Moisture is a main cause for weathering effects in historical façades made of natural stones. If applied correctly, hydrophobic agents can successfully protect these façades without altering the appearance. The major challenge is to ensure the required surface penetration. In-situ measurements for quality assurance on historical façades are still mainly done destructively through the extraction of core samples or superficially through the measurement of water ingress. An alternative approach has been found in nuclear magnetic resonance, which is able to visualize pore water. Applying this, single-sided NMR sensors provide a non-destructive tool for use at the building site. For this paper, four sandstones of different porosity and one limestone have been treated with two hydrophobic agents of different composition. The ingress of water in the treated samples has been measured by the single-sided sensor using three criteria for quality assessment, namely the hydrophobic depth, the hydrophobic efficiency, and the water absorption coefficient. The results are compared to those of common quality assurance measurements: the visual assessment and the gravimetrical water ingress on core samples as well as the superficial Karsten tube penetration test. It is shown that the single-sided NMR sensor provides a tool for the non-destructive quality evaluation of hydrophobic treatments, which is generally applicable to the building site using the quality criteria now found.
Keine, S., Holthausen, R. S., & Raupach, M. (2019). Single-sided NMR as a non-destructive method for quality evaluation of hydrophobic treatments on natural stones. Journal of Cultural Heritage, 36, 128–134. https://doi.org/10.1016/j.culher.2018.07.012