Restoration is the process of returning a building to its former state. Historic England define this state as being ‘...a known earlier state, on the basis of compelling evidence, without conjecture.’
Restoration work is most commonly undertaken on historic buildings; accurately recreating its form, features and character as it appeared at a particular time, while protecting its heritage value. Decay or alterations made to the building can be reversed as part of restoration works.
Restoration aims to achieve a high level of authenticity, replicating materials and techniques as closely as possible. Where necessary, modern works, such as replacing outdated utilities, or installing climate controls, alarm systems, and so on, are undertaken in a concealed manner where they will not compromise historic character.
Criteria that might be used to assess whether restoration works are acceptable include:
- The effect of restoration work on the overall heritage value of the building.
- The weight of evidence for need for the work.
- Whether the work respects the previous forms of the building.
- The implications of the work in terms of ongoing maintenance requirements.
The basic principles that should be taken into consideration, particularly for structural restoration:
- Having sufficient respect for the original materials.
- Respecting the valid contributions of all periods on the building.
- Replacement of missing parts must integrate harmoniously with the whole.
- Additions should not detract from the building, its setting or relation with surroundings.
- Use of traditional techniques and materials.
- Modern techniques and materials are admissible where traditional alternatives are not feasible.
Restoration techniques can be classified as either reversible or irreversible. Reversible actions may be preferable as they can be subsequently replaced without damage to the original fabric, for example if further restoration is required, or if better techniques or materials are developed in the future. Typical examples of reversible techniques include:
- External buttresses.
- Ties at arch springings.
- Rings at the base of domes.
- Prestressed unbonded stitches.
- Anastylosis of stone or marble monuments with dry joints.
- External ties.
- Improvement of the strength, stiffness and ductility of existing diaphragms.
However, it may not be possible to use reversible methods for interventions, and some interventions cannot be easily undone without causing damage to the existing structure. Typical examples of irreversible techniques include:
- Bonding-in of new bricks across cracks after grouting and cutting out to each side.
- Deep rejoints.
- Rebuilding of part of the facings of walls.
- Stitching of walls with prestressed rebars.
- Reinforcement of masonry with steel bars.
- Connection of marble or stone parts with bonded dowels.
- Skins of reinforced concrete on masonry.
- Strengthening of foundations.
The compatibility and durability of the replacement materials should be considered carefully. They should be compatible with the existing building in terms of chemical, mineralogical, physical and mechanical properties, as well as being aesthetically harmonious. Other points to consider are the strength, stiffness, bonding, thermal expansion, and permeability as well as problems such as efflorescence.
Common types of non-metallic materials for irreversible interventions are as follows:
- Stone and marble.
- Mortar and grouts (Portland cement, lime-cement, pozzolanic, epoxy resin).
Where high tensile strength or prestressing forces are required, steel is often used, as well as fibre composite cables.
Paintwork and renderings can degrade due to air pollution, acid rain, UV radiation, and so on. Historic paint analysis of old paint layers can determine the original pigmentation and enable a chemical recipe to be reproduced. Often, modern substitutes have to be used, as some paints were originally manufactured using harmful materials such as arsenic and lead.
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