The Building Conservation Directory 2020

30 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 C AT H E D R A L C O MM U N I C AT I O N S required to have extensive scaffold in place often makes it unfeasible. Furthermore, the removal and replacement of the masonry risks extensive damage to the original cladding. Due to the high proportion of new material required, this is generally considered restoration not conservation, conflicting with widely accepted conservation principles. Even if removal and replacement is acceptable, many original materials are no longer available, often because the quarry is no longer in operation or because manufacturing methods have changed. For example, new Carraraware terracotta was, until recently, almost impossible to source and now only one or two manufacturers can replicate it. CATHODIC PROTECTION Cathodic protection as it is known (CP), began to be used as a treatment for Regent’s Street Disease in the 1990s and since then at least 100 buildings of different scales, cladding and historical importance have been protected. The technique involves a series of electrodes (anodes) being installed into the building’s masonry. Where successful, this reduces corrosion to negligible levels. At its basic level, the principle is the same as attaching a more reactive metal to the hull of a ship so that this corrodes sacrificially, thus preserving the parent metal. Although such ‘galvanic’ systems have been installed to some natural stone structures, in practice they are not generally capable of protecting against Regent Street Disease. This is because the cladding of most steel framed buildings has such high resistivity that the lower voltages of galvanic anodes cannot emit, or distribute, the required level of current. A modified form of CP is therefore used whereby a DC current from a power supply is used to activate otherwise inert metal anodes. Another advantage of using an impressed current system such as this is that the current can be varied. Extensive experience of monitoring systems protecting this sort of structure shows that, in many locations (particularly the north west) seasonal variations require frequent alterations to the level of current being delivered. This is only possible with a controllable impressed current system. It is also common for galvanic anodes to be physically larger than impressed current units, which is an issue when trying to limit heritage impact by, for example, fitting them into narrow masonry joints. A disadvantage of impressed current systems is the frequency of monitoring required. According to published guidance, all CP systems require some level of monitoring, but the risks from excessive outputs are particular to impressed current. So long as the design and procurement of the system ensures that the control equipment has ‘open source’ functionality, the monitoring of the system can be tendered on an annual basis and thus market competition keeps costs low. Essentially, each type of system and what features it has is a case of ‘horses for courses’. A qualified, independent engineer should be appointed to review the structure, its condition, the available budget, proposed service life and planned access. From this they can determine which form of CP, which products and which installation approach is most suited to the building and the client’s needs. TYPES OF ANODE Choosing the most suitable anode is also important. Cheap and effective solutions for the deck of a reinforced concrete bridge are carbon paint or a thin titanium wire mesh with concrete overlay. These would not, however, be appropriate for historic masonry, as the façade would effectively be covered in a black paint or sprayed concrete. To minimise visual impact anodes are usually inserted into the existing fabric, as either ribbons which can fit between masonry joints or rods which can be drilled into the masonry. Mixed metal oxide (MMO) coated titanium ribbon is a relatively cheap form of anode as there are many suppliers of quality products. However, where steel members are buried deep in the wall, ribbon is sometimes unable to deliver the current to all the surfaces, as it is usually installed parallel with the surface of the masonry, typically either in a masonry joint or under interior plasterwork. An example is where a system is being installed from the internal face of the wall, which is often an excellent way to combine work with internal fit-out, reducing the cost of scaffold access and minimising disruption to the heritage facade. The downside is that, in most cases, only discrete rod anodes will be able to deliver the current deep into the masonry to protect the outward facing aspects of the steel frame. As with all good engineering design, the health, safety and environmental risks should also be assessed when choosing the type of anode. Rod anodes require many holes to be drilled which comes with hand- arm vibration considerations. Ribbon requires long lengths of deep chasing and, although grinder work is generally safer, grinder accidents are usually very severe. The designer should also consider innovative products such as geopolymer conductive mortar. What information there In- fills External face Different Types of CP 5 Cathodic Protection Impressed Current Galvanic Hybrid Rods/Discrete Carbon Paint Geopolymer Ribbon Sticky back zinc Patch-repairs / incipient Anode XP Patch Guard Mapei CC Duo Guard Fusion Ribbon Fig 2. Embedded steel work and the effect of corrosion Fig 3. Common types of cathodic protection Those procuring CP are sometimes confused by the similarity of systems which are designed for very different uses. The category shown in red are technically galvanic anodes, but they are shown separately as their purpose is not to protect the structure, but to ensure the long term durability of isolated concrete patch repairs. These are rarely applied to steel framed buildings