Reinforced Autoclaved Aerated Concrete is a low-density, closed cellular structure material, weighing around 25% of normal concrete. The texture of RAAC is similar to honeycomb and it contains steel reinforcing bar for added tensile strength. RAAC shares some of the typical ingredients of regular concrete, however it omits the heavy aggregate.

To manufacture RAAC, cement and sand are mixed together with water to form a slurry. Additives are introduced which cause the mixture to expand during the curing process. The slurry is then poured into a mould around reinforcing bars and placed in an autoclave to ‘cook’ in a pressurised steam environment and at high temperature.

Purcell’s guidance primarily aims to provide information to building and estate owners to help them deal with the issues of RAAC in the context of listed buildings, reducing safety risk. As heritage architecture experts we have aimed to provide readers with information on how to mitigate heritage impact in RAAC remediation strategies. 

RAAC in schools has been the major focus of news and published research, initiated by the Department for Education (DfE) following the collapse of multiple school roofs. RAAC can be found in all types of building however, putting individuals across all communities and groups at risk. Prior to Purcell publishing this guidance, there was no publicly available information relating to listed buildings or other designated heritage assets affected by RAAC, therefore, since publishing the document earlier in 2024 the guidance has been warmly welcomed, filling this void.

In the context of protected buildings, careful, well-planned investigations should be undertaken, avoiding damage to the buildings. A team of professionals with heritage building expertise should be selected to appropriately investigate and design solutions, protecting the character of the affected buildings. The team may include an architect, chartered building surveyor and structural engineer who will also be well placed to advise the building owner of choosing a suitable builder with relevant experience and expertise to the affected building. 

The Institution of Structural Engineers have published guidance for the professional assessment of RAAC. The appointed structural engineer will assign a risk category for RAAC panels according to a RAG (red/amber/green) risk rating. The risk level, together with the use of the listed building, will inform the nature of the remediation measures and if the area will need to close until a temporary or permanent solution has been carried out. 

For building owners, cost implications can accumulate quickly if they have to close their buildings due to RAAC identification and a structural engineer risk assessment finding as higher risk. Loss of revenue from subsequent closure can be reduced with a temporary solution, often involving propping, designed by a structural engineer and heritage architect to omit or minimise irreversible damage to the listed building fabric. 

Purcell’s guidance document provides 3 case studies where RAAC has been identified in listed buildings. The York Castle Museum case study is a particularly interesting instance of where RAAC has been found in a Grade 1 listed building of 18th Century construction. A carefully considered scheme for temporary propping was developed between Purcell architects and BDP structural engineers. 

The temporary solution avoided any irreversible fixings into the highly significant building fabric and enabled the client to re-open the museum after it was closed for 3 months following the identification of RAAC and assessment as higher risk by the structural engineer. Once propped, the client was able to focus their efforts on preparing the design for a permanent solution, without worrying about additional loss of revenue from forced closure.