Author: E. Morton (The Morton Partnership Ltd)
1 April 2016
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E. Morton (The Morton Partnership Ltd)
This article introduces engineers to the various techniques available to monitor movement in historic structures, from simple manual techniques which are less commonly used today, to sophisticated electronic systems. The form of monitoring will depend on the nature of the assumed movement, the funds available, and the possible consequences if the movement is progressive.
In conservation work and like-for-like repair on older masonry, lime mortar is the only recommended material. The thick, plain or lightly punctured walls that make up most historic buildings have few concentrations of load. Calculations of stress in such cases are often needless and, subject perhaps to the check of any critical element, we can generally lay aside our concerns about mortar strength. In contrast, the need to maintain a balance of moisture and flexibility in the body of an old wall is essential. Ignoring this will lead to the classic error of repointing old structures in brittle, impermeable Portland cement (OPC) mortar. The mortar provides the route for evaporation from the core and should be more permeable than the brick or stone. To reverse this by sealing the joints with a hard finish can only lead to trouble.
This article focuses on the phenomenon of 'bond timbers', which were commonly built into masonry walls from the late 17th to the early 19th century. Guidance is offered to engineers who may encounter these when working on an existing building.
Replacement of stone on historic buildings may be required for numerous reasons. These include age-related decay and weathering, poor workmanship in terms of material choice or setting, defective fixings, and structural failure. The main aim, in assessment, will be to retain the historic fabric where practical. However, the decision to replace will depend to a great extent on having a clear understanding of the significance of the stone, both individually and within the context of the element that it is part of, its predicted life or durability and its cost.