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School roof collapse prompts concrete plank warning

NCE stock health and safety

The collapse of a school roof has prompted a leading structural safety body to put out a warning for buildings built using the same reinforced autoclaved aerated concrete (RAAC) planks.

The report concludes that the collapse occurred with little warning and that a similar, near failure was reported in 2019 in a retail unit which incorporated the same type of concrete planks.

The school roof collapsed last year, but the conclusions from the investigation have just been published by Standing Committee on Structural Safety (Scoss).

The reports adds that the concrete planks in question - which were popular in construction between the 1960s and 1980s - are much weaker than traditional concrete due to the way they are made.

As the lifespan of the planks is estimated to be around 30 years, Scoss is now recommending that all planks installed before 1980 are replaced.

It is not the first time problems have arisen with the planks. According to the report there have been many failures of RAAC planks which had been installed during the mid-1960s with a large number of buildings demolished as a result.

The failures highlighted primary deficiencies such as incorrect cover to tension steel, high span-to-depth ratios, insufficient provision of crossbars to provide anchorage for the longitudinal steel, failure in performance of roof membranes and rapid worsening localised steel corrosion.

More recently, concerns about the planks have arisen due to rusting of the reinforcement leading to cracking and spalling of the AAC cover. In this case the cracking is thought to be associated with moisture and changing temperatures. There have also been excessive deflections due to creep and floor and roof planks acting independently, rather than as a single structural entity.

In some cases the report concludes that deflections had become appreciable leading to ponding of water, increasing the loading on the roof. Water penetration of the planks has also caused reinforcement corrosion.

In the case of the 2018 collapse, the report states that there was evidence of shear cracking adjacent to a support and possible indications of tension reinforcement stopping short of the support.

It adds that those concerned should conduct a risk assessment of the structure, potentially discontinuing the use of the space underneath until it had been strengthened or replaced.

Reinforced autoclaved aerated concrete

Autoclaved aerated concrete differs from normal concrete as it does not contain coarse aggregate and is made in factories using fine aggregate, chemicals to create gas bubbles, and heat to cure the compound.

It is relatively weak with a low capacity for developing bond with embedded reinforcement. It has been used in two main forms of structural elements; lightweight masonry blocks and structural units such as roof planks, wall and floor units.

When reinforced to form structural units, corrosion protection is provided by a bituminous or a cement latex coating, which is applied to the reinforcement before the planks are cast.

The reinforcement mesh is then introduced into the formwork and the liquid concrete mix added.

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Readers' comments (5)

  • Yet another article that has not been spell-checked, reread or edited - see paragraph beginning “More recently...”.

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  • A worthy report but I am "impacted" by NCE going "big" with a grammatical "fail". OK, I will get my coat...……..

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  • An interesting article that draws attention to a serious issue for buildings of that age and construction. Suggest that an image of RAAC concrete would have been beneficial for readers, to appreciate the appearance of this potentially unsafe system.

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  • At the start of my career, as a trainee draughtsman from 1970-74, I detailed more than 20 school roofs, mainly in Kent and Herts, using RAAC planks produced by Gascon Building Materials, and was also involved in supervising their installation on site. RAAC was at that time regarded as something of a "wonder material" - relatively lightweight, easy to install, and with good insulation properties etc. But at much the same time high-alumina cement was also in widespread use, which later proved a disadvantage for many structures. We live and (hopefully !) learn. We also detailed a complete pub in the same planks - the "Rifle Volunteer" in Dunstable if I remember rightly.

    Mike Paul (retired)
    Stuttgart,
    Germany

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  • Thank you for publishing this article NCE. A link to the full SCOSS Alert can be found here: http://bit.ly/SCOSS_Alert_RAAC

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