40 THE BUILDING CONSERVATION DIRECTORY 2024 CATHEDRAL COMMUNICATIONS Decisions regarding drying strategy should take this into account. Where possible, some additional samples from the masonry units should be taken to give a broader understanding of the wall as a whole. The quantity of material collected must be optimised: a minimum of two grams is needed for a sample to be representative of a sufficient volume of masonry while excessively large samples will extend equilibration time when assessing hygroscopic moisture content. Sampling can provide benchmarks for quasi-qualitative/relative measurements if corresponding measurements of the masonry are recorded with electrical meters at sampling locations, for example, recording capacitance and microwave measurements at locations where surface and deep samples are subsequently extracted. This can serve as project specific calibration for the electric meters, enabling monitoring of drying or wetting trends of the wall by relative measurements thereafter, provided the sampling covers a suitable range of moisture contents from dry to wet. Assessment by gravimetry In the laboratory, the MC of the samples is commonly quantified gravimetrically using the oven drying method. The initial mass of the sample is determined by weighing on a balance with a resolution of at least 0.1mg before the sample is then dried to a constant weight (when the difference between consecutive measurements taken at four-hour intervals is less than 0.5%) in an oven and re-weighed. Oven temperature is generally set at 103±2°C for expediency, but for materials with chemically bound moisture (such as gypsum and cement) temperature should be reduced to 60°C to avoid over-estimating the moisture content. The gravimetric MC is expressed as a percentage ratio of the mass of water lost during drying. It can be calculated in two ways, giving broadly similar results: • as a ‘dry weight basis’ – the mass of water divided by the mass of the dry sample (preferred by international standards) • as a ‘wet weight basis’ – the mass of water divided by the initial weight of the sample (recommended in BRE Digest 245 and the most common method in the UK). The two ways of expressing moisture content give broadly similar results and are convertible but not equivalent: MC calculated on the dry basis will always be slightly higher than on the wet basis, so it is important to state which method was used when reporting. Expressing moisture content by weight does not permit direct comparison between materials with different densities. To enable cross-comparison between materials, volumetric MC should be calculated, which requires the bulk densities of the various components of the wall to be known. Very rough estimates are possible on the basis of material type: typical bulk densities include mortar 1500–1900kg/m3 (with lime mortar at c 1700kg/m3), stone at 1900–2500kg/m3, and plaster at 900kg/m3. The different capacities for moisture retention between masonry materials also prevent direct comparison, for example, MC at saturation (open porosity fully filled with water) can vary between 4% and 25% between different brick types. BRE Digest 245 recommends an additional stage of investigation for which dried samples are conditioned at a relative humidity of 75% RH until they reach constant weight, which gives a hygroscopic MC when re-weighed. (75% RH represents an extreme value for occupied buildings but is simple to achieve with a saturated solution of table salt.) This step was intended as a means of estimating salt contamination of the masonry (and thus as a way of evaluating the contribution of groundwater to damp), but it can provide other useful information. Excessively high hygroscopic MC (greater than 2%) suggests high salt contamination and an associated risk of damage from salt crystallisation cycles. For samples with a lower hygroscopic MC, the measure can be used as an indication of the combined moisture storage characteristics of the material. By comparing it with the sample’s total moisture content, it is possible to put a figure on the proportion of moisture which is readily mobile within the pore structure, and therefore associated with water damage to internal finishes. The technique thereby allows MC by weight to be broadly characterised into nonmaterial specific bands of dry, wet and saturated on the basis of the available moisture not bound into the material. Assessment by calcium carbide testing Calcium carbide testing is a quicker alternative to gravimetric sampling for which the analysis (total moisture content only) is possible on site, but a sample size of around 8g is necessary. Specialised equipment measures the pressure within a sealed vessel of acetylene gas given off as calcium carbide reacts with water in drill dust or a pulverised sample. This is directly convertible to moisture content with the use of tables. Despite this being a relative measurement, the results are known to be very accurate. Carbide meters do tend to underestimate the MC of cementitious materials (since they do not fully account for hygroscopic MC in fine pore structures), but the method avoids the overestimation likely for cement if oven temperatures are too high. To conclude, while it might be argued that quantitative measurement techniques are generally under-used when assessing damp in masonry; in most cases intuition and experience will be enough to demonstrate the nature of any remedial work required. However, there are times when a more detailed understanding is necessary, requiring a combination of different measurement techniques to be employed. A further article in the next edition of the BCD will examine the practicality of recent developments in the field of moisture content monitoring (such as time domain reflectometry or TDR) and the use of embedded probes. Recommended Reading BRE, 2007. Digest 245 : Rising damp in walls. Diagnosis and treatment. BRE, 2002. Assessing moisture in building materials (Good Repair Guide 33 Parts 1–3). BRE BSI, 2017. BS EN 16682:2017 – Conservation of cultural heritage. Methods of measurement of moisture content, or water content, in materials constituting immovable cultural heritage. BSI, 2012. BS EN 16085:2012 – Conservation of Cultural property. Methodology for sampling from materials of cultural property. General rules Dill, M, 2000. A review of testing for moisture in building elements (errata 2000) (No. C538). CIRIA. Orr, S for Historic Scotland, 2021. Moisture measurement in the historic environment – Historic Environment Scotland Technical Paper 35 Pender, R, 2016. “Water in permeable building materials”, Building Conservation Directory Cathedral Communications Phillipson, M, Baker, P, Davies, M, Ye, Z, Galbraith, G, McNaughtan, A, McLean, R, 2007. “Moisture measurement in building materials: an overview of current methods and new approaches” Building Services Engineering Research and Technology 28,4 (2007) pp. 303–316 Wilhelm, K., Viles, H., Burke, Ò., 2016. “The Influence of Salt on Handheld Electrical Moisture Meters: Can They Be Used to Detect Salt Problems in Porous Stone?” International Journal of Architectural Heritage 10, 735–748. https://doi.org/ 10.1080/15583058.2015.1109733 Viles, H, Zhang, H & Orr, S. for Historic England, 2022: A Comparative Evaluation of Methods to Monitor Moisture in Historic Porous Masonry Materials – Historic England Research Report Series no. 2/2022 MATTHEW WELLESLEY-SMITH is a building surveyor at Hutton + Rostron Environmental Investigations Ltd (see page 32). A surveyor undertaking gravimetric sampling
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