8 top of the parapet wall and by applying mastic to the skyward joints of horizontal projections such as the cornice. Because it is impossible to eliminate moisture completely from the fabric of a building, it is not possible to arrest the process of steel corrosion totally without protecting it. For this reason itwas decided to stabilise the steelwork by means of cathodic protection. This involves the application of a small electric current to the steel to make the frame act as a noncorroding cathode. Although this technology dates from the middle of the last century, it has never been applied to a structure of this nature in the United Kingdom (a similar smaller structure has been cathodically protected in Dublin). Steel and iron corrosion is an electrochemical process where electrons move through an electrolyte (usually saltrich water) from an area called an 'anode' to one called a 'cathode' (see Figure 4). At the anode site the product of the reaction is commonly known as rust, and this grows and expands as the corrosion process proceeds - it is this characteristic ofrustwhichisofprimaryconcerntothe conservator as it generally results in the spalling of masonry in the vicinity of the corroded area. Other associated problems such as the loss of uncorroded crosssectional area, or a decrease in the structural integrity of the corroded element, are generally not an issue in load-bearing masonry structures, but may be a considerable problem in reinforced concrete structures. Steel which corrodes in this way can be cathodically protected by impressing a flow of electrons from an external 'sacrificial' anode towards the metal being conserved, which acts as the cathode (see Figure 4). This totally eliminates any possibility of corrosion of the conserved metal since the entire surface of the metal now acts as a cathode (hence the name 'cathodic protection') and there is no opportunity for the formation of anodes on it. In practice the cathodic protection system consists of long thin anode rods within the masonry fill and adjacent to the corroding steelwork. Each rod is connected into a circuit and direct current is impressed between these anodes and the steel frame itself. The frame thus acts as a large cathode and is therefore protected from further deterioration. Once the structure has been stabilised it isthen practical to carry out a restoration programme involving the replacement of cracked and repaired terracotta units with new blocks; these have already been manufactured by Shaws of Darwen. In addition to lead flashing the parapet and application of mastic to skyward joints, the paint and water repellent coating will be removed from the terracotta blocks, and the terracotta facade cleaned. It should be stressed that all of the cleaning and removal techniques have been assessed both in the laboratory and in situ to ensure that no damage is caused to the glaze or body of the terracotta units. For example, it was found that the terracotta had been treated at some time in the past with a protective coating. This had since failed, the once transparent application had become cloudy and cracked in appearance. It was decided to remove this coating for two main reasons; it was detracting visuallyfrom the integrity of the building and the coating did not serve, and could never have served, any useful purpose since the glazed tile is almost totally impervious to water penetration in the first place. Before effective removal could be carried out it was important to identify the chemical make up of the treatment so that the most suitable solvent could be chosen for its removal. From experience and by a process of elimination in the laboratory it was determined that the treatment was most probably a polyurethane-based water repellent. A product containing suitable organic solvents was chosen to remove the failed coating. The solvent was then successfully tested in situ on the building to ensure that its practical use did not lead to any nforeseen problems. It can be seen that the methodology adopted, a rigorous assessment of the causes of damage and the design of a strategy for their elimination; has resulted in a clear definition of the problems and their solutions. Recommendations have been chosen to minimise the extent of intervention -rather than 'dismantle and rebuild', the new strategy could be described as 'stabilise and restore'. This approach represents a less intrusive and more sympathetic approach to the conservation of early steel-framed buildings. Dr Aidan Duffy BA BAJ is an engineering graduate of Trinity College, Dublin, and is one of the principals of Carrig Conservation Engineering which is based in Dublin, but also has offices in Florence and Manchester (tel: 0161-928 8428); David Wrightson MCD BArch DipCons (loASS York) is a partner in Lawrence & Wrightson Architects and Landscape Architects, Voysry House, Barley Mow Passage, Chiswick, London W4 4PN. INFRA-REDTHEMOGRAPHY Robert Demaus discusses a technique with many uses for the Conservation officer. In previous articles ( Context 35, p28 and Context 39, p25), I discussed the applications of ultrasound and micro-drilling for the assessment of decay and structural weakness in timber. In this article, I will be looking at the use of infra-red thermography in a wide range of building diagnostic applications. Allbodies radiate energy continuously in the form of electromagnetic waves: the strength of this radiation in the infrared range is related to the thermal properties of the body, its surface temperature, and its emissivity. The components comprising that body absorb, transfer and reflect the energy in different ways depending on their composition. It is the thermographic camera's ability to identify these often minute differences that makes it such a powerful diagnostic and archaeological tool. The sensitivity of the latest equipment allows a huge amount of information to be collected about the original construction, historical changes to the structure, structural failures and decay and damp problems hidden behind the outer surface, as well as variations in that outer surface that are not visible to the naked eye. It can be seen therefore that the technique can be as valuable for its archaeological as for its structural information. Thermographic cameras are generally similar in concept and appearance to a large hand-held video camera. Wide angle, telephoto and macro lenses are available to suit different operating conditions and the colour images are stored on either video tape or discs for subsequent analysis and printing. The control unit has facilities to add spot and difference temperatures, isotherms, magnification and adjustments to image contrast. The essential and very expensive differences from a conventional video camera are that the camera contains a photo-voltaic temperature detector cooled to 80 K (approx.-193°C),andthelensesaremade of germanium dipped in gold. The very low temperature isneeded to achieve the sensitivity required (within 0·08°C at 30°C ambient temperature). Infra-red thermography has an CONTEXT49
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