Conservation of Masonry
Conservation of the built environment is a difficult subject to cover comprehensively because it involves a complex interplay between politics, history, legislation, sustainability, the construction arts and crafts, materials science and structural engineering.
In this short article only a flavour can be conveyed and an indication of where to look for more information. Figure 1 shows an example of how masonry can be conserved given sufficient attention to all the principles involved.
Over time and in different countries, districts etc. the principles behind the conservation of what has been built in the past have varied enormously between:
- complete indifference, where unwanted buildings were simply demolished for their materials and new ones built over the rubble (a fate of many Roman examples in the UK)
- complete negativity (such as the destruction of the monasteries by King Henry VIII)
- often misguided prettification/upgrading
- the current more-considered and responsible policy.
The philosophy applied currently in the UK in respect of important monuments is:
- minimal intervention
- protection against disaster and neglect
- use of identical replacement materials or the best compromise
- additions designed to be in keeping with the style of the original building or alternatively clearly distinguishable from it
- changes are recorded in a log
- the philosophy and management must be agreed with the regulating body such as a local authority, English Heritage, Heritage Scotland etc. before the start of projects.
Put more colloquially this is:
- if it ain’t broke don’t fix (or alter) it
- protect against fire, flood, rain, frost, vandalism, theft and neglect
- don’t replace Portland limestone with concrete blocks
- don’t change from art nouveau to art deco
- record what was done
- try to get all affected parties on-board including the public.
BRE Digest 502 (2007) gives a brief overview and a useful reference list.
Replacement Materials: Stone
Stone is a very variable natural product formed from a wide range of minerals by different consolidation processes. Because its properties, particularly durability, can have such a wide range, even within each sub-class, it is important to be able to accurately identify the type and source of the original rock when choosing replacements.
This is particularly important for the sedimentary rocks limestone and sandstone because they are the most widely used and have the greatest variability. It is best to start by trying to obtain information from the building owner’s own knowledge and records.
As stone is heavy and expensive to transport it is often quarried nearby and often whole villages and towns are built from the local material. Typical examples are the villages surrounding the Ham stone in Dorset. If available, original stone from redundant parts of the same building/s or rescued from local architectural salvage yards is ideal provided that it has not deteriorated.
Visual identification can be facilitated by comparison with examples held in stone libraries as listed in BRE Digest 508: Part 1 using information such as grain (fine to coarse), overall colour or patterning, degree of stratification (layering), orientation of any layers in the work, size and shape of uncut rubble stones and the degree to which the stone can be worked or polished.
If visual data are inconclusive then data from petrology – examination of thin sections under a microscope or X-ray diffraction – can be used to identify the minerals present and chemical analysis can determine the constituent elements.
Other properties such as density, strength, hardness and porosity may give further clues. Having identified the source quarry or an alternative, newly quarried stone may be suitable if an equivalent material is still in production.
Stone can vary quite markedly with quarry, position in the quarry and depth from which it is cut, so it will have to be tested in accordance with the methods given in standards such as BS EN 771-6 (2005) and BS EN 1467 (2003) to check the match with the original.
Replacement Materials: Clay Bricks, Terracotta Ware, Concrete and Calcium Silicate Units
Even though such bricks and blocks are man-made, the process of identification and/or determination of a suitable replacement product is broadly similar to that for stone, as are the sources of original material. Important visual clues are makers’ marks (usually impressed in the frog of mud or pressed bricks or sometimes stamped on a face of an extruded unit), overall colour, colour variation, appearance of any coatings or impressed patterning, perforation pattern, frog size and shape.
The size and proportion of clay bricks has varied since their original introduction by the Romans. Terracotta, which is hollow moulded clay ware, often of elaborate and madeto-measure shapes, was imported during the 15–17th centuries and made in England in the 19–20th centuries for use in prestige buildings. Concrete and calcium silicate units will only be found in unmodified buildings dating from around 1900 onwards.
If new replacement units are necessary they should be matched in accordance with the test methods given in standards such as BS EN 771-1 to 5 (2003). BRE Digest 508 Part 2 (2009) gives some helpful guidance on man-made units.
Replacement Materials: Mortars
Mortars are normally identified on the basis of six main characteristics:
- The type of binder, e.g. natural hydraulic lime.
- The main type of aggregate, e.g. silica sand.
