30 C O N T E X T 9 0 : J U LY 2 0 0 5 IAN BROCKLEBANK The problem with reinforced concrete Despite the relatively small number of people with expertise in the field, a methodology for the repair of historic reinforced concrete is developing. The principal problem with reinforced concrete is that, when used in a weathering environment, it contains within itself the seeds of its own destruction. It is poignant to those (such as myself) who are enthusiasts for lime that carbonation, the very same chemical reaction which makes lime mortars so durable, and so suitable for the maintenance of historic buildings generally, is the direct cause of this particular decay process. 1 When lime carbonates it slowly takes in CO 2 from the atmosphere and converts from calcium hydroxide (lime putty or bag lime) to calcium carbonate, the basic binder of limestone rocks. In the process it changes from being highly alkaline to having a pH close to neutral. Unfortunately, for reinforced concrete, this original high alkalinity is extremely effective in inhibiting the corrosion of the reinforcing steel, and its eventual loss, as the carbonation front gradually advances into the concrete from the surface, allows the reinforcement to corrode. The rust thus produced takes up as much as 10 times the volume of the parent steel, and the concrete is ruptured from the inside. As the first cracks form, air and water can more easily reach the steel so that, once started, the process can accelerate rapidly until severe failure results. Early reinforced concrete, such as that dating from before the second world war, is particularly prone to this mode of failure as the cement from which it was made is less strong and impermeable than that produced now. The compaction of the material when placed was often of variable quality, and the concrete cover to the reinforcement much less than would now be acceptable. As a result many early modern movement concrete structures, the best of which are often now listed, require careful periodic examination and possibly considerable remedial work. Over the past few decades the business of concrete repair has become a significant specialised sector of the building industry. Not only does considerable theoretical and practical knowledge already exist, but research is actively being carried out and the field is maturing rapidly. Most of this effort, however, has been aimed towards the cost-effective repair of major civil engineering structures such as motorway viaducts, bridges and oil rigs. The requirements of listed historic buildings are often little understood by people from such a background, and there is therefore the danger that repair works can be carried out crudely or overenthusiastically. Although very good repair of such buildings has been successfully carried out, such as the recent work toWells Coates’ 1934 Isokon Building in Hampstead, it would be misleading to suggest that all necessary problems have been solved. It seems reasonable to hope that a more coherent range of approaches can be established to suit the full range of early modern movement buildings which do, or which soon will, need conservation. Carbonation which has not yet reached the reinforcement can be hugely slowed by applying an anti-carbonation coating, in effect a high-build acrylic paint. This, as long as it is maintained, can bar the passage of atmospheric carbon dioxide. It is effective, practical and cost-effective where surface damage is minimal. Where the concrete was originally painted, it need not compromise the cultural value of the building in any way. Transparent versions are available for where the original concrete finish was designed to be shown, but those with a matt finish tend to make the concrete appear plasticky, and can not be used with any real enthusiasm. Where, however, the carbonation front has already reached the steel, the application of such a coating is really too little, too late, and more sophisticated and intensive treatment is required. Where reinforced concrete has already been damaged to the extent that cracks are open and sections of material physically damaged or missing, there is little alternative but to break out material to expose the reinforcement, treat the corrosion, and re-form the broken section in an appropriate repair material. Several companies offer repair systems for such situations, usually incorporating a standard cementitious repair mortar2, but in order to meet The Isokon Building looking superb following recent repair and restoration.The concrete was originally painted.
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