The Building Conservation Directory 2020

PROFESSIONAL SERVICES 1 29 C AT H E D R A L C O MM U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 0 CATHODIC PROTECTION OF STEEL-FRAMED BUILDINGS CHRIS WOZENCROFT B Y THE late 19th century advancements in building technology had enabled multi-storey and high-rise buildings to be constructed. These impressive buildings had structural skeletal frames made from steel and iron, with external masonry cladding forming a weather barrier. After about 1895, architects and engineers started to design these structures with the cladding tightly built around the metal frames. Although this form of construction has several advantages (such as the extra rigidity given by the bracing of the infill walls) with age the masonry became vulnerable to the corrosion of the embedded metal frame because the corrosion product, rust, is approximately 7–11 times the volume of the steel it replaces. Even relatively low levels of corrosion and section loss can therefore lead to large levels of cracking and displacement of masonry (figure 2). The problem has been termed ‘Regent Street Disease’ due to the high density of early 20th century steel-framed buildings in this area of London, although the same decay mechanisms also affect other types of structures, including older masonry buildings where iron and steel fixings (such as cramps) have been used. PREVENTION & TREATMENT In many cases, Regent Street Disease can be diagnosed early and prevented by controlling water ingress and the moisture content of the masonry. Regular survey and maintenance of drainage systems, roof coverings and/or flashings is the first place to start. In some cases, porous brickwork cladding has been successfully treated with siloxane water-proofing to the extent that Regent Street Disease in the brick-clad parts of the building has not appeared. However, making the surface of masonry waterproof can have significant detrimental effects. Not only can it lock moisture into a structure, but it also affects the physical properties of the surface, causing some types of stone to spall. Prior to the 1990s, advanced Regent Street Disease was treated by removing external masonry to allow the corroded metalwork to be cleaned and painted with a protective coating. This approach is not only very expensive, noisy and disruptive, but the time Fig 1 Gloucester Road, one of many London Underground stations built in the early 20th century with a steel frame clad in glazed terracotta blocks and a breeze concrete fill