Conserving Burns Monument
|The interior chamber in 2016, prior to grouting works, and the interior chamber in 2019 with visible evidence of initial drying of the formerly saturated masonry.|
Water penetration of traditional solid mass masonry is becoming an increasingly common issue. While traditional solid mass masonry has performed very well in past centuries, with the extremes of weather in such exposed locations as the west of Scotland, we are aware of increasing instances of water penetration in traditional masonry buildings, including those that have been subject to conservation repair, especially where repointing is involved.
Ayrshire in south-west Scotland experiences around 200 rainy days a year, contributing to around 1,500mm annual rainfall and an average relative humidity of 90 per cent. These are challenging conditions for any building to weather and provide a comfortable dry internal environment.
Burns Monument was constructed 1820–23 to a clever design by architect Thomas Hamilton Junior, based on the ancient Greek choragic monument of Lysicrates, and incorporating much symbolism commemorating poet Robert Burns’ achievements and his associations with freemasonry. It is constructed of traditional composite masonry of two skins of ashlar sandstone with a central core of lime mortar and rubble. The ashlar is of the finest quality, using durable Scottish sandstones cut and wrought in the finest stonemasonry practices of the time. It has always been a popular visitor attraction, and is now Category A listed by Historic Environment Scotland (HES) and visited by over 100,000 a year.
Water penetration and surveys
The property came into the care of the National Trust for Scotland (NTS) in 2008. Serious dampness to the interior masonry was evident from this point. Initial condition surveys revealed that a high proportion of mortar joints were either open or had been repointed with various mortars over a number of successive attempts, but underneath the surface skin the joints were open. Further investigation revealed that many of these voids led straight into the core masonry, providing a clear route for water to enter and saturate the structure.
Between 2008 and 2016, during a period of further surveys and monitoring, the effects of water penetration on the ashlar interior worsened significantly. It was clear that the issue was progressive, and was beginning to cause irreversible damage and disfigurement to the carved and embellished ashlar masonry interior.
Concurrent investigations of the archive record revealed that the original architect’s specification for the fine ashlar jointing mortar required white lead, linseed oil and chalk filler. This had evidently washed out over the years leaving voided mortar joints. We also realised that many beds of large masonry units had been left empty during construction. Pads of mortar placed only at either end allowed the masons to tap each ashlar stone into place, fitting accurately against adjacent stones; something they could not have achieved if the beds had been completely filled with mortar.
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 mitigate 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 shedding 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 . Existing grouting technology such as the clay-cup method  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.
Injection equipment and our proposed application 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 monument’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 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 development 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, grouting technique and expected volume of material that should be absorbed.
- Preparation immediately pre-grouting: Essential pre-wetting of all open joints and voids controlled 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 hydraulic lime grouts ensured an initial hydraulic set deep in the structure and provided sufficient durability at the surface to endure local conditions. Grouts with strengths of between 2 to 5 N/mm2 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:
- Highly fluid hydraulic lime grouts modified with synthetic plasticisers and colloidal agents balanced very high fluidity requirements with lowest possible water content (up to 86 per cent). These were shown to flow many metres through the structure at a ‘runny custard’ consistency. They are complete mortars as they contain very finely ground aggregate fillers (particle sizes not exceeding 0.1mm). Fluid grout was always injected from the bottom of a structure, flowing and settling by gravity only. Progress was made upwards in small stages of one or two stone courses at a time.
- Modified hydraulic lime grouts with thixotropic restricted flow characteristics for ease of placing in situations where only the outer portion of narrow ashlar joints required to be filled to depths of up to 150mm. Again, these are complete mortars, containing finely ground aggregates and very low water content (up to 46 per cent). Restricted flow grouts were applied from the top-down to avoid working over freshly placed grout.
- Joint finishing and aftercare: After initial setting, joint surfaces were finished using conventional lime work practices such as tamping and/or scraping the surface. Conventional good practice lime aftercare procedures were applied in common with all lime work.
Over the construction phase of 15 months, approximately 8,000 litres of hydraulic lime grout was injected into joints and interstitial voids throughout the monument, equal to about three per cent of its volume. A large proportion of this was delivered through the narrow nozzles via exterior mortar joints, but where opportunities arose where stones were removed, large volumes could be poured directly into the core to dissipate through the void network.
We observed environmental conditions improving within the monument immediately after all exterior joints were filled with grout. Despite the masonry being saturated from years of water penetration and from the grouting operation, interior surfaces began to appear drier and salt efflorescence started to occur, indicating net evaporation.
A temporary limewash was applied to internal surfaces for sacrificial protection of the finely carved and fragile ashlar surfaces against potentially damaging effects of crystallising salts at the surface. The surface is dry-brushed each week to remove these salt blooms.
We are continuously monitoring interstitial moisture levels using probes inserted into a network of mortar joints. The probes measure humidity and temperature conditions within the wall, allowing us to monitor and report on the drying-out phase over the next three years and beyond.
- Wood, C and Burns, C (2013) ‘Grouting Solid Masonry Walls’ in Historic Churches, Cathedral Communications, Tisbury.
- Scottish Lime Centre Trust (2013) How to grout ashlar masonry.
This article originally appeared in IHBC's Context 164 (Page 22), published by The Institute of Historic Building Conservation in March 2020. It was written by Kinlay Laidlaw, director of building surveying and heritage consultants Laidlaw Associates Building Surveying.
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