material dependences. Although Nuclear Magnetic Resonance (NMR) measurements are
commonly used to investigate the moisture content, distribution and transport in building
materials, it is usually treated as a laboratory method only. With the development of unilateral
devices, which allow to investigate extended samples, NMR becomes increasingly
interesting for field application.
The biggest challenge concerning building materials are the very fast T2 relaxation
times (10μs-100ms) and a complex T2 distribution. Additionally, unilateral NMR devices
usually suffer from poor SNR. This in combination with the demand to detect moisture
contents down to 0.5% of dry mass, makes the estimation of the initial amplitude very
inaccurate. Therefore, we are left to use the attenuated signals with moisture dependent
relaxation times resulting in non-linear and material dependent correlation curves between
the NMR signal and moisture content. We present a method to determine such correlation
curves based on a single T2 measurement close to saturation. This yields a substantial saving
of time (upto several weeks) compared to a manual, empiric determination.
When using a capillary bundle model and the Young-Laplace equation to describe
the drying behavior of porous media, larger pores are emptied before smaller ones. Since the
T2 distribution reflects the pore size distribution of the material, we introduce a moisture
dependent cut-off radius, respectively relaxation time. Subsequently, the expected relaxation
decay at a certain moisture content can be reconstructed.
Our results show a good agreement between the predicted and experimentally
determined correlation curves for different building materials. Our method is easy to use and
allows a fast build-up of a material database. Together with the technology of single-sided
NMR this could be a vital step towards a reliable and consumer suitable moisture measuring
tool.