Context 164 - May 2020

C O N T E X T 1 6 4 : M A Y 2 0 2 0 25 The war on damp the development phase and practice, many issues had to be overcome on-site, as it is impossible to replicate every construction and environmental scenario in the ‘lab’. Nevertheless, the develop- ment period helped enormously and provided the skills and understanding to overcome the issues as they arose. Grouting the monument The works were undertaken from March 2018 to July 2019, organised by a main contractor providing all access and temporary works to support NTS stonemasons to carry out the grouting works.The main contractor also carried out all other work, including new stone indents, structural repairs, leadwork, and renewal of drainage and other building services. In summary, the grouting operation involved the following stages: Joint preparation and recording Raking out defective mortars, soils and all debris from joints and beds. A selection of hand tools were used, including hacksaw blades, mortar rakes and panel saws with continuous water flushing essential to wash out debris and to form an effective cutting paste from water and grit. Water flushing and probing with fine rods and hacksaw blades after joint cleaning allowed us to understand the depth and width of voided joints and if the voids led into the core. All this information was recorded.This was essential to make decisions regarding choice of grout, grout- ing technique and expected volume of material that should be absorbed. Preparation immediately pre-grouting Essential pre-wetting of all open joints and voids con- trolled suction so that grout would flow as deeply as possible. Cleanliness throughout all processes was necessary to avoid interruptions to grout flow. Clay strips applied to joint faces prevented grout leakage and tell-tales (such as drinking straws) inserted at regular intervals relieved displaced air during joint filling and indicated grout flow. Grout selection Selection of proprietary hydrau- lic lime grouts ensured an initial hydraulic set deep in the structure and provided sufficient durability at the surface to endure local condi- tions. Grouts with strengths of between 2 to 5 N/mm 2 were specified as appropriate to each application or location: high durability and lower permeability on upper dome surfaces, parapets and cornices; lower strength, highly permeable grouts were chosen for less exposed, lower-level areas. Two ranges of grout fluidity were used: 1. Highly fluid hydraulic lime grouts modified with synthetic plasticisers and colloidal agents Opposite: The monument in 2019 on completion of grouting works A typical deeply voided joint Fully fluid grout injection