the cracking of many of the glazed terracotta units. lhis isparticularly severe on the pilasters which are located at ground floor level. In addition, and as a direct result of cracking and spalling of the terracotta in the past, there are large areas of plastic mortar repairs in different areas of the building. All of these repairs, together with some isolated areas of terracotta, have been painted with a red gloss paint with the intention of visually blending the repairs in with the red glazed terracotta. Other existing problems include the presence oflarge amounts of reprecipitated calcite under many projections (such as the cornice) and a surface coating (probably an organic water repellent) on the Courtfield Road facade. Previous engineering reports had concentrated on the problem of terracotta cracking and had attributed this to the corrosion, expansion and consequent movement of a steel frame inside the structure. The recommendation for dealing with this was to remove all terracotta and brick from around the steel frame, blast and protect the frame and replace the brick and terracotta. Due to an umber ofproblems with this strategy including its high cost and associated conservation philosophy difficulties (most terracotta would be irreparably damaged in the process), the client, London Underground Ltd (LUL),sought a second opinion. In July 1995 LULappointed a multidisciplinary team including architects, engineers, scientists, metallurgists and conservation specialists, in the form of Lawrence and Wrightson, Carrig, Mott McDonald and the LUL's own in-house staff, to investigate alternatives to the 'dismantle and rebuild' strategy. The approach adopted by the team was comprehensively to determine the causes of damage and to base all conservation strategies on eliminating these asfar as possible. The first, and most important, step involved determining the cause ofcracking of the terracotta through structural investigation. lhis task was facilitated by detailed original drawings of the steel frame (see Figure 2) which showed the precise location, dimensions and construction of each stanchion and beam in the station. The condition of these structural members was determined by opening up a number of areas where the terracotta was already badly damaged. The openings revealed that approximately 3 mm of rust had built up on the outside faces of the flanges and webs of the stanchions and beams. lhis rust build up had pushed against the tightly packed mortar and brick fill v v electrolyte(wetmortar); v v [7 v v ~ v fowof~ctrons v [7 [7~~ anode(+)' 'J ~ c l-7777777J'7T,,.. rust CONTEXT49 Figure 2: Typical constructional detail. around the steel (see Figure 3) and forced it apart, resulting in turn in cracking and fracturing of the glazed terracotta facing of the building. The cause of this is the ingress of moisture into the structure of the building, the wetting of the steel, the formation of corrosion cells and the ensuing deterioration of the steel by rusting. The primary locations of water ingress included the parapet and upper cornice. Significant amounts of reprecipitated calcite showed that water was entering the structure, dissolving the free lime and -calcite in the bedding mortar and terracotta void packing, and reprecipitating as hard white calcite where it exited the structure under the cornice. Another source of water ingress was damaged or blocked internal down pipes. The solution to arrest the ongoing corrosion and terracotta cracking totally is thus deemed to be the exclusion of water, as far as possible, from the structure. lhis is achieved by replacing all internal down pipes, lead flashing the Figure 3: Left: the constructiuon process Right: The cathodic protection of steel using a sacrifical anode. v v 7
RkJQdWJsaXNoZXIy MjgyMjA=