- Any other discernible ingredients such as plasticisers, added fine material, pozzolans and pigments.
- The hydraulicity of the mixture.
- The particle shape and size range (grading) of the aggregates (from sieve analysis).
- The porosity of the cured mortar.
The state of carbonation and the cube strength may also be helpful. Where required measurements should conform to tests specified in BS EN 998-2 (2003) or BS4551 (2005). The composition of most of the mortar types likely to be encountered in conservation work and guideline data on their identification are covered in more detail in BRE GBG66 (2005).
Selection of Replacement Materials
Ightham Moat, a National Trust property, is typical of the problems that might arise when choosing replacements for the complex cocktail of materials present. Having identified the original units the broad rules for replacing them are:
- Try to use un-weathered identical examples if available.
- Similar but stronger and/or harder replacements may be used for additions or wholesale replacement of elements.
- If anything, select slightly softer, lower-strength examples for patching walls because stronger harder materials often accelerate the weathering of the surviving original materials.
- Do not mix materials that have widely differing chemical compositions or movement characteristics without consulting specialist materials scientists. The wrong choice could cause a 10-fold increase in weathering damage or disfigurement.
Mortar should be easier to reinstate as it is always placed as a plastic material that sets to a solid mass where placed. Mortar should have as near the same composition and be of compatible texture, colour, porosity, hardness, vapour permeability, strength and durability as the ‘original’, providing that the original was of acceptable durability.
To avoid stress related damage the hardened, particularly repointing, mortar should never be stronger/harder than the units, and to minimise shrinkage cracking, washed well-graded sand should be used. BRE GBG 66 (2005) lists most of the applicable mortar types and there is useful guidance on selection in Tables 2 and 3 of BS8221-2.
The main methods used (in order of severity) are:
- demolition and reconstruction of whole elements, e.g. walls, arches, or parts thereof
- removal and replacement of whole units, e.g. blocks, bricks
- cutting out the whole face of a unit and replacement with a slip
- cutting out of areas of deteriorated units and replacement with a shaped piece
- plastic repairs with mortars formulated to match the characteristics of the units.
In all cases, except at ground level, safe access must be provided using scaffolding or lift systems, a building professional/engineer should evaluate any collapse hazards, propping should be used if necessary and, ideally, a trial of the proposed technique should be carried out before the main contract starts. Techniques for partial demolition, removal of units and cutting out of parts of units are given in BRE Digest 508 together with further references.
Cleaning of Masonry
Exposed masonry has always been subject to staining and defacement processes by interaction with the environment, water (moisture) being the main agent and probably industrial smoke the second. Since staining or surface deterioration changes the appearance from the original architectural concept, various cleaning methods have been developed based mainly on: mechanical action using brushes or abrasives; various water washing techniques; and treatment with weak to strong acids and alkalis. This is a very complex topic and only the basic rules can be covered here, which are:
- Before industrialisation in the 17th century and more recently following the clean air acts of the 1950s, biological growths are the most likely staining agent.
- During the most industrialised period (1750– 1950) smoke and sulphur dioxide were probably the worst offenders especially affecting limestones, lime-bound sandstones and porous mortars. Such staining/erosion may still be encountered.
- From 1950 to date there have been increased low-level effects of road splash and diesel exhausts.
- It is essential to be sure of both the masonry materials and the staining agent before choosing a cleaning method.
- Water is a relatively harmless cleaner but may cause further staining/damage if used in excess on, and not dried out from, porous materials.
- Strong chemicals, such as acids, should not be allowed to penetrate and remain in pore systems, as they almost certainly will cause accelerated soiling and deterioration.
- Most strong acids pose serious health and safety risks to operatives and should be avoided if at all possible.
- Research by Historic Scotland (2003) on sandstones has shown an up to 10-fold increase in deterioration rate following cleaning.
- Light brushing and grit blasting with softer abrasives such as crushed nut-shells is usually safe.
- Laser-based ablation cleaning is probably safe but it is expensive and slow.
- Do not use mechanical or chemical methods that remove surface layers from materials that depend on protective skins such as terracotta ware and weathered limestones.
- If there is any doubt or known risk, do not use any cleaning process, particularly on stones or mortars with poor durability. Trials of cleaning systems may give some guidance but are rarely of sufficient duration.