Non-destructive microwave techniques for the quantification and elimination of moisture in cultural heritage monuments

In this paper, we present our experimental methodology, which was developed to assess and eliminate the excess moisture levels in the masonry of a Roman archeological monument, namely the Mensa Ponderaria of Tivoli, which is an important landmark dating back to the first century A.D. The need to keep interventions to a minimum were crucial factors for choosing microwave techniques as a non-destructive method for removal and moisture infiltration measurements in the walls. We used a microwave generator connected to a horn antenna to increase the temperature locally at the point of treatment and achieve the elimination of moisture. Additionally, to quantify the amount of eliminated water, we used a free space transmission measurement by two patch micro-strip fed antennas with a resonance at 2.62 GHz connected to a vector network analyser (VNA). S-parameters were measured at a frequency range 2–3 GHz. The phase shift method was used to extract the effective dielectric permittivity by the phase of the transmission S21 parameter. In the final step, we derived the volumetric water content of the wall θ (%) based on the dielectric permittivity of the wall material (tuff). Measurements were performed at several points of the masonry, which were previously treated with microwaves or remained without treatment for comparison. We report an average of 139 gr in water extraction from the treated areas of the masonry with the mild treatment protocol we used, with the possibility of further increase with longer or higher-power drying cycles.