Reinforced aerated autoclaved concrete (RAAC) planks are now considered to be short-life materials where failure, while rare, could have disastrous consequences.
RAAC planks were first introduced into the UK in the mid-to-late 1950s and production stopped in 1982. By the 1980s, failures started occurring to RAAC planks. Where complete failure occurs, it occurs suddenly. This has brought RAAC planks greater public awareness. Property owners and managers need to take action to determine whether their buildings are at risk and what mitigation measures might be required.
RAAC is a form of steel-reinforced lightweight concrete, with a porous internal bubble-like structure. The reinforced planks are large structural units working independently, loads are not shared as with conventional concrete. Planks are up to around 2,400mm long and 600mm wide.
They can be found in many buildings including schools, hospitals, primary care facilities, infrastructure, libraries, shopping centres and commercial buildings. Most used for flat roofs, they are occasionally found in walls or tiled mansard structures.
According to the Building Research Establishment, the material has a life expectancy of around 30 years. If built to standard and kept dry, RAAC can last longer. However, rushed construction, less stringent quality control methods of the time and later re-roofing leave it vulnerable. This does not mean failure will occur beyond life expectancy, but the risk is increased.
RAAC planks are gaining notoriety with mainstream news organisations. The managers and operators of buildings will be well advised to take suitable steps to identify, investigate and mitigate risks.
The steel reinforcement is often coated with bitumen or cement latex to provide corrosion resistance. However, this doesn’t provide permanent protection or often is insufficiently covered. This and the porous structure of the concrete make it vulnerable to the effects of moisture. If the steel is weakened by corrosion, it can tear out of the plank and the plank can sag and crack; this deflection can be exacerbated by the weight of roof finishes and additional loads. The material is softer than normal concrete and crushed easily, so if the bearing is inadequate, it creates a risk of failure at the ends of the planks.
The first step for an estate manager is to identify which buildings may be at risk. Age of the property is the first consideration. Construction is the second. Visual confirmation of RAAC planks can be achieved by internal inspection (viewing the underside and above any suspended ceilings). Finding RAAC will trigger further investigation best handled by experienced building surveyors who can recognise problem areas and can advise how best to proceed.
Site inspection will look to: identify if there is transverse cracking to the soffits; find any cracking and disruption around the bearing and deflection of the planks (and any notable difference between adjacent planks); check the bearing widths are no less than 40mm; identify staining or evidence of corrosion of the reinforcement; and inspect the condition of the roof covering (past repairs or unresolved leaks) and ponding etc. This may trigger the need for more detailed investigations. Once RAAC is identified to a property, a detailed risk assessment is required considering the building use and the potential for harm, should failure occur. It may be that following site investigations and the risk assessment; the risks can be mitigated by management (repairs, re-roofing, reducing loads, monitoring and reinspection) or structural intervention.
RAAC planks are gaining notoriety with mainstream news organisations. The managers and operators of buildings will be well advised to take suitable steps to identify, investigate and mitigate risks.
Hayley Coles is an associate at Watts
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