Guidance from Purcell stresses the need to use heritage-trained architects, spend time and effort on detailed design, and collaborate with council conservation officers
With a growing number of public and private sector buildings likely to be affected by unsafe RAAC planks and panels, new advice backed by heritage experts reveals the best practice approaches to its investigation and remediation in listed buildings.
Created by conservation architect Purcell, the document is intended for buildings that have either been extended or undergone internal alterations incorporating Reinforced Autoclaved Aerated Concrete between 1950 and the 1990s, the period when the material was manufactured.
The guidance is endorsed by the Twentieth Century Society, the Society for the Protection of Ancient Buildings, and the Institute of Historic Building Conservation.
RAAC is a lightweight reinforced concrete, whose soft bubbly fill material becomes unstable due either to being cut during building modifications, or because of degradation caused by water ingress or overloading – or as a result of manufacturing defects. Consequently, steel reinforcing bars within the wall or floor panels can decay, rust and weaken.
Although issues with RAAC have been known about for over two decades, its potential for failure has only been recognised in the last five years, mainly due to structural failures in schools and other public buildings.
According to Purcell, avoiding any negative impact on listed status means appointing a consultant team, typically involving a structural engineer and either a conservation architect or chartered building surveyor with relevant experience of historic architecture.
Initial assessment of the building may need consultation with the local planning authority’s conservation officer to establish whether listed building consent is required for opening-up works. Materials containing asbestos should also be scoped out and tested for, as they could be disturbed by opening up or intrusive testing.
If RAAC is found to be present, detailed structural and condition assessment by the consultant team should, the guidance notes, involve a detailed survey of RAAC panels and the surrounding area to establish condition and inform the development of proposals for remediation.
If RAAC is adequately exposed, a risk assessment carried out by the structural engineer in line with IStructE guidance will advise the client of ongoing risks. If panels are concealed, the conservation officer will need to clear any intrusive opening up work, which the guidance states ‘should be planned to avoid or minimise any harm to the listed building’.
Once a risk assessment is completed, it is critical, says Purcell, to establish both the significance of the fabric where RAAC is located and how any remedial works may affect the listed building.
Key questions to consider as part of this include whether or not the panels are exposed –and/or part of the aesthetic appearance – or concealed; and how RAAC contributes to the visual appearance/form of the building and the adjacent materials. Furthermore, the nature and significance of the space that the RAAC occupies should be considered, alongside the depth of the panels and how a variation of that depth, due to replacement by another material, would affect the building and detailing.
Answers to these questions, combined with the risk level established as part of the identification process, will inform the approach to remediation.
According to Purcell, more time and effort should be put into detailed design for listed buildings. In addition, the combination of high resolution detail drawings and a carefully-written specification by conservation specialists ‘should achieve high quality building work required of listed buildings’.
Another key aspect is the contents of the space in which the RAAC is identified. Many listed buildings house important collections, objects and artwork, with will need to be temporarily relocated if they are at risk from any potential collapse before or during the course of remedial works.
Remediation actions must be agreed with the LPA conservation officer and building control officer and may involve: cordoning off areas of high risk, installing temporary propping, installing secondary/ pairing beams to bearing ends and/ or replacing individual panels with an alternative material. Seriously affected areas may require a long-term support structure or the replacement of entire areas of RAAC.
All these options may require listed building consent and close liaison with the local planning authority and wider stakeholders.
Long-term solutions for the replacement of RAAC need to be developed ‘with care and technical expertise’, says Purcell, as set out in Historic England conservation principles, policies and guidance. The building owner must also develop a management strategy for any RAAC panels assessed as having residual risks by a structural engineer.