Moisture investigations in heritage structures – the influence of masonry salt content on the reliability of dielectric measurements

Excessive moisture is one of the most common problems in heritage monumental buildings. This phenomenon is mainly due to the lack of original waterproofing in most buildings and the frequent failure of solutions implemented in later periods. As a result, moisture content measurements are among the most frequently performed diagnostic tests in this type of building. The greatest difficulty remains in the destructive nature of laboratory tests. In the case of the gravimetric method, it is necessary to collect material for testing, which results in the need to interfere with valuable historic substances. For this reason, conservation authorities are reluctant to grant permission for sampling, and even when permission is granted, the number of permissible measurement points is often significantly limited. For the above reasons, indirect, non-destructive methods are commonly used in diagnostic practice. The most common technique is the dielectric method, which does not require interference with the structure of the material being tested. However, it should be emphasised that the presence of salt in the material changes its dielectric properties, which can lead to significant measurement errors. The aim of this article is to determine whether and to what extent the salt content affects the reliability of moisture measurements performed using indirect methods.