The corrosion of history
Now that early steel-framed buildings are being listed, it is becoming apparent that there is significant corrosion of steel frames, causing damage to masonry and terracotta facings. Spalling, displacement or even dislodgement of ashlar may occur, and the obvious method of repair - stripping back to the frame - is out of the question. More than 100 such buildings in London alone are thought to need repair, at £750,000 each.
Buildings constructed before the Second World War are particularly at risk, both because corrosion has had longer to occur and because they were often built with the masonry tight against the frame (see diagram). For a number of reasons - lack of maintenance, broken gutters, thermal movement, etc - water tends to infiltrate, and corrosion starts. Although the frames were originally coated with various protective materials, these have not proved durable enough.
Published information is limited, and frequently, when repairs have been done, they have had to be repeated later. In 1993 English Heritage initiated research on such corrosion and its prevention, part-funded by the doe and undertaken by Taywood Engineering. The guidance document has now been published*. It both proposes remedies for old buildings and suggests measures to prevent problems in new ones.
The report covers the construction of early twentieth-century buildings, pinpoints common areas of deterioration and gives guidance on remedies available. It details best practice for repair, including that to listed buildings. For new buildings it emphasises the need to manage water ingress, the correct use of wall ties and coatings, and the incorporation of movement joints. Case studies are given of several recently treated buildings.
The beginning of corrosion is simple enough. It is all too easy to accidentally set up an electric cell by bringing together different materials, particularly metals, in the presence of water. Electricity flows and one of the poles of the cell - in this case the steel cathode - corrodes. Cathodic protection consists of wiring the building and applying a small voltage to make the electricity flow in the opposite direction, stopping corrosion. This does not rectify the steel surface expansion or loss of section that have already occurred.
The speed of corrosion varies, in part with the resistivity of the material in contact with the steel. When that material gets wet, its resistivity falls. If the material has a moisture content of more than 5 per cent, corrosion can start, and then accelerate. As the steel starts to rust, the material in contact with the steel is no longer the masonry but rather the rust. This has a very low resistivity. Even 2 per cent moisture content can promote corrosion.
Repair and prevention
Keeping the frame dry is the first line of defence. The report also comes down in favour of cathodic protection, the only system that can be retro- fitted to existing buildings. This process has been around for a long time, but has not been much used in buildings until recently.
Installation involves cutting into joints or making slots in the masonry near steel members and putting in conducting wires and tapes. These are connected to a control unit. Cathodic protection may be installed in sections, as the problems, and hence the voltages to be applied, vary from place to place. The control system both runs the system and provides feedback that the design voltages are being maintained. The electricity bill is small.
Installation is normally from the exterior of the building, though it can be invisible when completed. Repairs to existing damage may be done at the same time.
When a building, not very old, is being refurbished, it may be sensible to install a system but not to switch it on until needed.
John Rawson is an architect and writer
*The Prevention and Repair of Corrosion in Steel Framed, Masonry Clad Structures. Available on cd-rom, for pc Windows 95 or Mac, at £10. Hard- copy price £200. Contact Michelle Connolly at Taywood Engineering, tel: 0181 575 4161. For further information contact Ray Elliott at Taywood Engineering, tel: 0181 575 4849, fax: 0181 575 4215), e-mail: email@example.com