C O N T E X T 9 0 : J U LY 2 0 0 5 31 the requirements of the system guarantee these need to be applied to a minimum depth of cover and a visual match is by no means assured. Where depth of cover is not a problem, and the building would not be harmed by an applied paint finish, this can, however, conveniently be combined with an anti-carbonation coating, as above. Clearly, if the preservation of a significant part of the building’s cultural value requires the retention and matching of particular materials and surface finishes, it will be necessary to depart from standard systems and repair mixes. Although this may invalidate a manufacturer’s standard guarantee, such an approach should not be discarded out of hand, as results can still be very good. In particular, if the original cement characteristics are matched more closely than by a modern ordinary Portland cement (perhaps even by the use of a hydraulic lime), the mix may actually offer greater alkalinity. Repair finishing becomes most demanding where the concrete finish has been worked to expose the aggregate, as the level of craft skill necessary to achieve a good match under such circumstances is easily equivalent to that which may be expected of a highly talented and experienced stone conservator. It should nevertheless be recognised that such skills do exist, and can be developed. Corrosion is in all instances an electro-chemical process, and operates by the creation of an electrolytic cell of anode, cathode and electrolyte. This is not always apparent as the process in circumstances such as these works at a microscopic level, where minute anodic and cathodic areas exist side by side on the surface of the steel, with moisture in the surrounding concrete being the electrolyte. This does, however, mean that rehabilitation of concrete which has not yet become physically too damaged is possible by electrochemical methods, in this case either cathodic protection or realkalisation. Impressed current cathodic protection3 is a longestablished and well-proven technique. The small, localised, corrosion cells on the surface of the steel are effectively swamped by the imposition of a greater impressed current through a system of applied anodes, making the entire steel structure cathodic. In practice, the amount of electricity required is very small indeed. This system is of particular use in the repair and conservation of steel structures encased in masonry, where the masonry offers a suitable zone around the steel for the installation of the anode network, and all the connecting wires4. In self-finished concrete, however, this paraphernalia must either be applied to the surface of the concrete or chased into it which presents real aesthetic problems and limits its usefulness on culturally valuable structures. Realkalisation is an allied technique, but uses an external mesh anode temporarily fixed over the face of the concrete and surrounded by a poultice containing an electrolyte of water with an added salt. A higher current is applied for a matter of weeks only, the effect of which is to cause electrolysis of the water surrounding the cathode (the reinforcement), which is reduced to form hydroxyl ions (hydroxide) and hydrogen gas. The resulting hydroxide is alkaline, thus restoring the passive protection to the steel, after which the external anode and poultice can be cleaned away, leaving no significant visual alteration to the concrete. This technique has considerable potential for the long-term conservation of self-finished reinforced concrete, but in practical terms is still in its infancy, and does give rise to some complications which need to be taken into account. If the applied electrical current is too great, the gas pressure caused by the released hydrogen can cause separation of the bond between the steel and surrounding concrete, and the process can also make the reinforcing steel brittle, effects which may reduce the structural capacity of the overall material. Much of the salt needed to make the electrolyte sufficiently conductive remains in the concrete, and can cause efflorescence on the surface on completion. The amount of alkalinity provided is generally less than that originally offered by the calcium hydroxide in the concrete mix, and it may well be leached out unless steps are taken to provide adequate weather protection to the finished structure.5 The latter two points are currently often dealt with by applying an anti-carbonation coating in any case, as a way of playing safe, which unfortunately removes a major inherent virtue of the realkalisation technique: its minimal visual impact. Efflorescence problems tend to result from excessive treatment, as in fact, only the immediate area around the reinforcing steel needs to be repassivated, rather than the full depth of concrete cover as is sometimes assumed. Substantial research has been carried out into the use of hydrophobic coatings and impregnations to protect repaired concrete. These have been quite widely used by the Department of Transport, among others. They offer considerable potential, but have so far been little used in the conservation of historic structures. The usefulness of traditional weatherings and drips on building facades should not be underestimated where these can be added or enhanced discreetly without damaging perceptions of the architecture and its values. Chiswick Park station canopy, built in 1932. Technically correct patch repairs stand proud of the original concrete, and the standard repair mix does not adequately match. Recesses in the centres of bays originally contained concrete pavement lights, now lost.
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