Evaluation of historic masonry: Towards greater objectivity and efficiency

Internationally, historic masonry buildings constitute a significant proportion of the traditional built environment. In the UK alone, there are 450,000 listed buildings and 10.6 million pre-1945 structures, a high proportion of which have been constructed in traditional masonry. The value of the historic built environment is significant and as such it requires regular, well-considered and cost-effective survey and maintenance.
Survey for the sensitive repair of these buildings has generally been undertaken adopting visual methods. Whilst this is an important mechanism for determining construction characteristics, associated materials, condition, and subsequently the scope and cost of repairs, it has been shown to have limitations. Survey subjectivity has presented itself as an issue of concern and variability in evaluation is the starting point for cumulative errors that lead to unnecessary, costly, and philosophically contentious fabric interventions. Survey accuracy also requires adequate access, which is not always easy to facilitate and is a significant cost which can ultimately reduce frequency and scope of inspection. Scaffolding in particular represents a large portion of survey costs; and it may also cause irreversible physical damage to the masonry substrates, if tied into them. In addition, working at height can be dangerous for operatives during erection and operation.
Innovation in survey technology, now often referred to as reality capture technology, is prevalent. Reality capture technologies, including ground-based and aerial laser scanning and photogrammetry, promise more comprehensive and accurate data collection. This should ultimately support greater levels of objectivity in determination of fabric condition, in addition to making data acquisition safer, faster and more cost-effective. This chapter builds a case for the deployment of these technologies to support building surveying and maintenance decision making, with focus on historic masonry. The arguments particularly resonate when considering pro-active maintenance strategies that rely upon accurate, coherent information, as well as Building Information Modelling (BIM) for the organization and management of these data. The chapter also highlights the emerging awareness of restrictions encountered in the use of these modern survey technologies, that is: the extensive time and cost of processing the acquired data to extract information of ultimate practical value to surveyors. To show that these constraints can be alleviated even in complex situations, preliminary results are reported on the development of a novel system for analysing three-dimensional data from rubble stonework (acquired by laser scanning and photogrammetric means). The system delivers valuable information, arguably supporting more objective determination of regions associated with under-cutting masonry via excessive joist erosion, and quantification of pinning (or galleting) and repointing intervention required. This work shows how this kind of solution can be used to improve the efficiency and accuracy of measurement and costing works for rubble stonework that has traditionally been evaluated on an approximate square-metre basis.