THE issue of RAAC (Reinforced Autoclaved Aerated Concrete) reared its head again in Scotland recently, with the news that Aberdeen City Council had approved the demolition of more than 500 homes affected by RAAC in roof panels.
An estimated 366 council and 138 private properties in the Balnagask area of Torry are to be demolished on safety grounds, followed by a multi-million-pound rebuilding programme lasting many years.
The local authority accepted officers’ recommendations for dealing with RAAC following independent survey work, a detailed options appraisal, and community engagement. Several remedies were considered – including roof replacement – however demolition was judged to offer the ‘quickest and most cost-effective’ way to remove the ‘high risk’ posed by the lightweight concrete.
Stuart Fleming, director at Will Rudd Edinburgh, told Project Scotland that the realities of RAAC that are so problematic are, that, whilst it is a ‘fairly generic and recognisable’ product, its use in construction spanned decades around the mid 20th Century. “Therefore, quality control of original manufacture will have varied, the applicable codes of practice of the day have changed and evolved, and the age of the RAAC planks seen now, relative to their expected design life, varies greatly,” he added. “Whilst there are universal aspects to consider with each RAAC roof that exists, each one has its own prognosis.
“Structural engineers, like Will Rudd, who have experience in identifying, investigating and remediating RAAC, can inspect, measure, and appraise the condition and performance of the planks with a comprehensive data gathering exercise. We are then able to risk assess and give each individual plank a risk rating, using a traffic light system. From this we are able to provide discrete advice to inform stakeholders on whether to: keep the property in use and monitor, with recommendations for monitoring frequency; mitigate the risks with localised temporary works to permit the property to stay in use; put high-risk areas out of use, but delineate, so that other areas can remain in use; close the property to allow necessary mitigation/ remediation.
“From this we are also able to inform owners and stakeholders with their whole life cycle cost analysis, and there may be a case for replacement of the RAAC as part of broader considerations.
“Where there are long term objectives to sustain the property in question, we are often able to provide structural solutions, to strengthen, or re-support the RAAC roof, taking away the inherent risks, and giving the building a viable future service life. This can often be done with limited decanting, and disruption. It is also possible, and logical, where RAAC roofs are being retained and re-supported, the structural interventions can also facilitate sustainability and decarbonisation objectives such as supporting over cladding, additional insulation, and PV roof panels.”
Alan Ferns, director – structural engineering at East Kilbride-based Dougall Baillie Associates (DBA), revealed RAAC panels are generally found as 600mm-wide pre-cast concrete planks commonly used for flat roofs, sometimes pitched roofs, and occasionally walls and floors.
“The issues with RAAC panels stem from their extensive use in the construction industry for over 30 years on a variety of different projects from housing, schools and a variety of other institution type buildings,” he told Project Scotland. “The majority of the construction record drawings have been lost, the exposed ceilings have been covered up, in keeping with changing décor and tastes, and existing occupants or caretakers have either retired or moved on. Therefore, the location of these slabs within the buildings has not been kept up to date or worse, forgotten about.
“Further to this, builders’ work alterations and modifications may have been carried out on the slabs where holes have been cut or drilled for new services, without adequate trimming, therefore weakening slabs further. In addition, where panels have been commonly used for flat roofs and these have not been adequately maintained, water ingress can saturate the panels, potentially increasing the panel self-weight, corrode the reinforcement within the panel, and reduce material strength. This increase in weight and loss of strength can reduce load carrying capacity and failures of the slabs can be sudden, catastrophic and with no warning. Therefore, it is important that these slabs are identified, surveyed and a monitoring regime adopted.”
Alan added that there are a number of inherent issues with the manufacturing of the RAAC slabs themselves. DBA has extensive experience in the survey and identification of RAAC panels in housing and institutional type buildings over the last four years. The firm’s services include survey and identification of RAAC slabs, reporting on findings using a traffic light risk matrix to identify areas of concern, and excessive slab deflection. The business can suggest recommendations and actions for further monitoring, remediation and/or removal of identified elements. A drone was recently added to inspection methods, which has allowed the company to inspect areas that are difficult to access and also identify the condition of roof finishes, possible areas of water ingress, and ponding.
Referring to the recent Aberdeen case, whether alternatives to demolition could have been pursued, and what the challenges would have been in trying to repair the homes, Alan said, “My understanding of the Aberdeen properties in question was that the RAAC slabs had been identified in the roof construction. In very simplistic terms (and without knowing the full construction of the properties), the residents would need to be decanted from their homes and re-housed during the refurbishment works.
“The RAAC slabs forming the roofs would need to be propped to minimise local collapse and then carefully removed, either as whole units or cut down to more manageable sizes. Following the slab removal and making good of the wall heads and associated preparation works, a duo-pitched timber roof truss arrangement could be constructed on site and utilise the existing superstructure arrangements for support. A concrete tile or other lightweight finish could be utilised to weather-proof the property, with insulation and plasterboard ceiling finishes incorporated to make good the decoration and insulate the home.
“I appreciate the number of the homes in question, the timescales to carry out the refurbishments and the need to act quickly has been at the forefront of the decisions being made, but the wholescale demolition of all the properties seems quite heavy handed, especially when Scotland is currently in the middle of a housing crisis.”
