.. Failingconcreteof the BearRavineat DudleyZoo. 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 Overclaclding 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. Patchrepair 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 previous 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 Context65 March2000 historic buildings, yet somehow they are regarded as more acceptable. A patch repair can be permanent if clone 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. Surfaceappliedcoatingsandinhibitors 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. Cathodicprotection In this process an electrochemical system is permanently connected to the building to remedy chloride attack. The reinforcement isused to transmit a current through the stmcture 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 stmctures 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 term 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. Previousrepairs now failing at DudleyZoo Elephant House. Electrochemical re-alkalisationandchloride extraction These are non destructive, electro-chemical processesusuallyemployedwhere there is severe reinforcement corrosion. Realkalisation generates an alkaline solution which restores tl1epH value around the steel reinforcement and prevents future carbonation whilst chloride extraction redresses chloride attack.Although tl1eprocess 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 ofelementsor 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 thisan unrealistic option. There must also be a question as to the acceptability of this approach as a 'conservation' exercise. Donothingor recordanddemolish 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 isundertaken and useful comparisons on technique, appearance, reversibility,cost,longevityand 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. Melanie Cramp is an architect with the firm of Percell, Miller& Tritton at Ely, Cambs, and has recently completed her MA in architectural conservation at de Monifort University. 27
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