Melanie Clamp suggests a concrete philosophy for modern movement conservation.
"New or old in style? It will all soon be old, and neither better nor worse for that". David Watkin, Morality & Arcbitecture, p98
The decay of concrete architecture is one we witness every day without realising it, as so much of our built environment is constructed using some form of concrete, for example pavements, roads and bridges. This decay has taken a relatively short time to become apparent, and it does not display the time-honoured patina of historic masonry. Time and progress are so often encouraged and displayed in historic buildings. Calke Abbey in Derbyshire is not simply about the displaying of built fabric, but also the social decline of the aristocracy with a kind of decayed glamour. In the case of modem architecture however, such decay is often interpreted as an unfortunate failure of technology that detracts from the original concept of an undefiled building. Material decay simply does not fit the concept of 'pure' form that such architecture embodies.
Dudley Zoo.
Ancient v modern - the problem with traditional repair
The piecemeal consolidation and replacement of badly decayed material often considered acceptable for traditional masonry buildings is not necessarily an appropriate method for the conservation of concrete architecture.
Many historic buildings are constructed of multiple elements that are small in scale, which when they deteriorate can be repaired or replaced in a piecemeal fashion stone by stone, or brick by brick; but while the conservation of these earlier buildings is generally based on material fabric, there may be other values relevant to modem movement restoration which should be explored before deciding on the priorities for conservation. New ideas as well as techniques were being tested on many modem buildings and in some instances it may be appropriate to preserve the authenticity of the 'concept', particularly if it is well documented. it may be purity of form, constructional approach, structural innovation or social interest that is the key to the cultural significance of the building. At Dudley Zoo, for example, it appears that technical innovation was not as much an issue as speed of construction, so it is likely that getting details absolutely crisp and perfect was not necessarily the highest priority.
Concrete by its nature is monolithic, generally composed of a reinforced steel skeleton encased with concrete cast between temporary shuttering and in many early examples this performed as both structure and envelope. It is the interdependency of this composition that is critical and any failure within it compromises the integrity of the whole building. In such deterioration, problems generally occur from the inside out, and not the outside in as with most traditional structures. It is analogous to iron cramps within masonry walls, where current practice is to locate, cut out and replace with a nonferrous alternative. This technique is applied to concrete in the form of patch repair although the reinforcement is the structure, rather than an addition, and cannot be easily replaced. Patch repairs of this kind have been carried out to nearly all of Lubetkin's structures at Dudley Zoo over their 50 year life although the application of 'piecemeal consolidation' has not generally been successful and the repairs themselves have failed and are now considered to be defects. At Dudley, therefore, the same method used for traditional historic structures has proven not to be even a medium-term option.
The concealment of patch repairs behind an overcoat of paint or other finish provides an extra layer of protection from the elements but can result in a complete change in appearance, particularly where the concrete was originally 'fair-faced'. The 'as new' finish may be unintentional but it can be difficult to prevent, as numerous repaired buildings illustrate. The aim should be to portray a 'conserved' building and not a new one, and although as much original fabric should be preserved as possible, this often proves difficult with modem integral construction techniques. The idea of reversibility is also difficult to comply with when repairing concrete and overcladding is probably the most reversible and patch repair the least. However, overcladding can also modify the appearance more than any other technique
Level of intervention
Much as there are levels of conservation (preservation, restoration, reconstruction, adaptation, maintenance), repair techniques have been introduced for concrete that address the limitations of patch repair and have varying impacts on the structure. The chosen method will depend on the conservation approach to the building which will be typically influenced by the owner's objectives, cost, longevity and aftercare requirements together with the cause and extent of deterioration (visible and potential).
Diagnosis also differs from that for traditional masonry structures due largely to the pattern of decay and the nature of the material. The repair method will largely depend on the cause of the decay, whether this is physical (freezethaw cracking), mechanical or chemical (carbonation or chloride attack), or a combination. Careful inspection and testing is important for correct diagnosis. This is often determined scientifically by a specialist as the condition survey of the structure must identify concealed and potential decay. As well as basic ,opening up' to determine the depth of reinforcement cover, a cover meter survey will determine the position of concealed reinforcement. Testing is also required to identify various material properties including the presence of chlorides and carbonation. Inspections range from hammer surveys, to determine loose or hollow material, to removing quantities of material. Thorough testing can be invasive and expensive but is generally a good investment to enable more accurate costings and prevent spiralling costs once work is on site.
