Context 164 - May 2020

C O N T E X T 1 6 4 : M A Y 2 0 2 0 23 The war on damp Developing a repair strategy Great care was taken in the original design to shed water off the exterior surfaces. Original details such as water joints on cornices, sloping ‘flat’ surfaces and concealed joints help to miti- gate water penetration. The building has little evidence of alteration and only in recent decades has significant damage occurred to internal surfaces, so we thought it reasonable to infer that the structure successfully shed water and kept the interior reasonably dry for many years. We reasoned that if lime mortar could fully fill all open joints and masonry voids, we could restore the original design philosophy of shed- ding water from the exterior. However, when mortar joints are only some 1–3mm wide at the surface, yet voided to depths of 250mm or more, typical lime-rich ashlar mortar mixes and fine pointing keys could not fully fill these joints. Grouting with lime mortar has been used to great effect. Historic England has undertaken some work on this through its Damp Towers research project 1 . Existing grouting technol- ogy such as the clay-cup method 2 is too slow to grout narrow ashlar joints on a large scale, and the textured masonry joint faces and surface tension of grout create a high resistance to flow. Significant advantage can be gained by injecting grout using narrow nozzles pushed into the joints up to 150mm deep. This overcomes the resistance to flow through narrow outer joints and successfully delivers grout into the back of the joint, exactly where it is needed. In addition to fully filling all open joints, our repair strategy also required conservation heating to provide a moderate flow of warmed air into the interior chamber. This, combined with good levels of natural ventilation, helps the drying-out process by gently driving moisture out of the masonry and assisting evaporation from interior surfaces. Grouting trials Injection equipment and our proposed applica- tion techniques were extensively trialled in 2017 before any attempt was made to repair the building. The research and development phase was part-funded by a technical research grant from HES, in association with the NTS in-house stonemasonry team at nearby Culzean Castle. A stone-built test wall resembling the monu- ment’s masonry characteristics was built to trial our grouting methodology, and was refined and improved as testing progressed. Grout was injected into the test wall, which was then dismantled to observe grout flow and its extent of penetration into the voided core. All issues arising were considered, solutions found and tested further in subsequent trials. Of equal importance, the NTS stonemasons developed their skills in the processes and learned how to use the equipment to best effect. Refining the methodology and developing the requisite understanding and skills to competently implement the grouting process was crucial prior to commencing the operation on-site. Despite The interior chamber in 2016, prior to grouting works The interior chamber in 2019 with visible evidence of initial drying of the formerly saturated masonry References 1 Wood, C and Burns, C (2013) ‘Grouting Solid Masonry Walls’ in Historic Churches , Cathedral Communications, Tisbury 2 Scottish Lime Centre Trust (2013) How to grout ashlar masonry