Monday 9th May 2011; Deterioration Mechanisms, Mechanics of Masonry Construction & Structural Mechanisms of Decay

Week 5 of 12

Module 4: Deterioration Mechanisms; Methods of Survey & Analysis

This morning we received lectures from John Fidler, staff consultant and the corporate practice leader for preservation technology with consulting engineers Simpson Gumpertz and Heger and former Conservation Director of English Heritage, on an 'Overview of Deterioration Mechanisms'. John started by asking us what backgrounds we were all from and how many engineers there were on the course; the answer is none. He said it is essential to understand that groups of people from different professions will, in his experience, process information in different ways. This needs to be taken into account when addressing interdisciplinary groups with regard to conservation.

When making observations on a building we need to take into account that observations on any one day are a snapshot in time, but they do not represent changes that may or may not have occurred. Drawings and paintings are also a good source of information about historic structures, but they have to be treated with caution because the artist would often represent what they wished to see rather than what they could see. Ideally we would always revisit a site in different weather conditions and in different seasons, and where possible over a long period of time. As this is often not possible, it is always prudent to carry out historical research on a structure to locate old photographs and find information on past deterioration/interventions etc. It is essential that we distinguish between the symptoms and the causes of any deterioration. There is no point repairing the symptom if we have not identified the cause (‘there’s no point putting a plaster on a festering wound’ unknown).

One of the biggest issues in the field of stone conservation is that far too many people still don’t understand that stone is a natural material, which can vary massively quarry-to-quarry, and even block-to-block. We’ve been brought up on manufactured materials with guarantees and known strengths etc. Speaking to the quarriers is an opportunity too often overlooked, but they are the people who work with the stone from a particular area daily and therefore know much about its quality and variation. Apparently the historic names of quarries sometimes also give you information about the quality of the materials being produced. Anita told us that in Australia quarries are often given the terms ‘paradise’, ‘purgatory’ and ‘hell hole’ to indicate the quality of stone being produced.

One of the large problems we face with many of our sandstone buildings in Scotland is the issue of face-bedded/end-bedded stone (where a block is laid with its natural bedding planes vertical and parallel to the outer face of the building). John touched on this briefly, but unfortunately it was to say that there aren’t any repair technologies that have been found to solve the associated problems of spalling in the long term; they were built to fail.

This afternoon we received lectures from Giorgio Croci, Professor of Structural Engineering at La Sapienza University, Rome, who has been involved in major restoration projects, including the Colosseum in Rome and the 'Leaning Tower' in Pisa. His lectures were titled 'Mechanical Properties of Masonry Construction & Structural Mechanisms of Decay'. Giorgio opened by telling us that there are three types of structural issues that we need to consider in historic buildings; columns/walls/pillars (elements only under compression- lateral deformation is possible), arches (element only under compression- similar behaviour to a column) and beams (elements under tension and compression). It is worth remembering that the geometry and stresses within a building change with time due to movement, interventions and material alterations. A building must be treated as an organic structure to a certain extent.

Giorgio explained how structures that have some flexibility will be less affected by deformation than a brittle structure. He listed the three possible groups of remedial measures that can be considered where deformation occurs, these are; remedial measures on the soil, remedial measures to modify the stresses under the foundations and remedial measures on the structure.

We were given the case study of the famous ‘Leaning Tower’ of Pisa, for which Giorgio was one of the leading consultants in its stabilization. What I hadn’t realized is that the tower has been leaning ever since it was first constructed, and therefore retaining a lean is an important part of its heritage. Construction of the tower began in 1173AD when it was half built and in fact tilted in the opposite direction to the current tilt. It was completed in 1360, by which time it was tilting in its current direction by an overhang of the upper part by 3m (c.3 degree tilt). By the 1990s, when the stabilization work Giorgio was involved in began, the overhang had reached 5.5m (c.5.5 degrees tilt). The stabilization work included adding several tones of lead to the base of the oblique angle side of the tower, wrapping it at the centre with cables and attaching them to a separate structure, and finally removing soil from below the side of the oblique angle to straighten the tower (38 cubic metres was removed). The tower was partially straightened and is now stable and tilts at c.5 degrees (a 5m overhang). We will be visiting the tower in the 8^th week of this course during our study tour.