It is vital for the architect to understand the various concrete repair techniques available and appreciate their impact on the building fabric. The following list is not intended to be exhaustive, but introduces techniques currently available for the repair of concrete architecture. Some are invasive and require the removal of material whereas others need no physical intervention but have other effects. The consideration of these is no different from studying options for the cleaning of historic stonework and concrete repair requires no less skill than stone conservation.
Overcladding
Overcladding is probably the most intrusive in terms of appearance yet the most reversible of all the techniques available. It is expensive and visibly alters a building, meaning consents may be required, and this often means that it is dismissed. The main advantage is its longevity. Although it can reduce the amount of water penetrating to the reinforcement, it will not reduce chloride attack from within the concrete, and action should be taken to prevent problems of condensation.
Patch repair
The commonest approach to 'repairing' concrete simply involves the removal of damaged or potentially damaged material and replacement with a new high quality mortar, together with reinforcement if required. Defective concrete and reinforcement are generally mechanically removed and the area cleaned, while surface damage and pre- vious coatings are normally grit blasted, though high pressure water or suction blasting may also be used. Levelling and pore filling mortars are often applied as a protective measure to provide resistance to further carbonation and obscure the repair. This does, however, result in a finish which may be totally different from the original. Many early buildings had a rough texture finish to the concrete, some purposely exhibiting their original shuttering, and small patch repairs can result in alteration of their whole appearance. A compromise can sometimes be achieved by using a 'pore stopper' applied by brush to retain surface texture rather than using levelling coats. Patch repairs are generally obvious, but then so are stone repairs to historic buildings, yet somehow they are regarded as more acceptable. A patch repair can be permanent if done properly, but at Dudley Zoo it has not proven to be the case. Experience has also shown that it is difficult to control cost as further decay is found on site.
Surface applied coatings and inhibitors
Surface coating has similar effect on appearance and is often used alongside other methods such as patch repair, but it does not in itself address latent damage. The technique is relatively cheap but it requires regular renewal. Corrosion inhibitors can be applied in a number of ways and are absorbed into the surface via liquid or vapour diffusion to protect the reinforcement from water and oxygen. A painted application will affect the appearance, but an alternative, such as pellets inserted into the structure, can be less intrusive to the surface finish.
Cathodic protection
In this process an electrochemical system is permanently connected to the building to remedy chloride attack. The reinforcement is used to transmit a current through the structure to control corrosion, even if chlorides are present and alkalinity has been lost. It cannot repair reinforcement already corroded, but it is used to restrict damage to those areas potentially at risk. it is widely used across America for civil engineering structures and has been used in the UK for stonework conservation. Although the initial outlay for cathodic protection is moderate, this should be balanced out against long tenn monitoring and maintenance. Its external fitting affects appearance and, because all reinforcement should be in contact, it can require an amount of 'cutting in' to ensure that all sections receive current..
These are non destructive, electiochen-iical processes usually employed where there is severe reinforcement corrosion. Realkalisation generates an alkaline solution which restores the pH value around the steel reinforcement and prevents future carbonation whilst chloride extraction redresses chloride attack. Although the process does not in itself affect appearance, the surface is usually cleaned down after the poultice has been removed. Finishing coats are then applied. Again, this approach cannot reverse corrosion damage to the concrete and this is dealt with by patch repair. However, compared with other methods small amounts of material are removed but it is relatively expensive and its longevity is not known.
Replacement& elements or reconstruction
With recent architecture, documentation is often available to allow accurate reconstruction of individual elements or even a complete building. Demolition and reconstruction can create an exact copy of a building including its faults, and can be employed where other methods would result in a complete change of character. In principle it provides a 'permanent' , stable and almost maintenance free alternative without affecting appearance. In practice, however, compliance with current regulations and the cost makes this an unrealistic option. There must also be a question as to the acceptability of this approach as a 'conservation' exercise.
Do nothing or record and demolish
Do nothing means leave for now and hope that repair techniques or alternative uses will become available in the future. The risk is that, in extreme cases, it has been an excuse for blatant neglect that eventually results in demolition. Demolition is, perhaps, the last act of conservation when a building is considered unstable and unusable and it acknowledges that a building has a specific lifetime. Before demolition, it should be fully recorded including producing accurate 'as built' drawings.
Guidance on available methods of and impacts of concrete conservation and repair are required, but are unlikely until much more work is undertaken and useful comparisons on technique, appearance, reversibility, cost, longevity and aftercare can be assessed. There are a number of specialist contractors available to do quality work and these, with bodies such as the Concrete Repair Association, are largely responsible for most current published work on the subject.