Restoration is the conservation 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 applied to historic buildings; accurately depicting 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 for a high level of authenticity, replicating materials and techniques as closely as possible. Modern techniques should be used in a concealed manner where they will not compromise the historic character, such as replacing outdated utilities to modern, or installing climate controls, alarm systems, and so on.
Some criteria are often used to assess whether restoration works are acceptable:
- Measuring the effect of restoration work to the overall heritage value of the building.
- Sufficient evidence for need for the work.
- 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, include the following:
- Sufficient respect for the original material.
- Respect for 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 are preferable for structural restorations.
- Modern techniques and materials are admissable where traditional alternatives are not feasible.
Restoration techniques can be classified as either reversible or irreversible. Reversible actions may be preferable in case they are proven inefficient or of low durability, as they can then be replaced without damage to the original fabric. Also, in case that better techniques or materials are developed at some point 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.
Often it may not be possible to use reversible methods for interventions, and it may prove the case that 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 incorporated steel bars.
- Interconnections of marble or stone parts with bonded dowels.
- Skins of reinforced concrete on masonry walls or upper side of vaults.
- Strengthening of foundations.
The compatibility and durability of the replacement materials should be considered carefully. They should be compatible to 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, coefficient of thermal expansion, permeability and problem of 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.
Problems can be posed to paintwork and renderings 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, substitutes will need to be provided, as some paints were manufactured using harmful materials such as arsenic and lead.
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