Historic Churches 2019

32 BCD SPECIAL REPORT ON HISTORIC CHURCHES 26 TH ANNUAL EDITION MORTAR ANALYSIS A view from the lab Mike Barham I N THE field of conservation the importance of materials analysis is well established. Identifying the composition of heritage building materials informs sympathetic repair and restoration of historic buildings and avoids damage arising from the use of incompatible materials. The necessary degree of investigation varies depending on the type and scale of a project, but laboratory testing is frequently involved. For the assessment of the mortars used in the past, several laboratory methodologies are available, and some of the more common ones are outlined in the table below. The essential question is, precisely what level of information is required from the testing? Answers to this question allow a planned approach and scheduling of appropriate procedures. Achieving the requisite level of information often involves a combination of complementary techniques balanced by a pragmatic, cost effective approach when tailoring laboratory testing to objectives. OBJECTIVES Testing is often required to inform ‘replication’ of the original mortar for repair or restoration works. Given suitable samples and supporting information, laboratory testing will indeed yield pertinent information but, use of this as a basis for a mix design is beset with pitfalls and difficulties. Any analytical results will relate only to the mortar sample(s) at the specific time of the testing undertaken, and there is no guarantee that the samples represent the mortar as a whole, or its condition at any particular time in its service life. The fact is that, in many cases, the current condition of the mortar may well differ significantly from that mixed and placed in the original construction or used for past repairs. Certain factors may have impacted the mortar over time, from changes in weathering/exposure conditions to leaching and depletion of lime binder with re-deposition elsewhere in the mortar fabric and the effects of adverse chemical reactions arising from environmental or other circumstances. It is mostly a question of ‘best endeavour’ using test data in conjunction with other facts and consideration of wider issues. Where a mortar has failed, replicating the problem would be pointless. In this case the objective should be to identify COMMON LABORATORY ANALYSIS TECHNIQUES METHODOLOGY INFORMATION YIELD Initial visual examination + basic indicative testing Extent of carbonation/reversion (indication of whether air lime has fully cured), assessment of colour, indication of porosity X-ray diffraction (XRD) + supportive chemical analysis Confirmation of binder (eg lime or cement); – if lime, its type (air lime, natural hydraulic lime, lime modified with additions to impart ‘chemical set’ or improve hardness), calculation of indicative mix proportions, identification of contaminants, reaction products etc Aggregate separation / recovery / examination and particle size distribution (PSD) via sieve analysis Details of basic aggregate type(s) and ‘grading envelope’ Petrographic examination by thin-section microscopy Detailed confirmation of aggregate composition, overall binder condition and distribution, detailed information relating to effects of weathering and/or adverse chemical reactions, confirmation of the form of lime binder, presence and type of pozzolanic components. Also, where relevant, information enabling correcting/refining of calculated mix proportions (avoiding potential over-estimation of binder content) Biological testing (where plasters and mortars contain animal hair) Confirmation of the presence/absence of ‘active’ anthrax Mortar investigations of some form are always required prior to carrying out masonry repairs or repointing (All photos: Mike Barham Kiwa CMT Testing unless otherwise stated)