Module 5: Conservation Interventions and Treatments; Criteria for Selection and Implementation
This morning Ian Hume, a conservation structural engineer from the UK, joined us to speak about 'Structural Repairs'. I was very pleased to see Ian Hume teaching on this course because he is one of our trainers at the Scottish Lime Centre Trust where he teaches a course on the Structural Repair of Historic Buildings. The main subject of Ian's lectures was 'cracks' and their interpretation. We get very worried if we see a crack in a building but Ian has told us to assess each situation separately to identify if the crack is worth worrying about. A good example of the extreme contrast in danger is that if a crack appears on an oil rig which is 0.1mm wide we may have reason to worry, whereas on a historic building it is possible to have a crack up to 1 metre wide and it will not necessarily be a concern (although I would imagine this is only the case if we're talking about a ruin!). The key things to identify are; is the cracking live? Is the cracking structural or due to other factors? Is it a seasonal or cyclic cracking? Where cracks are present in a structure we do expect them to have some movement due to thermal expansion and contraction and as long as the crack is not getting wider in the long term this is most likely not a concern. A problem faced with seasonal cracking however is that a ratchet effect can occur whereby debris falls into the crack when it's at its widest and it is then not able to contract to its starting position and the crack will widen.
To identify if a crack is active or inactive you should always measure it over a period of time (bearing in mind seasonal movement) but there are also pieces of evidence you can look for such as does the crack look fresh/sharp (new?) or are the edges rounded (old?), is it full of debris/cobwebs (old?), if an internal wall is cracked is the cornice also cracked? Some cracks may have existed in a building since it was constructed due to the subsidence that occurred from the weight of the construction materials- this should not be a concern.
Although it is possible to calculate a certain level of stresses and strains on an historic building, Ian believes they they are not definitive and should only be used as a guide. Temporary works are extremely important and often they can be in place for a very long time so they need to be well thought out and durable. This can be difficult because good attachment to the structure is not always possible. Ian was very clear on the fact that we should always employ an engineer with experience of historic buildings; as a young engineer in the 1970s he severely over engineered an historic structure which he believes could, even now, survive a nuclear attack but was not carried out in a manner sensitive to the site (see image below).
Ian showing an image of one of his first jobs as a conservation structural engineer where he believes he might have over done the reinforcement... (just the walls are original).
This afternoon Gionata Rizzi spoke to us about Architectural Repairs. One of the biggest questions seems to be how can we repair something without faking it? In relation to mortar Gionata suggests that one way we can make it clear for future that a mortar is not an original would be to use polypropylene fibres (in place of hair which give a mortar strength). This way if the surface of the mortar is scraped back the fibres would be visible and it would be clear that this is a modern material. A building that has come up in conversation a couple of times now is the Temple of Athena Nike which is located on the Acropolis in Athens, Greece and has recently been restored. Although a large proportion of the original blocks still exist, many parts were missing and the recent restoration included highly skilled masons carving the missing elements out of a stone found to match the original to complete the wall once again. There is no question about the skill involved to do this but Gionata asks; does this fake the discovery of the missing pieces? Should the missing elements have been recreated in mortar rather than stone? Our eyes are drawn to the restoration before the original, is this wrong? The image I have added below of one of the walls is poor but if you click on the link above you will find many links to images and websites dedicated to this structure and can decide for yourself.
A restored wall of the Temple of Athena Nike.