The Building Conservation Directory 2024

39 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 PROFESSIONAL SERVICES 1 elevated readings for contaminated masonry even when it is relatively dry. Another problem is the difficulty of inserting pins into most masonry materials and thus high sensitivity to surface moisture from condensation. By taking multiple readings, meters can be used to reveal variations in the surface distribution of moisture and salt, but a high reading is meaningless without additional context. In relation to assessing masonry damp, resistance meters are most useful for: • preliminary scoping of damp distribution, by measuring the MC of embedded timbers or other timber in close contact with the wall (such as skirting boards); • monitoring wetting and drying trends via embedded dowels. Capacitance meters Capacitance meters work by inducing an electric field in the material and then measuring impedance (how the material within the field affects the field strength). Impedance is proportional to the dielectric constant, a property of materials which is heavily dependent on MC. The depth of penetration of the electrical field is limited: a maximum depth of 20mm may be claimed, but it is usually less. In many cases, readings will be of the plaster layer only, not of the underlying masonry, and results will be affected by the interface between plaster and masonry due to the variation in density and potential presence of voids. Small variations in plaster thickness can give misleading measurements. The angle at which the meter is applied to the wall and the roughness of the surface can have a significant impact on results. Microwave meters Microwave meters measure the same electrical properties as capacitance but via higher wave frequencies which increases depth penetration and reduces the influence of salts on measurements. Originally developed for assessing concrete, they are most accurate for homogenous materials of a substantial thickness which have a flat surface. Handheld microwave meters assess the attenuation of an electrical field projected in from the surface to a depth of between 10 to 300mm. Relative values are recorded as a weighted average of the measurement field, with the masonry nearest the plate accorded the greatest impact on the reading. Each type of microwave probe has a minimum wall thickness below which the energy being reflected at the opposite surface will distort the results. Like capacitance, microwave measurement can also give misleading results for masonry with a rough surface. Voids in a wall will strongly affect results (slightly blown plaster will give artificially high readings), as will walls with layered materials with differing MCs, such as brick and mortar. The impact of density also means that microwave readings cannot be compared across different materials. QUANTITATIVE MEASUREMENT Determining absolute moisture content involves measuring the quantity of water in a sample, and therefore requires invasive sampling of the masonry. The number of samples needed and their locations depend on the aim of the investigation. Preliminary qualitative and quasi-quantitative assessment can be used to guide the strategy. Where the intention is to help pinpoint an elusive moisture source, samples are often taken at different heights above floor level and at regular intervals across a wall. They can also be taken from various depths in the walls to provide a profile of moisture content through the wall, which is helpful for determining the contribution of condensation to a damp problem. Repeat sampling can be employed to monitor a trend in drying/wetting over time. It is important to remain aware that sampling gives snapshots of moisture content in the wall at the time of sampling, not a complete picture. This is especially relevant following an acute event like flooding or a temporarily blocked hopper: the moisture content is likely to vary substantially within a relatively small area, according to the routes the water took into and through the structure (via minor cracks, interfaces and voids) and to the properties of the material associated with initial water absorption, such as the proportion of large pores. These factors can vary significantly across the masonry, not only between blocks and mortar, but from stone to stone and brick to brick resulting in localised areas of saturated masonry in the midst of relatively drier areas. Drying times defined on the basis of the worst case measurement from such a wall is likely to be an overestimate; for this reason alone, it is advantageous to combine measurement techniques. Drilled samples The least destructive means of sampling is by drilling into the internal surface using a 9 to 13mm masonry bit, and capturing the drilling dust in a sealed container which can then be transferred to the laboratory for analysis. At least two drill bits should be used to avoid overheating (and the consequent evaporation of water in the sample) and also to prevent cross-contamination of samples between wetter and drier areas (residues of damp dust will adhere to the bit). Sampling should be at a consistent depth across all locations to enable valid cross-comparison. Samples should be as representative as possible of the material at the chosen depth band. However, the effect of the very property being assessed can skew the results: drier masonry is generally easier to drill and is more readily expelled from drill holes (as dust) than wetter material (granular, through paste to slurry) so where taking a sample from a substantial wall thickness the extracted material can include a disproportionate quantity of the drier material. To minimise damage, samples are usually taken from the mortar if visible. However, this may exaggerate overall moisture levels in stone walls as the mortar will have a greater moisture content than the stone and accounts for a smaller volume of the wall. Results of non-invasive moisture and salt mapping using relative electric measuring techniques