Water Content Inspection in Timber Structures by Microwave Reflectometric Method

A non-destructive technique (NDT) based on microwave reflectometry has been applied to perform subsurface investigation on architectural structures like masonries and timber members. The ability of the approach has been proven to locate subsurface defects in the deep up to 10-20 cm into the host material. The recognized defects are of structural nature as cavities of air inside masonry or into wooden beam, detachments of plaster, or presence of buried metal objects. In this contribution we are investigated the effectiveness of the method to detect the presence of humidity into wooden artworks, in particular timber members. The measuring system consists of a vector network analyzer and a wide band antenna (ridged antenna) operating in the range 1.5-6 GHz. A model based on TEM approach is determined to evaluate constraints on the detectability of high humidity areas inside wood, as well as its size (maximum resolution) and its depth inside the medium's (maximum non un-ambiguous range). The reflection coefficient, measured in the frequency domain in amplitude and in phase, depends on permittivity of the material under investigation. In the case of wood the permittivity variations are affected from the presence of variations of density (i.e. knots and caries) and from the presence of humidity. The equivalent time-domain response is reconstructed using an Inverse Fourier Transform of the baseband signal, this approach allows the locating of defects in depth. The size of the area subject to defects is estimated by scanning the surface - step by step - along lines, the lines are properly selected after a preliminary visual inspection. The method was tested in laboratory on samples properly realized to model a lamellar timber structure and to simulate the decay due to water infiltration. The method was applied together other techniques for the structural health assessment of the timber roof in the Giotto's bell tower, Florence.