Historic Churches 2019

34 BCD SPECIAL REPORT ON HISTORIC CHURCHES 26 TH ANNUAL EDITION probably of similar strength initially, but the weathering sustained in one instance would have been sufficient to cause the loss or alteration of soluble or reactive components, resulting in the now friable nature of the mortar. This information enables the specification of a mix designed to reduce porosity or alternatively, the use of suitable barriers, such as lead weatherings on wall heads and cornices, to prevent future leaching which would damage repointed joints. This case also illustrates the advantages of using complementary techniques in second phase testing. Specific methods may well provide qualifying information (for example trace indicators of pozzolanic components may still be detectable) where a fuller understanding is needed to inform mix design or other measures to mitigate post restoration weathering. AVAILABILITY OF HISTORIC MATERIALS When trying to match historic materials with modern equivalents, it is important to consider whether they really are equivalent. For example, certain samples were found to contain a pozzolanic component, possibly furnace bottom ash (FBA). However, if sourcing FBA for use in a repair mix, caution is needed as the characteristics and properties of current FBA are likely to differ significantly from that produced in former times. Variables include the type of coal/fuel burnt, contaminants present, burning temperatures/conditions, age and storage conditions prior to use, all of which can impact quality and performance. Matching historic types of lime can be similarly difficult. There will have been variations between limes at the time of construction and this is also the case with modern limes. Care should be taken when cross referencing historic classifications as there is often a marked difference with perceived current counterparts, particularly with hydraulic lime. Modern versions generally give higher (compressive) strengths and are denser in comparison. These issues may have a bearing on mix design. SAMPLE DOCUMENTATION Because background information was communicated prior to analysis, the lab technicians knew the period of the mortar and were better able to tailor the analysis accordingly. It came as no surprise, for example, that some of the samples were confirmed as pozzolan modified mortars as this was consistent with other church restorations carried out in Victorian times, a period where the effects of ash/coal additions would have been known. Reporting was correspondingly more informative. The scope of reporting on samples when information is not forthcoming is inevitably restricted. I would always encourage a more collaborative approach with sharing of information to maximise outcomes. Ideally this philosophy would apply throughout a project right through to the person wielding the hammer and chisel, core drill or pointing trowel. Finally, the integrity and validity of laboratory testing relies, ultimately, on the availability of suitable representative samples of the mortar(s) and a documented sampling plan. Mortar samples should be obtained via suitable methods including club hammer and chisel penetrating well into mortar joints with careful recovery of sections or pieces, removal of whole masonry units and recovery of mortar across whole bedding and perpend joints or dry over- coring. Minimum sample mass for basic analysis is 100g and, for more extensive testing, approximately 250g is required. Samples should be sealed in suitable containers, labelled with location details. Record photographs and site sampling observations are invariably useful and should be included wherever possible. There is much to learn. Perhaps the most important objective of all is being able to conclude a project moving to the next piece of work sound in the knowledge that we have given our collective best. MIKE BARHAM is chief chemist at Kiwa CMT Testing ( mike.barham@kiwa.com) , with a career spanning 45 years, a major part of which has concerned testing of cement-based construction materials and historic lime and gypsum materials. He is part of a working group reviewing procedures for concrete analysis. ACKNOWLEDGMENT: THe author wishes to acknowledge a cumulative debt to Bill Revie for his input and assistance over the years demonstrating, on many occasions, his seemingly inexhaustible knowledge in addition to carrying out key elements of testing. SAMPLE TYPE A: AIR-LIME MORTAR SAMPLE TYPE B: MODIFIED MORTAR BINDER High calcium non-hydraulic lime binders probably in the form of lime putty Lime binder made from a magnesian limestone and mixed in dry hydrate form; large lime inclusions in some samples indicated ‘balling’ of the hydrate during mixing ADDITIVES None Pozzolanic additives including a mixture of ash (possibly furnace bottom ash, FBA) AGGREGATE Fine natural quartz sand with added crushed limestone fragments A small proportion of fine quartz sand and, on occasion limestone, coal and ash Cement pointing which has failed revealing original lime mortar beneath. The weather-beaten window sill of a church in Stirling: the original mortar is not always the most appropriate one. (Photo: Jonathan Taylor)